ALBERT

Description: Lakes are major depositional systems for which the related depositional processes have long been considered relatively simple. Breaking this statement, this study presents a detailed analysis of deposits in Lake Saint-Jean, the third largest natural lake in Québec. In addition to postglacial deltaic and coastal depositional systems fringing the lake, current-controlled features such as a large subaqueous prograding wedge and three sediment drifts have been identified in its central portion based on two-dimensional (2-D) acoustic high-resolution subbottom profiles. The large subaqueous prograding wedge is a 4-km-long and up to 15-m-thick heterolithic shelf-like construction in the southeastern part of the lake. The three sediment drifts are 0.1–0.5-km-long and 2–5-m-thick mud mounds distributed on the lake floor in the central portion of the lake. Diatom analyses and radiocarbon dating show that the development of these current-controlled features occurred during the lacustrine phase, after the disconnection with the postglacial marine Laflamme Gulf at 8.5 cal. k.y. B.P. Depositional facies show evidence of recurrent bottom-current activity. Related deposits alternate with pelagic sedimentation stages characterized by the settling of mud and biogenic accumulations. We investigated the origin of bottom currents using a numerical simulation (SYMPHONIE, an oceanographic model), with the aim of modeling wind-induced lake-scale water circulation. Simulations suggest that the subaqueous prograding wedge and the three sediment drifts result from wind-induced bottom currents generated by storm events having wind speed greater than 10 m s –1 . Such strong winds are able to significantly affect sedimentation in the central portion of Lake Saint-Jean. The resulting wind-induced sedimentary features were integrated into a refined lacustrine depositional model that summarizes the evolution of a group of water bodies referred to as "wind-driven water bodies." This study applies a new tool for lake strata characterization and highlights the potential difficulty in differentiating them from marine deposits in the geological record.

Description: In our complementary geochemical study (Part 1), the Malaysian granitoids of the Southeast Asian tin belt were divided into a Middle Permian to Late Triassic I-type–dominated Eastern province (Indochina terrane) and a Triassic to Early Jurassic transitional I/S-type Main Range province (Sibumasu terrane), separated by the Bentong-Raub suture zone which closed in the Late Triassic. Previous geochronology has relied on only a few U-Pb zircon ages together with K-Ar and whole rock Rb-Sr ages that may not accurately record true magmatic ages. We present 39 new high-precision U-Pb zircon ion microprobe ages from granitoids and volcanics across the Malay Peninsula. Our results show that ages from the Eastern province granitoids span 289–220 Ma, with those from the Main Range province granitoids being entirely Late Triassic, spanning 227–201 Ma. A general westerly younging magmatic trend across the Malay Peninsula is considered to reflect steepening and roll-back of the Bentong-Raub subduction zone during progressive closure of Paleo-Tethys. The youngest ages of subduction-related granites in the Eastern province roughly coincide with the youngest ages of marine sedimentary rocks along the Paleo-Tethyan suture zone. Our petrogenetic and U-Pb zircon age data support models that relate the Eastern province granites to pre-collisional Andean-type magmatism and the western Main Range province granites to syn- and post-collisional crustal melting of Sibumasu crust during the Late Triassic. Tin mineralization was mainly associated with the latter phase of magmatism. Two alternative tectonic models are discussed to explain the Triassic evolution of the Malay Peninsula. The first involves a second Late Triassic to Jurassic or Early Cretaceous east-dipping subduction zone west of Sibumasu where subduction-related hornblende and biotite–bearing granites along Sibumasu are paired with Main Range crustal-melt tin-bearing granites, analogous to the Bolivia Cordilleran tin-bearing granite belt. The second model involves westward underthrusting of Indochina beneath the West Malaya Main Range province, resulting in crustal thickening and formation of tin-bearing granites of the Main Ranges. Cretaceous granitoids are also present locally in Singapore (Ubin diorite), on Tioman Island, in the Noring pluton, of the Stong complex (Eastern Province), and along the Sibumasu terrane in southwest Thailand and Burma (Myanmar), reflecting localized crustal melting.

Description: Liquefaction during the 2010 moment magnitude (M w ) 7.1 Darfield earthquake and large aftershocks (known as the Canterbury earthquake sequence) caused severe damage to land and infrastructure in Christchurch, New Zealand. Liquefaction occurred at M w -weighted peak ground accelerations (PGA 7.5 ) as low as 0.06 g at highly susceptible sites. Trenching investigations conducted at two sites in eastern Christchurch enabled documentation of the geologic expressions of recurrent liquefaction and determination of whether evidence of pre–Canterbury earthquake sequence liquefaction is present. Excavation to water table depths (~1–2 m below surface) across sand blow vents and fissures revealed multiple generations of Canterbury earthquake sequence liquefaction "feeder" dikes that crosscut Holocene-to-recent fluvial and anthropogenic stratigraphy. Canterbury earthquake sequence dikes crosscut and intrude oxidized and weathered dikes and sills at both sites that are interpreted as evidence of pre–Canterbury earthquake sequence liquefaction. Crosscutting relationships combined with 14 C dating constrain the timing of the pre–Canterbury earthquake sequence liquefaction to post–A.D. 1660 to pre–ca. A.D. 1905 at one site, and post–A.D. 1415 to pre–ca. A.D. 1910 at another site. The PGA 7.5 of five well-documented historical earthquakes that caused regional damage between 1869 and 1922 are approximated for the study sites using a New Zealand specific ground motion prediction equation. Only the June 1869 M w ~4.8 Christchurch earthquake produces a median modeled PGA 7.5 that exceeds the PGA 7.5 0.06 g threshold for liquefaction. Prehistoric earthquakes sourced from regional faults, including the 1717 Alpine fault M w ~7.9 ± 0.3 and ca. 500–600 yr B.P. M w ≥ 7.1 Porters Pass fault earthquakes, provide additional potential paleoseismic sources for pre–Canterbury earthquake sequence liquefaction. The recognition of pre–Canterbury earthquake sequence liquefaction in late Holocene sediments is consistent with hazard model-based predicted return times of PGAs exceeding the liquefaction triggering threshold in Christchurch. Residential development in eastern Christchurch from ca. 1860 to 2005 occurred in areas where geologic evidence for pre–Canterbury earthquake sequence liquefaction was present, highlighting the potential of paleoliquefaction studies to predict locations of future liquefaction and to contribute to seismic hazard assessments and land-use planning.

Description: The Siberian Traps are one of the largest known continental flood basalt provinces and may be causally related to the end-Permian mass extinction. In some areas, a large fraction of the Siberian Traps volcanic sequence consists of mafic volcaniclastic rocks. Here, we synthesize paleomagnetic, petrographic, and field data to assess the likely origins of these volcaniclastic rocks and their significance for the overall environmental impact of the eruptions. We argue that magma-water interactions, including both lava-water interactions and phreatomagmatic explosions in vents, were important components of Siberian Traps magmatism. Phreatomagmatic episodes may have generated tall water-rich eruption columns, simultaneously promoting removal of highly soluble volcanic gases such as HCl and potentially delivering additional sulfur to the upper atmosphere.

Description: The Central Asian orogenic belt formed by accretion subsequent to the contraction of the paleo–Asian Ocean, and its southeastern segment terminated along the Solonker suture zone, amalgamating the Mongolian arcs and the North China craton by the end of the Early Triassic. Since typical regional collisional features are absent, its tectonic evolution remains speculative. An investigation into the variability of detrital zircon U-Pb ages and Hf ratios in Permian strata along a southeast-northwest transect from the Mongolian arcs to the North China craton reveals distinct differences: Northern basins carry a broad Mesoproterozoic to latest Precambrian age signature, and their sedimentary source terranes are of mixed juvenile to crustal magmatic origin. In contrast, southern basins contain detritus from the North China craton, and their sources are of dominantly crustal contaminated magmatic origin. Provenance analysis suggests that in the early Paleozoic (ca. 429 Ma), the paleo–Asian Ocean was consumed along the Uliastai arc and the North China craton, initiating the formation of the northern and southern accretionary orogens, respectively. By the end of the middle Carboniferous, the Mongolian arcs were consolidated after the accretion of the Uliastai arc. In the late Carboniferous (ca. 314 Ma), the Hegenshan back-arc basin opened, detaching the northern accretionary orogen. While subduction continued there, it may have temporarily ceased in the south after the accretion of a microcontinent (ca. 300 Ma). By the end of the Middle Permian (ca. 269 Ma), back-arc basin closure led to the formation and obduction of the suprasubduction zone Hegenshan ophiolite. Eventually, the paleo–Asian Ocean closed by double-sided subduction. Such accretionary wedge-wedge collision would not involve continental deep subduction, unlike other continental collisions. The early stages of the sequence likely resembled a Pacific-type scenario dominated by the subduction of oceanic lithosphere, including a Japanese Sea–type back-arc basin opening. The late stages involved the accretion of large-scale continental blocks, ultimately leading to final ocean closure, which may be similar to the archipelago-type setting of present-day Southeast Asia.

Description: We report geochemical data from (meta-)sedimentary and igneous rocks that crop out in the Ford Ranges of western Marie Byrd Land and discuss the evolution and reworking of the crust in this region during Paleozoic subduction along the former Gondwanan convergent plate margin. Detrital zircon age spectra from the Swanson Formation, a widespread low-grade metaturbidite sequence, define distinct populations in the late Paleoproterozoic, late Mesoproterozoic, and Neoproterozoic–Cambrian. The late Paleoproterozoic group records magmatism derived from a mixed juvenile and crustal source. By contrast, the late Mesoproterozoic group yields Hf isotope values consistent with derivation from a juvenile Mesoproterozoic source inferred to be an unexposed Grenville-age orogenic belt beneath the East Antarctic ice sheet. For the Neoproterozoic–Cambrian population, Hf isotope values indicate reworking of these older materials during Ross–Delamerian orogenesis. New U-Pb ages from the Devonian–Carboniferous Ford Granodiorite suite across the Ford Ranges reveal an extended period of arc magmatism from 375 to 345 Ma. For four younger samples of Ford Granodiorite, Hf and O isotope values in zircon suggest involvement of a larger (meta-)sedimentary component in the petrogenesis than for two older samples. This contrasts with the secular trend toward more juvenile values documented from Silurian to Permian granite suites in the Tasmanides of eastern Australia and Famennian to Tournasian granite suites in New Zealand, pieces of continental crust that were once contiguous with western Marie Byrd Land along the Gondwana margin. The differences may relate to an along-arc change from the typical extensional accretionary mode in eastern Australia to a neutral or an advancing mode in West Antarctica, and to an across-arc difference in distance from the trench between the New Zealand fragments of Zealandia and western Marie Byrd Land. Upper Devonian anatectic granites in the Ford Ranges most likely record reworking of early Ford Granodiorite suite members during arc magmatism.

Description: Coastal dune accumulations at locations around the world have formed during sea-level highstands, rising sea levels, falling sea levels, and fluctuating sea levels. Initially, episodes of maximum Pleistocene dune activity in Bermuda, which is the type location for eolianite, were correlated with glacio-eustatic regressions. This model had been abandoned by the 1970s in favor of a rising sea-level model for Bermuda’s north shore and autogenic sediment-supply model for the south shore. We report the results of a detailed investigation of the intercalated mid-late Pleistocene beach and coastal dune deposits on the islands of Bermuda, and we test the existing models for these deposits. We contend that the north shore model is invalidated by past misidentification of eolian deposits as transgressive beach deposits. On the south shore, facies analysis of the marine isotope stage 7 (MIS 7) Belmont Formation reveals that coastal deposition was divided into two phases. S1, which includes foreshore and shoreface progradation, is predominantly marine and coincided with rising relative sea level. S2, which commenced with protosol development on top of the marine section prior to burial by advancing dunes, is nonmarine. We conclude that the two successions S1 and S2 resulted from forcing by glacio-hydro-isostatic relative sea-level change spanning a highstand (rising and then falling), rather than from transgressive or autogenic processes. The finding that, in Bermuda, the construction of protective coastal dunes depends on a falling relative sea level has potential implications for many vulnerable shorelines that face rising relative sea levels associated with climate change.

Description: Thin-skinned gravitational gliding and spreading drive deformation on salt-bearing passive margins. Such margins typically have an updip extensional domain kinematically linked to a downdip contractional domain. However, calculating magnitudes of extension and shortening in salt-bearing margins is difficult because the initial widths of diapirs are uncertain. Extension and shortening may be cryptic, being hidden in widening or shortening of diapirs. This uncertainty can lead to controversy in regional analysis. The Santos Basin, offshore Brazil, contains a prime example of this uncertainty in the form of an enigmatic structure known as the "Albian Gap," a zone up to 75 km wide within which the Albian section is missing. The Albian Gap has been variably interpreted as the product of post-Albian extensional faulting (the extension model) or as an Albian salt structure evacuated in response to loading by post-Albian sediments (the expulsion model). We evaluate these two models by: (1) structurally restoring a regional seismic-reflection profile across the Albian Gap using both models; (2) quantitatively analyzing the geometry of the Upper Cretaceous rollover overlying the Albian Gap; and (3) synthesizing and critically evaluating arguments previously advanced in support of extension or expulsion. We propose a revised model for the evolution of the Albian Gap that invokes Albian thin-skinned extension and post-Albian salt expulsion. Our approach shows that critical analysis of geological observations from borehole-constrained seismic-reflection data can be used to assess the relative roles of the key processes in the deformation of salt-bearing passive margins.

Description: Numerous geochronological and geochemical studies of the Late Triassic granite plutons in the Qinling orogen have been conducted over the past few decades. These studies have extensively discussed the genesis and correlations of granite plutons with the collisional processes between the North and South China blocks. However, several contradictory conclusions on the tectonic settings of these plutons (subduction, syncollision, postcollision, or intraplate) have been reached. Moreover, in all these studies, compressional or extensional structures have been always considered to control the magma emplacement, but no direct evidence has been presented so far. In order to clarify the emplacement mechanism of these Late Triassic plutons and avoid the ambiguities from purely geochemical studies, we conducted a multidisciplinary structural study on the Mishuling pluton in West Qinling, one of the biggest Late Triassic plutons in the orogen. New single-grain zircon dating confirmed that the Mishuling pluton was emplaced at 212 ± 2 Ma. Gravity investigations show an overall wedge shape to the pluton: A deep root is located in its western part, the thickness gradually decreases to the east, and it finally pinches out at its eastern extremity. We studied internal structures of the Mishuling pluton through field, microscopic, and AMS (anisotropy of magnetic susceptibility) observations, which show that the pluton did not undergo penetrative secondary deformation, and preserved cryptic internal fabrics were mainly acquired during the final stage of emplacement. In the western part of the pluton, internal fabrics display a concentric pattern. In the eastern part, the lineations mainly plunge west, while the foliations form an analogous W-plunging synform, mainly dipping SW in the northern part and dipping NW in the southern part. The three-dimensional shape and internal structures of the Mishuling pluton are concordant with the wrench tectonics of country rocks that developed during the late stage of the Late Triassic collision between the North and South China blocks. We propose an integrated emplacement model involving both the three-dimensional shape and internal fabrics of the Mishuling pluton, and regional structures of the country rocks. In this model, the magma rose in the western part and then spread eastward parallel to the regional strike-slip movements.

Description: Bleaching of red beds by the migration of reducing pore fluids is a widespread phenomenon, with most previous work on the topic focused on detailed studies in the southwestern United States, and to a lesser extent Germany. Herein, we report on widespread bleaching of Cretaceous red beds in the Neuquén Basin and the relationship of this bleaching to hydrocarbon migration. In the Cerro Granito area of the Huincul High, Neuquén Basin, the basal deposits of the Neuquén Group (Candeleros and Huincul Formations) are bleached. This alteration was apparently controlled by fluid and host-rock composition as well as by the intensity and span of the fluid-rock interaction. Red, fine-grained sandstones and mudstones of the Candeleros Formation were deposited in fluvial and swamp environments and contain authigenic hematite, Na-rich corrensite, micro- and mesoquartz, calcite, and analcite. Coarse-grained sandstones and mudstones of the Huincul Formation, which overlie the Candeleros Formation, were deposited in a braided fluvial system, under more humid and acidic conditions than the Candeleros Formation. Sandstones are similar in composition and texture to the Candeleros sandstones, except they contain less basic volcanic detritus and lack corrensite. Bleached facies in both formations contain bitumen and clusters of calcite + bitumen concretions, and partially dissolved detrital clasts and most cements, including early Fe-oxides, which resulted in an increase in secondary porosity and bleaching of the red beds. Alteration of the Candeleros Formation is confined to medium- to coarse-grained sandstone in which corrensite is partially dissolved and its interlayer charge is satisfied by Ca instead of Na, as in corrensite from the red sandstones. Extensive dissolution occurred in the Huincul Formation, which favored the precipitation of new authigenic minerals such as smectite, mixed-layer chlorite-smectite, and pyrite. Bleaching and associated alteration most likely resulted from interaction of hydrocarbons with the red beds. Hydrocarbon paleomigration in the Cerro Granito area occurred in higher-permeability portions of the Candeleros and Huincul Formations. The seal provided by the Lisandro Formation was breached during uplift driven by Tertiary tectonism, resulting in the exhumation of the Huincul and Candeleros Formations. This study highlights the importance of a variety of alterations, including complex clay mineral paragenesis, contemporaneous with bleaching of the red beds. Such alterations can potentially be used to provide evidence for the passage of chemically reducing fluids associated with hydrocarbons in other regions, including cases where the original red color has been completely removed.

Description: We studied the petrogenesis of mafic igneous rocks in the Famatinian arc in the western Sierra Famatina (NW Argentina), an Early Ordovician middle-crustal section in the proto-Andean margin of Gondwana. Mafic rock types consist of amphibolite, metagabbro, and gabbro, as well as pod- and dike-like bodies of gabbro to diorite composition. Field relations together with geochemical and isotopic data for the mafic rocks of the western Sierra de Famatina (at 29°S) define two contrasting suites, which can be correlated with similar assemblages noted in other parts of the orogen. Amphibolite, metagabbro, and gabbro bodies are mostly the oldest intrusive rocks (older than 480 Ma), with the host tonalite and post-tonalite mafic dikes being slightly younger. The older mafic suite is tholeiitic to calc-alkaline and isotopically evolved, except for most of the amphibolite samples. The younger suite is calc-alkaline, typically displaying subduction-related geochemical signatures, and it is isotopically more juvenile. Whole-rock chemical composition and isotopic analyses are compatible with a progressive mixing of different isotopic reservoirs. Pyroxenite (±garnet) was likely the dominant source of the older gabbroic magmas, whereas peridotite dominated in the source of the younger suite, implying that the mafic magma experienced a progressive shift toward more juvenile compositions though time (over 20 m.y.). Pyroxenite-derived melts could have been generated by lithospheric foundering followed by upwelling of primitive melts by adiabatic decompression of mantle wedge peridotite.

Description: Pedogenic carbonate is common in arid and semiarid soils, and though its stable C and O isotope composition has been shown to reflect local environmental conditions, questions remain about the quantitative nature of this relationship and the implications for paleoclimate applications. To further address these questions, a climosequence of four Holocene soils in Fish Lake Valley, Nevada (western United States), was instrumented and examined over the course of more than a year. The annual precipitation along the transect ranges from ~80 mm yr –1 to ~220 mm yr –1 . Results show that the C and O stable isotope compositions of soil CO 2 , H 2 O, and carbonate change systematically with elevation and climate. However, there was considerable temporal variability in the conditions that affected carbonate isotope values. At the lowest elevation, CO 2 13 C and H 2 O 18 O values were similar to that in equilibrium with carbonate nearly year-round. At the midelevation sites, spring through summer CO 2 13 C and H 2 O 18 O values appeared to most closely match the 13 C and 18 O values of pedogenic carbonate. At the highest elevation, the C and O isotopes of carbonate did not reflect the soil CO 2 13 C and H 2 O 18 O values measured during the period of study, but they did appear to reflect soil respiration rates during the late spring. The transect shows that arid soil carbonate 13 C values strongly reflect variations in soil respiration rates and the resulting concentrations of soil CO 2 . These results also indicate that depth profile sampling may be required to adequately constrain respiration rates, which affect the interpreted atmospheric CO 2 concentrations.

Description: New mapping demonstrates that active emergent thrust faulting is occurring within the fold-and-thrust belt north of the deformation thrust front in the NW Himalaya. The 〉60-km-long Riasi fault system is the southeasternmost segment of a seismically active regional fault system that extends more than 200 km stepwise to the southeast from the Balakot-Bagh fault in Pakistan into northwestern India. Two fault strands, the Main Riasi and Frontal Riasi thrusts, dominate the fault system in the study area. The Main Riasi thrust places Precambrian Sirban Formation over folded unconsolidated Quaternary sediments and fluvial terraces. New age data and crosscutting relationships between the Main Riasi thrust and the Quaternary units demonstrate that the Main Riasi thrust accommodated shortening between 100 and 40 ka at rates of 6–7 mm/yr. Deformation shifted to the southern Frontal Riasi thrust splay after ca. 39 ka. Differential uplift of a 14–7 ka terrace yields a range of shortening rates between 3 and 6 mm/yr. Together, shortening across the two strands indicates that a 6–7 mm/yr shortening rate has characterized the Riasi fault system since 100 ka. Geodetic data indicate that an 11–12 mm/yr arc-normal shortening rate characterizes the interseismic strain accumulation across the plate boundary due to India-Tibet convergence. These data combined with rates of other active faults in the Kashmir Himalaya indicate that the Suruin-Mastgarh anticline at the thrust front accounts for the remainder 40%–50% of the convergence not taken up by the Riasi fault system. Active deformation, and therefore earthquake sources, include both internal faults such the Riasi fault system, as well as rupture of the basal décollement (the Main Himalayan thrust) to the thrust front. Limited paleoseismic data from the Riasi fault system, the historical earthquake record of the past 1000 yr, the high strain rates, and partitioning of slip between the Riasi fault system and the thrust front demonstrate that a substantial slip deficit characterizes both structures and highlights the presence of a regionally important seismic gap in the Kashmir Himalaya. Slip deficit, scaling relationships, and a scenario of rupture and slip on the basal décollement (the Main Himalayan thrust) parsed onto either the Riasi fault system or the thrust front, or both, suggests that great earthquakes (Mw 〉 8) pose an even greater seismic hazard than the Mw 7.6 2005 earthquake on the Balakot-Bagh fault in Pakistan Azad Kashmir.

Description: The Campanian Ignimbrite eruption (39 ka) was the most powerful eruptive event of the Campi Flegrei caldera (southern Italy). This event coincided with the onset of a cold climatic phase and the Paleolithic transition from Neanderthals to modern humans. The eruption started with a sustained column that emplaced a widespread pyroclastic fall deposit covering an area 〉4000 km 2 within the 15 cm isopach. For the first time, we present the complete longitudinal variation from the coarse and 10-m-thick proximal (down to 15 km) sequence, through the well-stratified pumice lapilli deposit in medial areas (30–80 km), to the distal tephra (hundreds to thousands of kilometers distant). The Plinian pumice fall deposit shows a strong lateral heterogeneity due to variations in stratification, grading, and abundance of components. All Campanian Ignimbrite fall layers display Plinian dispersal. Variation of grain size with stratigraphic height suggests that the convective plume was far from steady state. Initially, the plume dispersed 1.3 km 3 of tephra toward the ENE (N75°E) from a height of 29 km (layer A). A gradual increase in intensity resulted in inverse grading of layer B. Column height increased from 26 to 37 km at a vertical velocity of 3.6 m/s. It had a main dispersal axis to the east (N97°E) and a secondary lobe to the southeast (N137°E). During this phase, a maximum volume of 1.73 km 3 of tephra was emplaced. Accessory lithics concentrated in layer C are possibly due to vent clearing after partial blockage of the vent. The 33-km-high column dispersed ejecta to the east (N95°E), with only 0.2 km 3 of tephra erupted during this phase. During the successive gradual decline in eruption intensity (column height decreased from 38 to 32 km at a velocity of 4.2 m/s), a normally graded layer (D) with a volume of 1 km 3 accumulated to the east (N95°E). The sustained column phase ended with a pulsating and partially collapsing column that reached 23 km in height and dispersed 1.1 km 3 of a stratified and lithic-rich succession (layer E) to the southeast (N112°E). If we include the distal co-Plinian deposit, a total volume of 7.8 km 3 (dense rock equivalent [DRE]) of magma was released (containing 0.09 km 3 of accessory lithics). Tephra mass for the single layers is on the order of 10 11 kg, and the total mass is ~2 x 10 12 kg (2 x 10 13 kg including the co-Plinian ash). Mass discharge rates ranged from 0.9 to 6.7 x 10 8 kg/s. The calculated magnitude of the sustained column phase is 6.3. The duration of the Plinian phase of this eruption, based on the ratio of two parameters, erupted mass divided by discharge rate, is estimated to have been ~20 h (including co-Plinian ash). This study shows that Plinian deposits are not always homogeneous and, as for pyroclastic current deposits, can show an articulate architecture. Only the complete reconstruction of vertical and lateral variations in components, stratification, and grading might provide insights into the temporal and spatial evolution of the sustained plume.

Description: The 1.95-km-thick Cassia Formation, defined in the Cassia Hills at the southern margin of the Snake River Plain, Idaho, consists of 12 refined and newly described rhyolitic members, each with distinctive field, geochemical, mineralogical, geochronological, and paleomagnetic characteristics. It records voluminous high-temperature, Snake River–type explosive eruptions between ca. 11.3 Ma and ca. 8.1 Ma that emplaced intensely welded rheomorphic ignimbrites and associated ash-fall layers. One ignimbrite records the ca. 8.1 Ma Castleford Crossing eruption, which was of supereruption magnitude (~1900 km 3 ). It covers 14,000 km 2 and exceeds 1.35 km thickness within a subsided, proximal caldera-like depocenter. Major- and trace-element data define three successive temporal trends toward less-evolved rhyolitic compositions, separated by abrupt returns to more-evolved compositions. These cycles are thought to reflect increasing mantle-derived basaltic intraplating and hybridization of a midcrustal region, coupled with shallower fractionation in upper-crustal magma reservoirs. The onset of each new cycle is thought to record renewed intraplating at an adjacent region of crust, possibly as the North American plate migrated westward over the Yellowstone hotspot. A regional NE-trending monocline, here termed the Cassia monocline, was formed by synvolcanic deformation and subsidence of the intracontinental Snake River basin. Its structural and topographic evolution is reconstructed using thickness variations, offlap relations, and rheomorphic transport indicators in the successive dated ignimbrites. The subsidence is thought to have occurred in response to incremental loading and modification of the crust by the mantle-derived basaltic magmas. During this time, the area also underwent NW-trending faulting related to opening of the western Snake River rift and E-W Basin and Range extension. The large eruptions probably had different source locations, all within the subsiding basin. The proximal Miocene topography was thus in marked contrast to the more elevated present-day Yellowstone plateau.

Description: Submerged shorelines hold much potential for examining the interplay between the rate of sea-level rise and geomorphic setting, and informing the development of models of contemporary shoreline behavior. This paper describes the sedimentary architecture of a submerged barrier shoreline complex off Durban, South Africa. The complex consists of several barriers that have survived the postglacial transgression and associated erosive ravinement processes. The main shoreline sequence (–60 m) dates to 11,690 ± 90 calibrated (cal) yr B.P. and rests on a Pleistocene lagoonal deposit (35,395 ± 592 cal yr B.P.). The entire barrier shoreline complex is truncated by a strongly diachronous wave ravinement surface. The ravinement surface seaward of the main –60 m shoreline is steep, but the gradient declines across and landward of the subordinate landward shoreline complexes. The steep ravinement surface is attributed to fast sea-level rise (possibly associated with meltwater pulse 1B), while the gentle ravinement surface is associated with a subsequent slowing of the rate of sea-level rise (to 0.15 mm yr –1 ). Preservation of the main barrier and back-barrier deposits through overstepping is associated namely with rapid transgression and increased back-barrier accommodation. The smaller barriers landward of the main barrier were preserved through overstepping related to gentle antecedent gradients, despite more intense ravinement during a very slow rise in sea level (slowstand). This process was assisted by a sediment deficit. The resulting post-transgressive drape is also influenced by antecedent topography created by the barriers themselves; damming along the landward sides of overstepped barriers thickens the drape and creates a temporal disconnect between the migration of the shoreface and more landward elements of the littoral system. In examining the rates of shoreline migration associated with the overstepped barrier system, these are far greater than those calculated for the predicted rates of shoreline change on similarly steep coastal profiles. Future rates of shoreline migration appear to be insufficient to cause overstepping, and rollover or erosional responses are more likely.

Description: Proterozoic rocks exposed in the Rocky Mountains of New Mexico record the history of the assembly of North America, providing insight into crustal growth, accretion, and recycling. Here, we present six well-constrained Lu-Hf garnet ages showing that regional amphibolite-facies metamorphism and deformation occurred between 1.46 and 1.40 Ga. The samples come from the kyanite, sillimanite, and andalusite zones of the "Al 2 SiO 5 triple-point terrane" of northern New Mexico. Porphyroblast-matrix textures in dated samples provide definitive evidence for garnet growth during three phases of deformation associated with crustal thickening during the development of a regional-scale north-vergent contractional orogenic belt. The development of first-generation deformation fabrics is constrained to a period between 1.46 and ca. 1.45 Ga; second-generation deformation fabrics are constrained to the period 1.42–1.40 Ga; and third-generation fabrics were formed after ca. 1.40 Ga. Our results demonstrate that crustal-scale structures and regional metamorphism in northern New Mexico are both the result of a Mesoproterozoic orogenic event, the Picuris orogeny. We found no evidence for amphibolite-facies metamorphism during the Paleoproterozoic Mazatzal or Yavapai orogenies in northern New Mexico.

Description: Drilling through the edifice of Rangitoto, the youngest and largest volcano in the "monogenetic" Auckland volcanic field, reveals the multistage eruptive and magmatic history of a small basalt shield volcano. Activity commenced calendar year (cal.) 6000 cal. yr B.P., involving minor effusive and pyroclastic volcanism until 650 cal. yr B.P. This period either represents an early, less productive phase of a single polygenetic volcano, or, alternatively, Rangitoto is better described as a volcanic complex that includes one or more buried edifices concealed by the main structure. A voluminous shield-building phase occurred 650–550 cal. yr B.P., erupting isotopically uniform subalkalic basalts (Mg# 60–64). Four batches of magma distinguished by trace-element chemistry were erupted sequentially, but they lack genetic connection via fractional crystallization or assimilation. Two of the magma batches display linear trends of decreasing incompatible trace-element abundance and increasing ratios of moderately incompatible to highly incompatible elements with decreasing age. This is consistent with cycles of progressive partial melting at the source. The final phase of activity (ca. 550–500 cal. yr B.P.) was explosive and less voluminous, producing scoria cones at the summit. This phase involved more diversity in magma compositions, including more mafic subalkalic basalt, and alkali basalt, pointing to sourcing of magmas simultaneously from different depths in the mantle. Rangitoto volcano contributes to a growing body of evidence showing that major periods of volcanism in "monogenetic" basalt fields occur at centers that have experienced multiple eruption episodes. Changes in magma composition accompany changes in eruption style, but a lack of an obvious shared pattern in magmatic evolution at various volcanoes points to the localized mantle heterogeneity and conduit systems. Hazard scenarios for regions traditionally classified as "monogenetic" need to encompass the possibility of prolonged episodes of activity and reawakening of volcanoes, a significant implication where infrastructure is built on such regions.

Description: The presence of potential microbial trace fossils (endolithic microborings) has been well documented in oceanic basaltic pillow lavas, hyaloclastites, tuffs, and transitional subglacial marine lavas in the past 30 yr. Despite their evident abundance in oceanic to subglacial environments, they have not been observed in continental basalts that were not erupted in marine or subglacial settings. To expand the record of putative endolithic microborings in volcanic rocks to nonmarine, continental lacustrine environments, we examined hydrovolcanic pyroclastic deposits in the Fort Rock volcanic field, central Oregon. This study presents the textures, mineralogy, and geochemistry of basaltic tuffs containing possible endolithic microborings comparable in morphology, size, and distribution to those described in earlier oceanic and subglacial basalt studies. We observed a variety of tubular and granular textures that show evidence of biogenic morphologies and behavior, and a primary geological context that expresses their age and syngenicity. Petrographic relationships with secondary phases (chabazite, nontronite, calcite) indicate that the construction of microtunnels occurred in saline, alkaline fluids at temperatures of 25–80 °C. In addition, positive correlations were observed between the extent of aqueous (abiotic) alteration and both the abundance of microtunnels and morphological type. These correlations suggest that microtunnels were more readily formed where there was greater abiotic alteration-fluid flux and that the resulting change in chemical composition of those fluids may have had a direct influence on the formation process or possibly the type of constructing microbe. This work adds to understanding of factors controlling microtunnel formation and is the first account of putative endolithic microborings in a continental lacustrine setting. This new information may also have implications in the search for habitable extraterrestrial environments, such as on Mars.

Description: The Eastern Mediterranean is a land-locked basin, a remnant of the Neotethys Ocean. It was formed in the Permian–Triassic as a result of the drift of the Tauride block from the Afro-Arabian margin of Gondwana. Herein, we show that rather than being a genuine Afro-Arabia crustal fragment, the Tauride block is underlain by late Neoproterozoic Cadomian basement, which differs significantly from the Neoproterozoic "Pan-African" basement of NE Africa from which it was detached. Resembling other Cadomian terranes of western Europe, the Tauride basement is chiefly a graywacke succession deposited in a mid- to late Ediacaran back-arc basin formed on the periphery of Afro-Arabia, above the southward-subducting proto-Tethys Ocean. The back-arc region was deformed and metamorphosed to various degrees and intruded by latest Ediacaran–Cambrian granites and volcanics during the Cadomian orogeny. Unlike the protracted (~300 m.y.) Neoproterozoic crustal evolution recorded in Afro-Arabia, the Cadomian basement of the Taurides evolved briefly, over ~50 m.y. We show that the entire cycle of sedimentation, metamorphism, and magmatism in the Tauride basement took place in the late Ediacaran–Cambrian and lagged after Neoproterozoic Pan-African orogeny and igneous activity in Afro-Arabia. The Cadomian orogeny accreted the Taurides, and adjoining peri-Gondwana Cadomian terranes, with an already-consolidated Afro-Arabian continent. Permian–Triassic rifting of the Eastern Mediterranean occurred close to the transition between these two domains. Rifting was thus inherited from, and superimposed on, late Ediacaran structures formed in front of the current Afro-Arabia margin of Gondwana during Cadomian orogeny. The boundary between the Cadomian edifice and the Pan-African crust of Afro-Arabia appears to lie presently on the southern margin of the Mediterranean, extending from Morocco in the west to Arabia in the east. Hence, the continental margin of the Eastern Mediterranean, including in the Levant Basin, is probably underlain by a thinned Cadomian crust.

Description: The Selenga River Delta, Lake Baikal, Russia, is ~600 km 2 in size and contains multiple distributary channels that receive varying amounts of water and sediment discharge. The delta is positioned along the deep-water (~1600 m) margin of Lake Baikal, a half-graben–styled rift basin, qualifying it as a modern analogue of a shelf-edge delta system. This study provides a detailed field survey of channel bed sediment composition, channel geometry, and water discharge. The data and analyses presented here indicate that the Selenga Delta exhibits downstream sediment fining over tens of kilometers, ranging from predominantly gravel (coarse pebble) and sand near its apex to silt and sand at the delta-lake interface. We developed an analytical framework to evaluate the downstream elimination of gravel within the multiple distributary channels. The findings include the following. (1) The Selenga River Delta consists of at least eight orders of distributary channels. (2) With increasing channel order downstream, channel cross-sectional area, width-depth ratio, water discharge, boundary shear stress, and sediment flux systematically decrease. (3) The downstream elimination of gravel in distributary channels is caused by declining boundary shear stress as a result of water discharge partitioning among the bifurcating channels. (4) Over longer time scales, gravel is contained on the delta topset due to frequent and discrete seismic events that produce subsidence and accommodation, so that coarse sediment cannot be transported to the axis of the Baikal Rift basin. The distribution of sediment grain size in deltaic channels, as related to hydrodynamics and sediment transport, plays a critical role in influencing stratigraphy, because the sustained tectonism leads to high preservation potential of the delta topset sedimentary deposits. Therefore, the Selenga River Delta provides an opportunity to explore the interactions between modern deltaic sedimentation processes and tectonics that affect the production of basin stratigraphy.

Description: The Neoproterozoic outcrop belt of the Death Valley region, California, preserves an oblique cross section of the Noonday Formation, a mixed carbonate-siliciclastic platform that hosts distinctive basal Ediacaran cap carbonate–affiliated sedimentary structures, stromatolite textures, and 13 C carb values. The Noonday platform encompasses two depositional sequences that reveal two cycles of relative sea-level change within strata conventionally considered to record a single, rapid, postglacial sea-level rise. In updip localities, facies of the first depositional sequence record the transition from a carbonate ramp to a stromatolite-bearing, "tubestone"-textured, reef-rimmed platform; downdip, localities seaward of the reefal escarpment variably preserve a thin and condensed onlapping foreslope wedge. Base-level fall exposed the reef crest to karstic dissolution and propagated submarine incised valleys into the seaward margin of the reef. Overlying strata record the backfilling of a submarine incised valley and reestablishment of a back-stepping, carbonate-dominated ramp prior to a second subaerial exposure event that defines the contact between the Noonday and Johnnie formations. We address the relative contributions of syndepositional tectonism and recovery from low-latitude deglaciation in dictating Noonday platform architecture and the intra–Noonday Formation sequence boundary. Noonday Formation deposition coincided with extension of the Laurentian margin during disaggregation of the Rodinian supercontinent. Within this framework, previous work has suggested that the intra–Noonday Formation sequence boundary records growth faulting that reinforced differential topography, uplifting reef-rimmed horsts—exposing the reef crest to karstic dissolution—and downdropping grabens. However, we trace the intra–Noonday Formation sequence boundary seaward of the reef crest and demonstrate that, for a time, wave base was situated downdip of the reef escarpment on putatively downdropped fault blocks. Thus, if the Noonday margin were undergoing extension, then the creation of the intra–Noonday Formation sequence boundary required a concomitant decrease in accommodation due, perhaps, to postglacial isostatic uplift attendant with low-latitude deglaciation. We speculate that Noonday Formation sequence architecture records (1) immediate deglacial flooding, (2) shoaling and exposure due to isostatic rebound induced by either a hiatus in meltwater flux or rapid ice-sheet collapse against a background of global deglaciation, and (3) resumed flooding following complete deglaciation. As rift-related tectonism could amplify or counter glacial isostasy, inferences of the amplitude of local postglacial sea-level change will require robust estimates of syndepositional extension across the Noonday margin.

Description: Fill terraces along rivers represent the legacy of aggradation periods that are most commonly attributed to climate change. In the North Fork of the San Gabriel River, an arid bedrock landscape in the San Gabriel Mountains, California, a series of prominent fill terraces was previously related to climate-change–induced pulses of hillslope sediment supply that temporarily and repeatedly overwhelmed river transport capacity during the Quaternary. Based on field observations, digital topographic analysis, and dating of Quaternary deposits, we suggest instead that valley aggradation was spatially confined to the North Fork San Gabriel Canyon and was a consequence of the sudden supply of unconsolidated material to upstream reaches by one of the largest known landslides in the San Gabriel Mountains. New 10 Be-derived surface exposure ages from the landslide deposits, previously assumed to be early to middle Pleistocene in age, indicate at least three Holocene events at ca. 8–9 ka, ca. 4–5 ka, and ca. 0.5–1 ka. The oldest and presumably most extensive landslide predates the valley aggradation period, which is constrained by existing 14 C ages and new luminescence ages to ca. 7–8 ka. The spatial distribution, morphology, and sedimentology of the river terraces are consistent with deposition from far-traveling debris flows that originated within, and mined, the landslide deposits. Valley aggradation in the North Fork San Gabriel Canyon therefore resulted from locally enhanced sediment supply that temporarily overwhelmed river transport capacity, but the lack of similar deposits in other parts of the San Gabriel Mountains argues against a regional climatic signal. Our study highlights the potential for valley aggradation by debris flows in arid bedrock landscapes downstream of landslides that occupy headwater areas.

Description: Despite the hazard caused by near-surface destructive horizontal displacements during earthquakes, field evidence for coseismic slip along horizontal discontinuities is exceptionally rare, mainly due to the lack of adequate exposure and markers. However, within the seismically active Dead Sea Basin, the late Pleistocene Lisan Formation contains vertical clastic dikes that are sheared laterally at maximum depths of 15 m, and thereby provide unique profiles of such horizontal displacement. In order to investigate how coseismic horizontal shearing is distributed near the surface, we document an ~1-m-thick brittle shear zone, consisting of up to 11 slip surfaces that can be traced for tens of meters in the Lisan Formation. Displacements along individual slip surfaces are up to 0.6 m, and the total displacement across the shear zone is up to 2.0 m. Displacement profiles and gradients indicate that the brittle shear zone formed by simple shear, and deformation was associated with slip partitioning and transfer between primary and secondary slip surfaces. Evidence for concurrent displacement along slip surfaces during a single event indicates that the brittle shear zone was formed during a coseismic event subsequent to 30 ka. We consider the mechanical effect of seismic-wave–related transient stress, which, when added to the initial static effective stress, may result in concurrent horizontal shear failure along detrital-rich layers in the Lisan Formation. The exceptional quality of exposures and markers enables us to document, for the first time, the details of near-surface horizontal shearing, and indicates that displacement along horizontal bedding planes is a viable mechanism to absorb coseismic deformation in well-bedded near-surface strata.

Description: Since the first application of cosmogenic nuclides to the study of glacial history and processes in 1990, increasing numbers of studies have used a variety of cosmogenic isotopes to quantify the exposure age and erosion rate of glaciated landscapes. However, obtaining chronological data from glaciated landscapes once covered by cold-based, nonerosive ice is challenging because these surfaces violate assumptions associated with simple cosmogenic exposure dating. Nonerosive glacial ice fails to completely remove nuclides produced during previous periods of exposure, leaving behind rock surfaces with complex, multigenetic nuclide inventories. Here, we constrain the glacial history, landscape evolution, and efficiency of subglacial erosion in the Pangnirtung Fiord region of southern Baffin Island using over 300 paired analyses of in situ cosmogenic 10 Be and 26 Al. Simple exposure ages are 6.3–160 ka for 10 Be ( n = 152) and 4.3–124 ka for 26 Al ( n = 153). Paired bedrock-boulder samples have discordant ages, simple exposure ages generally increase with elevation, 10 Be and 26 Al ages for the same sample disagree, and both boulders and bedrock yield multimodal age distributions—all patterns suggestive of limited subglacial erosion. Measured 26 Al/ 10 Be ratios indicate that about one third of the samples in the data set experienced at least one period of pre-Holocene exposure followed by burial with limited erosion. Modeled two-isotope minimum-limiting exposure durations are as high as hundreds of thousands of years, and minimum-limiting burial durations range up to millions of years, implying that parts of southern Baffin Island have been preserved beneath nonerosive glacial ice for much, if not all, of the Quaternary. A subset of the samples contains few nuclides inherited from prior periods of exposure and is thus useful for constraining the chronology of the most recent deglaciation. Using these samples, we infer that deglaciation of most of the landscape occurred ca. 11.7 ka and that the Duval moraines, a prominent feature of the last deglaciation, formed ca. 11.2 ka.

Description: The fault history of the Mill Creek strand of the San Andreas fault (SAF) in the San Gorgonio Pass region, along with the reconstructed geomorphology surrounding this fault strand, reveals the important role of the left-lateral Pinto Mountain fault in the regional fault strand switching. The Mill Creek strand has 7.1–8.7 km total slip. Following this displacement, the Pinto Mountain fault offset the Mill Creek strand 1–1.25 km, as SAF slip transferred to the San Bernardino, Banning, and Garnet Hill strands. An alluvial complex within the Mission Creek watershed can be linked to palinspastic reconstruction of drainage segments to constrain slip history of the Mill Creek strand. We investigated surface remnants through detailed geologic mapping, morphometric and stratigraphic analysis, geochronology, and pedogenic analysis. The degree of soil development constrains the duration of surface stability when correlated to other regional, independently dated pedons. This correlation indicates that the oldest surfaces are significantly older than 500 ka. Luminescence dates of 106 ka and 95 ka from (respectively) 5 and 4 m beneath a younger fan surface are consistent with age estimates based on soil-profile development. Offset of the Mill Creek strand by the Pinto Mountain fault suggests a short-term slip rate of ~10–12.5 mm/yr for the Pinto Mountain fault, and a lower long-term slip rate. Uplift of the Yucaipa Ridge block during the period of Mill Creek strand activity is consistent with thermochronologic modeled uplift estimates.

Description: Banded iron formations (BIFs) are iron- and silica-rich chemical sedimentary rocks formed throughout the Archean and Paleoproterozoic Eras. The presence of hematite (Fe 2 O 3 ) and magnetite (Fe 3 O 4 ) in BIFs has led to the widespread assumption that Fe(II) oxidation must have occurred in the ancient oceans via either a biological or chemical mechanism. However, it is unclear whether the ferric iron now present in BIF represents the original ferric oxyhydroxide [e.g., ferrihydrite, Fe(OH) 3 ] precipitated in the water column, or if it is the result of later-stage circulation of oxidizing fluids through the sediment pile. In this study, we conducted high-resolution microscopic investigations on BIF from the 2728 Ma Abitibi greenstone belt located in the Superior Province of the Canadian Shield and the 2460 Ma Kuruman Iron Formation in South Africa to ascertain the timing and paragenesis of the hematite. Three types of hematite are identified by high-resolution electron microscopic characterization and selected area electron diffraction: (1) 3–5 nm ultrafine hematite particles in the iron oxide–rich bands (H1); (2) submicrometer subhedral to euhedral hematite crystals randomly distributed in the chert matrix of transitional zones between iron oxide– and chert-rich bands (H2); and (3) needle-like, radial and fibrous hematite that replaced stilpnomelane or carbonates and is distributed along fractures or layer boundaries (H3). We interpret the first two types as primary minerals dehydrated from precursor ferric oxyhydroxides. H1 remains ultrafine in size, while H2 has undergone an Ostwald coarsening process facilitated by internal fluids produced during amorphous silica to quartz transformation. H3 is a later-stage mineral formed by external fluid-mediated replacement of iron silicates or carbonates. These results indicate that a significant fraction of the hematite in the BIF originated from ferric oxyhydroxide precursors. Importantly, this implies that photosynthetic Fe(II) oxidation, by either a direct or indirect biological mechanism, did exist in seawaters from which some BIF material was deposited.

Description: Cosmogenic radionuclides (CRNs) are commonly employed to quantify both the production rates and residence times of mobile regolith. Meteoric and in situ CRNs have different accumulation mechanisms and can independently constrain landscape evolution rates. Here we use both in situ and meteoric 10 Be to investigate where in the regolith 10 Be is stored, and to quantify production rates and residence times of mobile regolith on active hillslopes in Gordon Gulch, within the Boulder Creek Critical Zone Observatory (CZO), Colorado, USA. Our data reveal that two-thirds of in situ 10 Be in regolith is produced within saprolite, and at least one-tenth of the meteoric 10 Be inventories is stored in saprolite, highlighting the importance of consistent terminology and identification of the mobile regolith–saprolite boundary. We find that mobile-regolith production rates are on average 3.1 cm/k.y., and residence times are between 10 and 20 k.y. A notable exception exists at the depositional north-facing footslope, where residence times likely exceed 40 k.y. Close agreement between the meteoric and in situ results indicates that upper- and mid-slope positions are consistent with steady, uniform lowering of the landscape. In addition to comparing the two methods, we develop a one-dimensional analytical model for the 10 Be concentration fields on an active, steady-state catena with uniform erosion. We then compare model predictions with measurements to evaluate how well our sites adhere to the steady-state assumption underlying the calculations for mobile-regolith residence time and production rates. Such comparisons suggest that calculated residence times and lowering rates are likely no closer than ±25% of the geomorphic reality.

Description: This study describes a peculiar, yet common type of fracture showing a staircase trajectory, which forms in rocks with moderately weak planar anisotropies. The staircase fracture trajectory is given by alternating fracture segments oriented parallel to (LaP) or at an angle (ramp) with respect to the lamination/layering. The analyses has been accomplished on travertines, which are continental microbial/hydrothermal deposits having a typical poorly stratified yet strongly laminated texture. In these rocks, porosity and permeability have a high across-lamination variability and are mostly controlled by an interconnected and locally corroded array of permeable layers, fractures, and faults. Structural analysis integrated with analytical modeling provided a conceptual model of staircase fracture localization as a function of the travertine lamination dip. Lamination-parallel fracture segments localize within the porous laminae, mostly at the interface with tight laminae. Ramp-type fracture segments cut the lamination, connecting lamination parallel segments. Two types of staircase fracture can be modeled. The first group develops in subhorizontal to gently dipping deposits (lamination dip 〈 30°) corresponding to low-energy depositional environments. The second group relates to staircase fractures developed in moderately to steeply dipping laminations (lamination dip 〉30°) and corresponds to high-energy environments. Major discoveries of hydrocarbon have been recently made in continental (lacustrine) microbial carbonates in the Brazilian South Atlantic margin, some of which exhibit a texture similar to those usually observed in travertines. Understanding of the lacustrine carbonates is still at an early stage. Given that in modern rift settings, vent-related thermal (travertine) and nonthermal (tufa) carbonates are a major component, the proposed conceptual model of staircase fracture localization contributes to the preparation of a model for the potential occurrence of high-permeability pathways in hydrocarbon and geothermal microbial reservoirs.

Description: Carbonate aquifers are some of most challenging to characterize because dissolution can greatly enhance permeability, but its effects are often difficult to determine. This study analyzes data from caves, wells, and tracer tests to explore the extent of solution channel networks and the factors that influence their development. The nonlinear dissolution kinetics of calcite, mixing of waters with different CO 2 concentrations, and unstable dissolution fronts all promote the development of solution channels, which are widespread in unconfined carbonate aquifers. Fractures are important for guiding channels at a local scale, but hydraulic gradients are the dominant control at a regional scale. Channels provide continuous, large-aperture pathways that result in rapid groundwater flow. Small channels are much more abundant than large channels, and often account for most of the permeability measured in wells. Caves represent the largest channels; they are more common in limestone than in dolostone, and the development of caves rather than smaller channels is also favored where there is sparse fracturing, low matrix porosity, and the presence of sinking stream recharge rather than percolation recharge. Solution channel networks have fractal properties, and their presence explains why carbonate aquifers have higher permeability than aquifers in any other rock type.

Description: The Taimyr fold-and-thrust belt records late Paleozoic compression, presumably related to Uralian orogenesis, overprinted by Mesozoic dextral strike-slip faulting. U-Pb detrital zircon analyses of 38 sandstones from southern Taimyr were conducted using laser ablation–inductively coupled plasma–mass spectrometry to investigate late Paleozoic to Mesozoic sediment provenance and the tectonic evolution of Taimyr within a regional framework. The Pennsylvanian to Permian sandstones contain detrital zircon populations of 370–260 Ma, which are consistent with derivation from the late Paleozoic Uralian orogen in northern Taimyr and/or the polar Urals. Late Neoproterozoic through Silurian ages (688–420 Ma), most consistent with derivation from Timanian and Caledonian age sources, suggest an ultimate Baltica source. Southern Taimyr represents the proforeland basin of the bivergent Uralian orogen in the late Paleozoic. Triassic sedimentary rocks contain detrital zircon populations of Carboniferous–Permian (355–260 Ma), late Neoproterozoic to Early Devonian (650–410 Ma), and minor Neoproterozoic (1000–700 Ma) ages, which suggest a similar provenance as the Carboniferous to Permian strata. The addition of a Permian–Triassic (260–220 Ma) zircon population indicates derivation of detritus from Siberian Trap–related magmatism. Jurassic samples have a dominant age peak at 255 Ma and a distinct reduction in Carboniferous–Permian and late Neoproterozoic to Early Devonian input, suggesting that erosion and contributions from Uralian sources ceased while greater input from Siberian Trap–related rocks of Taimyr dominated. Comparison of these results to the published literature demonstrates that detritus from the Uralian orogen was deposited in Taimyr, Novaya Zemlya, and the New Siberian Islands in the Permian, but not in the Lisburne Hills or Wrangel Island. In the Triassic, Taimyr, Chukotka, Wrangel Island, the Kular Dome in the northern Verkhoyansk of Siberia, Lisburne Hills, Franz Josef Land, and Svalbard shared sources from Taimyr, the Siberian Traps, and the polar Urals, indicating that there were no geographic barriers among these locations prior to opening of the Amerasia Basin. Detritus from the Uralian orogen in Taimyr was shed northward into the retroforeland basin and was then transported farther 20–30 m.y. after Uralian orogenesis. The widespread distribution of material eroded from Taimyr and the polar Urals during the Triassic is likely due to the arrival of, and sublithospheric spreading associated with, the Siberian mantle plume head at ca. 250 Ma. The subsequent motion of the lithosphere relative to the plume-swell likely caused a northwestward migration of the uplifted regions. Taimyr and the polar Urals were probably affected. In the Jurassic, detrital zircon spectra from Taimyr, Chukotka, the Kular Dome, and Svalbard show great differences, suggesting that these locations no longer shared the same provenance from Taimyr and the Urals. The restricted distribution of detritus from Taimyr and the Urals indicates that erosion of the Uralian orogen was reduced. In the Late Jurassic, the depositional setting of southern Taimyr probably changed from a foreland to an intracratonic basin.

Description: We used airborne light detection and ranging (LiDAR) data to reevaluate the single-event offsets of the 1920 Haiyuan Ms 8.5 earthquake and the cumulative offsets along the western and middle segments of the coseismic surface rupture zone. Our LiDAR data indicate that the offset observations along both the western and middle segments fall into groups. The group with the minimum slip amount is associated with the 1920 Haiyuan Ms 8.5 earthquake, which ruptured both the western and middle segments. Our research highlights two new interpretations: First, the previously reported maximum displacement of the 1920 earthquake was likely due to at least two earthquakes; second, our results reveal that the cumulative offset probability density (COPD) peaks of the same offset amounts on the western and middle segments do not correspond to one another one-to-one. We suggest that any discussion of the rupture pattern of a certain fault based on the offset data should also consider fault segmentation and paleoseismological data. Therefore, the COPD peaks should be computed and analyzed on fault subsections and not entire fault zones to study the number of paleoearthquakes and their rupture patterns.

Description: The study of shock pressure indicators can provide important clues for understanding the cratering process, though the estimation of shock pressures in weakly shocked rocks is commonly difficult. In this study, we selected a very young and well-preserved impact structure, the Lonar crater in India. The crater, devoid of any tectonic overprint, can be assumed as pristine. We used a combination of rock magnetic and microfracture studies to estimate shock pressure in the crater rim. On the basis of present results, the magnetic fabrics are interpreted to be of magmatic origin related to the Deccan basalt emplacement. The high-coercivity component of the natural remnant magnetization in the crater rim basalt is similar to that in the unshocked basalt. The lack of any shock-related magnetic overprint on the crater rim basalt is, therefore, evident in the Lonar crater. In contrast, radial and concentric microfractures observed in basalts at the crater rim and farther away show symmetric distribution with respect to the crater. The concentric microfractures consistently overprint the radial microfractures. We infer that the radial and concentric microfractures were developed during propagation of the early compressional and the late decompressional shock wave components, respectively. The results of our rock magnetic and microfracture studies, when interpreted in light of published experimental and numerical simulation studies on the Lonar basalt, reveal that the shock pressure in the Lonar crater rim was less than 0.5 GPa but greater than 0.2 GPa. This shock pressure was high enough to produce fractures but too low to affect the magnetic fabrics. These results give new information on the relationship between shock pressure and resulting microfractures.

Description: The intercalation of silty units and coarse-grained units represented by conglomerates and breccia characterizes a Lower Miocene terrestrial sedimentary sequence in the North Croatian Basin, a part of the southwestern Pannonian Basin system. These sediments were previously interpreted as alluvial sediments, where the silty units would reflect deposition on a floodplain. However, in this study, we show new results that support a different interpretation of the genesis of the silty units. The units, which vary in thickness between 6 and 180 cm, are mostly composed of structureless loose silt. They are brownish yellow to yellowish brown in color and do not contain fossils. Scanning electron microscopy indicated that quartz grains show fracture faces, conchoidal fractures, V-shaped percussion marks, linear steps, and conchoidal crushing features. Such microtextures together with the macroscopic characteristics of the silt units indicate that they were deposited by wind. Therefore, this study reports the first occurrence of Miocene loess outside of China. Silt-sized particles were probably produced by salt-weathering processes on salina-type lake flats during long arid periods. Alluvial deposition was controlled by a more humid climate, so the intercalation of eolian silty units with alluvial conglomerates and breccias reflects alternation of arid and more humid periods in the early Miocene. This agrees with regional paleoclimate studies that show cyclicity in the climate, with a dry cycle and orbital-scale climate variability controlling paleoenvironmental and sedimentary changes in the area during the early Miocene.

Description: Differences in marine terrace elevations across the Pacific coast of North America have long been assumed to be a result of differences in the rates of tectonic motion. However, other processes, particularly glacio-isostatic adjustment, lead to regional variations in sea levels. In this study, we compiled the elevations of marine isotope stage (MIS) 5e (ca. 119–129 ka), 5c (ca. 106 ka), and 5a (ca. 84 ka) terraces across the Pacific coast of central North America and compared these regional variations in elevation with model predictions of glacio-isostatic adjustment after correcting for tectonics. These predictions are generally consistent with the observed trends in the elevations of the terraces and show that this process created up to 20 m of coeval variation in local sea levels along the Pacific coast of central North America (between 20°N and 45°N) during MIS 5c and MIS 5a, but less, ~4 m, during MIS 5e. Accounting for glacio-isostatic adjustment reduces the variability in uplift rates calculated at individual locations using different-aged terraces as datums. Ignoring glacio-isostatic adjustment leads to overestimated uplift rates by an average of 40%, but up to 72%, across the Pacific coast of central North America. An understanding of regional variations in glacio-isostatic adjustment–corrected sea levels also contributes to the correct identification of marine terraces with mistaken ages.

Description: The Mesoproterozoic Voisey’s Bay intrusion is a small (~10 km 3 ) mafic intrusive complex forming part of the 20,000 km 2 Nain plutonic suite of Labrador (Canada) and hosting a world-class orthomagmatic Ni-Cu sulfide ore deposit. An extensive database of drill-core and down-hole televiewer data provides a wealth of subsurface three-dimensional information on the intrusion. The intrusion consists of the upper Eastern Deeps Chamber and lower Western Deeps Chamber, connected via a complex network of dikes. The Eastern Deeps Chamber can be divided into six units that are distinguished based on geometrical styles, dominant lithologies, and the occurrence of bodies of wall rock partly separating units. Unit I, occurring at the highest levels of the chamber, is tabular and consists dominantly of olivine gabbro with minor normal-textured (orthocumulate) troctolite. Units at midlevels of the Eastern Deeps Chamber (IIA, IIB, IIC) are sill-shaped bodies consisting of normal troctolite (i.e., orthocumulate, exhibiting tiling of plagioclase laths) with local variably textured troctolite (i.e., mesocumulate, locally pegmatitic, locally dynamically recrystallized). Units from the lowermost level (IIIA, IIIB) are irregularly shaped and consist of variable troctolite, breccias containing centimeter-sized xenoliths of host-rock paragneiss, and strongly sulfide-mineralized rocks. At least three different feeder dikes deliver magmas associated with unit IIIB. The overall dike system has an upward-curving geometry such that the dike dips shallow upward and as they approach the Eastern Deeps Chamber. This is analogous to fault geometries produced during caldera collapse or cauldron subsidence. Evolving emplacement styles reflect internal igneous processes and intermittent normal-sense activity along pre- to synemplacement brittle wall-rock structures. High-temperature normal-sense shear zones are prominent within the chamber within 100 m of its basal contact and are interpreted to have formed in response to late-stage normal faulting of the chamber floor. The geometry and internal structure of the Eastern Deeps Chamber are indicative of top-down incremental emplacement associated with floor subsidence. Our findings provide a kinematic and dynamic framework for petrogenetic models of Voisey’s Bay and other intrusion-hosted Ni-Cu sulfide deposits.

Description: The North Tanzanian divergence zone along the East African Rift is characterized by active faults and several large volcanoes such as Meru, Ol Doinyo Lengai, and Kilimanjaro. Based on systematic morphostructural analysis of the Shuttle Radar Topographic Mission digital elevation model and targeted field work, 14 debris avalanche deposits were identified and characterized, some of them being—to our knowledge—previously unknown. Our field survey around Mount Meru allowed previous "lahar" deposits to be reinterpreted as debris avalanche deposits and three major collapse events to be distinguished, with the two older ones being associated with eruptions. We used topographic lineaments and faults across the North Tanzanian divergence zone to derive the main tectonic trends and their spatial variations and highlight their control on volcano collapse orientation. Based on previous analogue models, the tectonic regime is inferred from the orientation of the collapse scars and/or debris avalanche deposits. We infer two types of regime: extensional and transtensional/strike-slip. The strike-slip regime dominates along the rift escarpment, but an extensional regime is inferred to have operated for the recent sector collapses. The proposed interpretation of sector collapse scars and debris avalanche deposits therefore provides constraints on the tectonic regime in the region. It is possible that, in some cases, movement on regional faults triggered sector collapse.

Description: A detailed tectonic analysis of the magmatic evolution of the Early to Middle Ordovician west-facing Popelogan arc in New Brunswick and adjacent Maine is presented based on combining new U-Pb zircon radiometric age dates (thermal ionization mass spectrometry and sensitive high-resolution ion microprobe) with existing age constraints on the various magmatic events recognized in this arc system. The Popelogan arc had a life span of nearly 20 m.y. (ca. 476–457 Ma) and becomes progressively younger toward the northwest. Existing lithogeochemical and isotope data combined with field relationships and new zircon inheritance data indicate that the Popelogan arc was built on the leading edge of Ganderia and represents a predominantly continental calc-alkaline arc. Incursions of arc to within-plate–like tholeiitic magmatism correspond to rifting events temporally and spatially linked to trenchward migration of the arc magmatic axis, isolation of arc ribbons, and formation of mafic oceanic and transitional crust in the associated Tetagouche backarc basin. Arc-trench migration and backarc basin opening were caused by a retreating subduction zone. Slab retreat prior to 467 Ma was at least partially accommodated by arc migration, but thereafter, it was mainly accommodated by extension and spreading in the associated Tetagouche backarc basin. New U-Pb zircon radiometric ages and lithogeochemistry of calc-alkaline felsic tuff beds interlayered with the oceanic backarc rocks of the Fournier Supergroup provide a direct link between the Tetagouche backarc rocks and coeval calc-alkaline arc activity in the Popelogan arc. The oldest incursion of arc tholeiite (470–467 Ma) corresponds to cessation of the ca. 476–470 Ma calc-alkaline Meductic phase of the Popelogan arc, the onset of the calc-alkaline Balmoral phase (467–457 Ma) farther to the west, and formation of oceanic and mafic-dominated transitional crust in the Canoe Landing Lake Formation of the Tetagouche backarc basin. A less-well-constrained second incursion of tholeiitic magmatism between 465 and 459 Ma corresponds to rifting of the Tetagouche arc ribbon and formation of the ophiolitic Devereaux complex in the Fournier Supergroup. Cessation of calc-alkaline arc magmatism and eruption of transitional and alkalic mafic volcanic rocks between 459 and 455 Ma may represent slab breakoff following accretion of the Popelogan arc to composite Laurentia, and/or ridge subduction immediately prior to arc-continent collision.

Description: Zircon U-Pb analyses of 18 orthogneisses from the Clearwater and Priest River complexes of northern Idaho have identified important exposures of Neoarchean and Paleoproterozoic basement rocks in northwest Laurentia. All samples have ages that fall into two tightly defined age ranges: in the Neoarchean from 2.67 to 2.65 Ga and in the Paleoproterozoic from 1.88 to 1.84 Ga. The Neoarchean orthogneisses show no evidence of older components, whereas some of the Paleoproterozoic orthogneisses have xenocrystic zircon cores with ages overlapping with those of the Neoarchean gneisses. Collectively, these two packages of rocks represent the two main periods of crust formation in northwest Laurentia, one wholly juvenile in the Neoarchean, and the other a mixture of juvenile Paleoproterozoic and inherited Neoarchean components. Based on existing data, the ages of orthogneisses in the Clearwater and Priest River complexes are identical and probably represent one continuous crustal block; we refer to this basement domain as the Clearwater block. The Paleoproterozoic 1.86 Ga magmatism described here is distributed throughout the northwest Laurentian margin and does not coincide with the proposed trend of the Great Falls tectonic zone. Therefore, a model of a single linear arc preserved within the Great Falls tectonic zone is inadequate in describing the majority of the known 1.86 Ga crust in the region.

Description: Mass-transport deposits (MTDs) are gravity-induced units that represent an important component of modern and ancient deep-water stratigraphic successions. MTDs have been widely documented in the literature, but a comprehensive compilation of quantitative morphometric parameters associated with their external architecture is still lacking. This work presents a morphometric database that contains 332 data points that document the length, area, volume, and thickness of MTDs from different geologic ages and a variety of continental margins around the world. The compilation contains data collected from interpretations done by the authors in eastern offshore Trinidad and the Gulf of Mexico as well as from data mining from the peer-reviewed literature. Preliminary results indicate that there is a good correlation between a series of parameters that include the area, length, and volume of MTDs. On the other hand, the correlation between thickness and volume seems to be harder to document mainly due to lateral variations in thickness that are typical within MTDs. Data analysis suggests that previous qualitative classification of attached and detached MTDs can be validated by using a quantitative approach. This validation suggests that morphometric parameters associated with the architecture of MTDs can be used as a hint to link geologic setting, deposit geometry, and potential causal mechanisms. In addition, the defined morphometric relationships that were encountered between the different morphometric parameters (e.g., length and area) are useful to predict MTD dimensions in areas of the subsurface where data are limited and/or data quality is low. Likewise, these morphometric relationships can be used in outcrop studies where exposure of the MTD units is also limited.

Description: The Cambrian Long Mountain Granite, exposed in the western Wichita Mountains, Oklahoma, is red, granophyric, alkali feldspar granite. The red granite abruptly transitions into a green granite in the subsurface; both red and green granites have similar geochemical signatures. Anisotropy of magnetic susceptibility analysis shows that the green granite retains a primary magnetic fabric that is consistent with the sill-like emplacement of the Wichita Granite Group. Demagnetization of green granite specimens yields a characteristic remanent magnetization (ChRM) residing in magnetite with east declinations and moderate down inclinations. The paleopole (9.0°N, 314.6°E) is interpreted as a primary Cambrian thermal remanent magnetization or an early magnetization related to deuteric/hydrothermal alteration. The paleopole position is consistent with some other poles for Cambrian igneous rocks in southern Oklahoma but is to the southwest of the Cambrian part of the apparent polar wander path. The red granite contains abundant secondary hematite that occurs in fractures, as grain boundary coatings, and along cleavage and exsolution planes in alkali feldspars. The iron in the hematite appears to be sourced from the oxidation of primary magnetite and ilmenite and the breakdown of silicate ferromagnesian minerals. The red granite has about two orders of magnitude lower magnetic susceptibility and natural remanent intensity than the green granite. The magnetic fabric is interpreted as an alteration fabric. The ChRM of red granite has southeast declinations and shallow inclinations and is interpreted as a chemical remanent magnetization (CRM) residing in hematite. The paleopole (44.9°S, 304.9°E) falls near the 290–300 Ma segment of the North American apparent polar wander path, which is consistent with inferred timing of exposure of the Long Mountain Granite. The CRM is interpreted to have been acquired during alteration by low-temperature weathering fluids near the surface during the late Paleozoic. The results from the red granite are not consistent with alteration caused by widespread paleoclimatic conditions in the Late Permian, and they are interpreted as related to local tectonic and/or weathering events.

Description: Understanding the source to sink relationship through time between the Eastern Kunlun Range, one of the major mountain belts in the northern Tibetan Plateau, and the actively deforming Qaidam Basin to the north has important implications for unravelling the growth history of the Tibetan Plateau. In this study, U-Pb dating (laser-ablation–inductively coupled plasma–mass spectrometry) of detrital zircons from 22 sandstone samples (Paleocene to Holocene) collected from four sections within the southwestern Qaidam Basin is combined with provenance analysis and new seismic profile interpretation to investigate the mountain building of the Eastern Kunlun Range and its effects on the development of the Qaidam Basin. The U-Pb age distributions of detrital grains from Paleocene strata are characterized by a major component of Paleozoic to late Proterozoic ages. Furthermore, carbonate debris containing foraminifera have been recognized in the Paleocene conglomerate sequences. We thus suggest that the Eastern Kunlun Range was already exhumed prior to the Paleocene. The southward onlaps of Paleocene to Oligocene strata observed on seismic profiles and the appearance of a Mesozoic component in the detrital zircon age spectra of Eocene to Oligocene strata indicate that the Qaidam Basin was widening southward during that early Cenozoic period. Well-developed post-Oligocene growth strata and the increasing proportion of Mesozoic and Paleozoic U-Pb ages in detrital zircon grains from late Neogene strata demonstrate that the relief of the Eastern Kunlun and Altyn Tagh Ranges increased, leading to isolation and narrowing of the Qaidam Basin, from Miocene to the present. The inferred pulsed deformation in the Eastern Kunlun Range highlights the complex growth history of the Tibetan Plateau.

Description: The Neruokpuk Formation is a Neoproterozoic and Cambrian turbiditic succession in northwesternmost Yukon (Canada) and northeastern Alaska (USA), part of a latest Proterozoic to Early Devonian slope and basin succession that is correlated in detail with strata in Selwyn Basin of the northern Canadian Cordillera. It includes quartz-lithic sandstone, locally containing altered detrital feldspar and muscovite indicating that a metamorphic source contributed detritus to the unit. The muscovite yields disturbed Ar-Ar spectra suggesting ages of 1800–1900 Ma. Detrital zircon distributions are dominated by 1800–2000 Ma grains with subsidiary populations of 1000–1600 Ma, 2300–2500 Ma and 2600–2800 Ma grains, consistent with a hybrid provenance dominated by a Laurentian cratonic source. Additional populations are derived from recycled Mackenzie Mountains and possibly Wernecke Supergroups. Integrating the geochronology with the regional stratigraphic setting, structural history, and geochemistry leads to the conclusion that the Neruokpuk Formation was deposited near its present location as part of the autochthonous northwest Laurentian continental margin. Therefore, the eastern part of Arctic Alaska, underlain by the Neruokpuk Formation, has a history that is distinct from the allochthonous western part(s) of the Arctic Alaska terrane. However, the rest of Arctic Alaska is structurally and stratigraphically linked to the eastern part by Late Devonian time.

Description: Magma ascent and emplacement in compressional tectonic settings remain poorly understood. Geophysical studies show that volcanic plumbing systems in compressional environments are vertically partitioned into a deep level subject to regional compression and a shallow level subject to local extension. Such vertical partitioning has also been documented for the plumbing systems of mud volcanoes, implying common, yet unresolved, underlying processes. In order to better constrain the mechanisms governing this depth partitioning of emplacement mechanisms, we studied the structure and evolution of the Cerro Negro intrusive complex emplaced in the Chos Malal fold-and-thrust belt in the foothills of the Neuquén Andes, Argentina. The Cerro Negro intrusive complex consists of sills and N-S–striking dikes that crosscut the sills. The most prominent structures in the study area are N-S–trending folds, and both E- and W-vergent thrusts. We provide new U-Pb ages of 11.63 ± 0.20 Ma and 11.58 ± 0.18 Ma for sills and 11.55 ± 0.06 Ma for a dike, which show that the Cerro Negro intrusive complex was emplaced in a short period of time. Our ages and field observations demonstrate that the emplacement of the Cerro Negro intrusive complex was coeval with the tectonic development of the Chos Malal fold-and-thrust belt. This implies that the dikes were emplaced perpendicular to the main shortening direction. The systematic locations of the dikes at the anticlinal hinges suggest that their emplacement was controlled by local, shallow stresses related to outer-arc stretching at the anticlinal hinge. We conclude that folding-related outer-arc stretching is one mechanism responsible for the vertical partitioning of igneous plumbing systems in compressional tectonic settings.

Description: Insight into the spatial and temporal changes of slip-rate is essential to understand the kinematic role of large strike-slip faults in continental collision zones. Geodetic and geologic rates from present to several million years ago along the Karakorum fault range from 0 to 11 mm/yr. Here, we determine the first late Quaternary slip-rate at the southern end of the linear Bangong-Chaxikang segment of the Karakorum fault, using cumulative offsets (20–200 m) of fans and terraces at three sites, as well as 74 new 10 Be surface-exposure ages to constrain the age of these offset geomorphic markers. The rate is 〉3 mm/yr at sites Gun and Chaxikang, and it is 〉1.7–2.2 mm/yr at the Gar fan site. Together with rates obtained along the southernmost Menshi-Kailas segment, the Karakorum fault slip-rate seems to increase southeastward from south of Bangong Lake to Kailas (from 〉3 to 〉8 mm/yr). These Karakorum fault slip-rate data (〉3–8 mm/yr), together with the total length of the fault (〉1000 km) and its initiation age (〉13–23 Ma), confirm that the Karakorum fault is the major fault accommodating dextral strike-slip motion NE of the western Himalayas. The dextral Karakorum fault in the south and the conjugate left-lateral Longmu Co–Altyn Tagh fault system in the north are thus the major strike-slip faults of western Tibet, which contribute to eastward extrusion of Tibet.

Description: The combination of structural data from the source area and descriptive data from the deposit’s carapace, as well as remote sensing and statistical analysis, allows a better understanding of the collapse, fragmentation, and propagation processes of the Frank Slide rock avalanche. The in situ observed conservation of the stratigraphic sequence of the Turtle Mountain anticline’s normal limb in the deposit is interpreted as the consequence of the collapse mode, involving simple shearing of the mass accompanied by a rotational movement, i.e., a "simple shear" parallel to the topography, and a breakage of the hinge, followed by overthrusting of the normal limb on the inverse limb. Coherence between the block size distributions of the source area and the deposit carapace is interpreted as a demonstration of the primary control of preexisting fracturing on the fragmentation processes. Remote-sensing indexes allow us to uncover a priori hidden morphological features preserved on the surface of the deposit, i.e., longitudinal and compressional features, as well as lobes. Their location on the carapace provides evidence of lateral heterogeneity in the propagation, highlighted by three different types of propagation behaviors. This comprehensive study not only provides elements that contribute to an understanding of the Frank Slide rock avalanche, but it also provides insight into essential parameters to take into account in further modeling of these types of phenomena.

Description: Thick sulfate evaporite accumulations are absent from Proterozoic strata between ca. 2000 and ca. 1000 Ma, and detailed sedimentologic, stratigraphic, and geochemical data for the oldest Neoproterozoic thick marine sulfate evaporite successions are largely lacking. The middle Neoproterozoic Ten Stone Formation (Little Dal Group, Northwest Territories, Canada) consists of ~500 m of pelagic lagoonal gypsite and anhydritite (rocks consisting of the minerals gypsum and anhydrite) deposited shortly before the ca. 811 Ma Bitter Springs carbon isotope anomaly in an intracratonic basin that developed prior to breakup of Rodinia. The thickness of regional stratigraphic subdivisions of this formation, defined by subtle silt- and carbonate-bearing intervals, indicates a minor terrigenous source in the southeast and a silled connection to the open ocean in the northwest. Deposition of the Ten Stone Formation began with abrupt, tectonically triggered subsidence and restriction, and ended equally abruptly, as shown by stratigraphic contacts across which lithofacies corresponding to strikingly different paleoenvironments change sharply, with no evidence for hiatus or erosion. Stratigraphic cyclicity in the evaporite succession is minimal owing to isolation of bottom-hugging, dense lagoonal brine from overlying waters. Deposition of the Ten Stone Formation in a basin that experienced intermittent, basin-scale tectonic adjustments, as recorded by details of its stratigraphy, supports the interpretation that the Mackenzie Mountains Supergroup accumulated in an extensional, tectonically active intracratonic basin whose structure resembled a lower-plate extensional system. The absence of halite from the Ten Stone Formation contrasts with its abundance in the stratigraphically lower, gypsum-free Dodo Creek Formation, suggesting that deposition of the lower to middle Little Dal Group spanned a major oxygenation event, during which the sequence of evaporite mineral precipitation from seawater changed from halite-first to sulfate-first in response to rapid accumulation of atmospheric oxygen and concomitant increase in the global marine sulfate reservoir. The limited range of sulfur isotope values in a new data set spanning hundreds of meters of gypsite indicates a strongly and persistently oxidizing mid-Neoproterozoic atmosphere, an abundance of sulfate in seawater, and marine oxygenation extending below storm wave base. The mineralogy, sedimentology, stratigraphy, and geochemistry of the Ten Stone Formation are virtually indistinguishable from those of thick, Phanerozoic "deep-water" (below wave-base) evaporite successions, and indicate that the tectonic, climatic, and geochemical conditions required for deposition of thick successions of marine sulfate evaporites were well established prior to ca. 811 Ma. Thick sulfate evaporite successions in equivalent stratigraphic positions just below the Bitter Springs carbon isotope excursion elsewhere in Laurentia, as well as on the Congo craton, and in South Australia attest to the global impact of the rapidly increased seawater sulfate reservoir prior to Rodinia’s breakup. High relative burial rates of organic matter prevailed before the breakup of Rodinia and led to oxygenation of the atmosphere-ocean system, growth of the seawater sulfate reservoir, and, in association with a warm and arid climate, deposition for the first time in Earth’s history of thick sulfate evaporites in the middle Neoproterozoic, ~100 m.y. before the first Cryogenian glacial episode. The Neoproterozoic Oxygenation Event may have taken place in several steps, the first of which preceded the Bitter Springs anomaly.

Description: Paleointensity is one of the least determined parameters in geomagnetism, yet this information has the potential to address many fundamental geophysical problems that bear on the evolution of the Earth’s core and lower mantle. We test two important hypotheses that affect our understanding of how paleointensity has changed in Earth’s past: (1) the Mesozoic Dipole Low (MDL) hypothesis and (2) the inverse relationship between geomagnetic polarity reversal rate and paleointensity. We report paleointensity results determined using both the Coe modified Thellier method and the IZZI protocol on the medium-grained hornblende-biotite granite of the Middle Jurassic Shamrock batholith in the southern Singatse Range, western Nevada (USA). Previous zircon U-Pb dating gives an age of batholith emplacement at ca. 165.8 ± 0.4 Ma. This age estimate is coincident with a period of unusually high reversal frequency (~10 reversals/Ma) in the Middle Jurassic. Remanence in the Shamrock batholith is held primarily by exsolved submicron magnetite inclusions in plagioclase. The laboratory unblocking temperature range for the well-defined thermal remanent magnetization is narrow, between ~540 and 575 °C, and median destructive fields are between 40 and 50 mT, thus we infer that the age of remanence acquisition is approximated by the U-Pb age estimate for pluton emplacement. Two sets of quality criteria were used to assess the paleointensity results. The first set yielded an 87% success rate while the second set using more stringent criteria resulted in only a 20% success rate. Mean paleointensity values for the Shamrock batholith using the "loose" and "strict" quality criteria are 34.0 ± 6.4 µT ( N = 13) and 33.6 ± 9.6 µT ( N = 3) respectively, with errors reported at one standard deviation. After correction for both remanence anisotropy and cooling rate, the mean values that pass "loose" and "strict" criteria are respectively 17.9 ± 2.7 µT and 17.5 ± 2.6 µT. The uncorrected and corrected paleointensity values for the "strict" estimates yield respective mean virtual dipole moments (VDMs) of 62.7 ± 18 ZAm 2 (Z = 10 21 ) and 32.7 ± 4.9 ZAm 2 . The uncorrected value is within error of the VDMs reported for plagioclase grains isolated from ca. 160 Ma basalts from Ocean Drilling Program Site 801, however these values have an absolute difference ranging from ~6 to 20 ZAm 2 , while the corrected VDM value is more consistent with the estimated average Jurassic field strength of ~29 ZAm 2 . Using a recent estimate for the long-term stable field strength of ~42 ZAm 2 , our corrected values add further credence to both the MDL and the inverse relationship between geomagnetic polarity reversal rate and dipole moment hypotheses.

Description: 〈span〉Various styles of ore deposits may form from a single magmatic-hydrothermal system. Identification of a possible genetic link between different ore types in a region is not only of critical importance for a better understanding of the magmatic-hydrothermal processes, but can also help in successful mineral exploration. Both iron oxide-apatite (IOA) and iron skarn deposits are closely associated with intrusive rocks of intermediate to felsic in composition, but whether these two ore types can form from the same magmatic intrusion remains poorly understood. In this study, we present a comparative study between a newly identified subsurface IOA ore body located at the apex of a diorite porphyry and the iron skarn ore bodies located immediately above it in the Jinniu volcanic basin of the Daye district, Middle-Lower Yangtze River metallogenetic belt (MLYRMB), eastern China in order to highlight a genetic link between these two styles of mineralization. The IOA ores are dominated by Ti-rich magnetite with variable amounts of fluorapatite, diopside, and actinolite. This mineralogical assemblage is distinctly different from the iron skarn ores, which consist mainly of Ti-depleted magnetite and subordinate pre-ore garnet and diopside, and post-ore quartz, chlorite, calcite, and pyrite. In addition, magnetite from the IOA ores is characterized by well-developed ilmenite lamellae and has high concentrations of Ni, V, Co, and Ga, consistent with high temperature crystallization, whereas magnetite grains from the iron skarn ores usually exhibit oscillatory growth zones and contain much lower Ni, V, Co, and Ga, indicating their formation under relatively low temperatures. Titanite and fluorapatite from the IOA ores have U-Pb ages of 132.5 ± 2.4 Ma to 128.4 ± 3.0 Ma, which match a titanite U-Pb age for the associated iron skarn ores (132.3 ± 2.0 Ma), and are consistent with zircon U-Pb ages for the ore-hosting diorite porphyry (130.4 ± 0.7 Ma to 130.3 ± 0.5 Ma). This age consistency supports a possible genetic link among the diorite porphyry, IOA ores, and iron skarn ores. We propose that the IOA and skarn ores are the products of two consecutive mineralization stages of the same magmatic-hydrothermal system, involving a high-temperature, hypersaline fluid coexisting with the diorite porphyry magma during emplacement and a subsequent low temperature, diluted hydrothermal fluid. Other IOA and iron skarn deposits of similar ages (130 Ma) are found in a series of volcanic basins in the MLYRMB, which forms one of the world’s largest IOA metallogenic belts. The close association of the two ore styles identified at Daye provides a useful exploration guide for IOA and iron skarn deposits both on a local and regional scale.〈/span〉

Description: 〈span〉Microbially induced sedimentary structures (MISS) are reportedly widespread in the Early Triassic and their occurrence is attributed to either the extinction of marine grazers (allowing mat preservation) during the Permo-Triassic mass extinction or the suppression of grazing due to harsh, oxygen-poor conditions in its aftermath. Here we report on the abundant occurrence of MISS in the Lower Triassic Blind Fiord Formation of the Sverdrup Basin, Arctic Canada. Sedimentological analysis shows that mid-shelf settings were dominated by deposition from cohesive sand-mud flows that produced heterolithic, rippled sandstone facies that pass down dip into laminated siltstones and ultimately basinal mudrocks. The absence of storm beds and any other “event beds” points to an unusual climatic regime of humid, quiet conditions characterized by near continuous run off. Geochemical proxies for oxygenation (Mo/Al, Th/U, and pyrite framboid analysis) indicate that lower dysoxic conditions prevailed in the basin for much of the Early Triassic. The resultant lack of bioturbation allowed the development and preservation of MISS, including wrinkle structures and bubble textures. The microbial mats responsible for these structures are envisaged to have thrived, on sandy substrates, within the photic zone, in oxygen-poor conditions. The dysoxic history was punctuated by better-oxygenated phases, which coincide with the loss of MISS. Thus, Permo-Triassic boundary and Griesbachian mudrocks from the deepest-water settings have common benthos and a well-developed, tiered burrow profile dominated by 〈span〉Phycosiphon〈/span〉. The presence of the intense burrowing in the earliest Triassic contradicts the notion that bioturbation was severely suppressed at this time due to extinction losses at the end of the Permian. The notion that Early Triassic MISS preservation was caused by the extinction of mat grazers is not tenable.〈/span〉

Description: 〈span〉The largest extinction in Earth history, in the latest Permian, was followed throughout most of the Early Triassic by a prolonged period of ecologic recovery. What factors delayed biotic recovery are still under debate and partly revolve around impacts of global warming on primary marine productivity. We examined N isotope records from the Festningen section on Spitsbergen, Arctic Norway, to examine changes in nutrient availability through the Early to Middle Triassic along the northern margin of Pangea. Our results show progressive decline in N availability throughout the Griesbachian, leading to severe nutrient limitations through the remainder of the Early Triassic, until returning to a highly productive continental margin in Middle Triassic time. These results are consistent with other studies from northern and western Pangea and thus show regional nutrient limitations occurred in what should have been the main zone of marine primary productivity. Such nutrient limitation likely stressed primary production and consequently contributed to prolonged marine recovery. We suggest this was driven by high ocean temperatures depressing the marine nutricline.〈/span〉

Description: 〈span〉The tectonic affinity of the terranes and microcontinents within the Central Asian Orogenic Belt (CAOB) remains controversial. The Altai-Mongolian terrane (AMT), as a representative tectonic unit in the Mongolian collage, plays a vital role in reconstructing evolution history of the CAOB. The well-preserved early Paleozoic sedimentary sequence covering in this terrane could be considered as a fingerprint to track its provenance and tectonic affinity. Here, we present new whole-rock geochemistry, detrital zircon U-Pb dating, and Hf isotopic analysis for the metasedimentary rocks from the Mongolian Altai in order to shed new light on the tectonic affinity of the AMT. The youngest detrital zircon ages and the regional intrusions constrain the depositional time of the Mongolian Altai sequence to between Late Silurian and Early Devonian, which is consistent with the Habahe group in the western Chinese Altai. The features of whole-rock geochemistry and the cumulative distribution curves of the detrital zircon age spectra indicate that the Mongolian Altai sequence was probably deposited in an active continental setting during early Paleozoic. The zircon age spectra of our samples are all characterized by a main age group in the early Cambrian (peak at 541 Ma, 522 Ma, 506 Ma and 496 Ma, respectively), subdominant age populations during the Tonian, as well as rare older zircons. The nearby Lake Zone of Ikh-Mongol Arc most likely provided plenty of early Paleozoic materials, the subdominant Neoproterozoic detrital zircons could be supplied by the felsic intrusions along the western margin of the Tuva-Mongol microcontinent, and the sparse older zircons may be derived from its basement material. The Precambrian age distribution of the AMT is quite similar to both the Tarim and Siberia cratons, but the Siberia Craton displays a closer resemblance in Hf isotopic composition with the AMT. Thus, we believe that the Siberia Craton contains a closer tectonic affinity with the AMT, and that the Tuva-Mongol microcontinent possibly rifted from the western margin of this craton after the Tonian. To the south of the AMT, recent studies indicated the Yili and Central Tianshan blocks in the Kazakhstan collage of the western CAOB likely have a closer affinity with Gondwana. Therefore, the microcontinents in the CAOB most likely derived bilaterally from both the Siberia Craton and the Gondwana supercontinent. Moreover, our Hf isotopic compositions indicate two significant continental growth events in the Tonian and early Paleozoic, respectively.〈/span〉

Description: 〈span〉The Neoarchean crust-mantle interaction and crustal evolution of the North China craton are controversial and are instructive of the processes of continental crust growth and cratonic evolution. We present here a systematic study of the petrology, geochemistry, and geochronology of Neoarchean granitoids from the eastern North China craton to elucidate their petrogenesis and tectonic setting. The rocks were collected from the Jielingkou, Anziling, and Qinhuangdao plutons, and an amphibole-monzoporphyry dike in the Qinhuangdao pluton. Samples from the Jielingkou pluton, consisting dominantly of monzodiorite and diorite with minor monzonite and granodiorite, contain 52.2−64.4 wt% SiO〈sub〉2〈/sub〉, 2.46−4.52 wt% MgO (Mg# = 0.41−0.54), 3.76−5.77 wt% Na〈sub〉2〈/sub〉O, and K〈sub〉2〈/sub〉O/Na〈sub〉2〈/sub〉O ratios of 0.29−0.71. The Anziling pluton samples, comprising syenite and monzonite, display slightly higher SiO〈sub〉2〈/sub〉 (60.9−66.7 wt%) and K〈sub〉2〈/sub〉O/Na〈sub〉2〈/sub〉O ratios (0.70−1.11), but lower MgO (1.54−2.33 wt%) and Mg# (0.40−0.47) values, compared to the Jielingkou rocks. The Qinhuangdao pluton samples, consisting mainly of granite and minor syenite and granodiorite, with some diorite and monzoporphyry dikes, are characterized by the highest SiO〈sub〉2〈/sub〉 values (75.7−76.9 wt%) and K〈sub〉2〈/sub〉O/Na〈sub〉2〈/sub〉O ratios (0.73−1.41) and lowest MgO content (0.14−0.32 wt%) among the studied samples. The amphibole-monzoporphyry dike has intermediate SiO〈sub〉2〈/sub〉 (56.3 wt%), high MgO (3.79 wt%), Na〈sub〉2〈/sub〉O (5.55 wt%), and Mg# (0.45), and low K〈sub〉2〈/sub〉O/Na〈sub〉2〈/sub〉O ratio (0.66). Zircon U-Pb laser-ablation−inductively coupled plasma−mass spectrometry dating showed that all plutons have a ca. 2.5 Ga crystallization age. Zircon crystals have mildly positive ε〈sub〉Hf(〈/sub〉〈span〉t〈/span〉〈sub〉)〈/sub〉 values (+0.24 to +5.45) and a depleted mantle model age (T〈sub〉DM1〈/sub〉) of ca. 2.7 Ga. We interpret the granitoid rocks as sanukitoid-related, Closepet-type granites, potassium-rich adakites, and potassium-rich granitoid rocks that crystallized in the late Neoarchean (2.5 Ga) and were derived from partial melting of mantle peridotite that was metasomatized with the addition of slab melt, including both thickened alkali-rich juvenile lower crust and juvenile metamorphosed tonalitic rocks. Mantle plume activity ca. 2.7 Ga is thought to have been responsible for the early Neoarchean tectono-thermal event in the eastern North China craton. This activity resulted in a major crustal accretion period in the craton, with subordinate crustal reworking at its margins. A steep subduction regime between ca. 2.55 Ga and ca. 2.48 Ga led to the remelting of older crustal material, with subordinate crustal accretion by magma upwelling from a depleted mantle source resulting in late Neoarchean underplating. This crustal reworking and underplating resulted in the widespread ca. 2.5 Ga plutons in the eastern North China craton. Continental crust growth in the North China craton thus occurred in multiple stages, in response to mantle plume activity, as well as protracted subduction-related granitoid magmatism during the Neoarchean.〈/span〉

Description: 〈span〉The most striking structural features in the interior of the Shan Plateau, southeast of the eastern Himalayan syntaxis, are a series of NE-trending faults that exhibit sinistral movement and an arcuate geometry. Their origin and tectonic evolution remain poorly understood. Furthermore, a switch in slip sense is recorded along many of these faults, but the timing of kinematic reversal is still unclear, hindering an understanding of the causal geodynamic mechanisms. We conducted an integrative study of apatite and zircon (U-Th)/He thermochronology, 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar geochronology, and structural and geomorphic analysis to decipher the evolution of two major NE-trending faults: the Nantinghe and Dayingjiang faults. At least three deformation stages are identified within the Nantinghe fault zone, including top-to-the-SE/ESE thrusting, dextral ductile strike-slip shearing, and sinistral movement. Zircon and apatite (U-Th)/He data, collected from the northeastern terminus of the Nantinghe fault, reveal rapid cooling in the early Miocene. Combined with the 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar data from sinistrally sheared mylonite, left-lateral movement on the Nantinghe fault is inferred to have initiated as early as ca. 20 Ma. The Dayingjiang fault reactivated as a sinistral brittle fault along the dextral Yingjiang shear zone. A two-stage thermal history is identified along the shear zone, with prominent cooling during dextral ductile shearing in the early- to mid-Miocene followed by a lower-magnitude cooling episode at ca. 11 Ma caused by sinistral transtension along the Dayingjiang fault. The evolution of the Nantinghe and Dayingjiang faults suggests that the NE-trending fault system in the Shan Plateau may have developed along preexisting structures and underwent diachronous slip-sense inversion in the late Cenozoic. The northward advance of the eastern Himalayan syntaxis caused a major change in both the regional stress field and fault geometries in the eastern India-Eurasia oblique convergence zone, contributing to the inversion of fault kinematics.〈/span〉

Description: 〈span〉This study focused on uppermost Cretaceous sedimentary rocks deposited in the Himalayan region and around the core of peninsular India just before the eruption of the Deccan Traps. Detailed stratigraphic and sedimentological analysis of Late Cretaceous successions in the Himalayan Range together with literature data from the Kirthar fold-and-thrust belt and central to southeastern India document a marked shallowing-upward depositional trend that took place in the Campanian−Maastrichtian before the Deccan magmatic outburst around the Cretaceous-Tertiary boundary. Topographic uplift of the Indian peninsula began in Campanian time and is held responsible for thick sediment accumulation associated with shorter periods of nondeposition in peripheral areas (Himalayan Range, Kirthar fold belt, and Krishna-Godavari Basin) than in the central part of the Deccan Province. Surface uplift preceding Deccan volcanism took place at warm-humid equatorial latitudes, which may have led to an acceleration of silicate weathering, lowered atmospheric 〈span〉p〈/span〉CO〈sub〉2〈/sub〉, and climate cooling starting in the Campanian−Maastrichtian. The radial centrifugal fluvial drainage in India that is still observed today was established at that time.〈/span〉

Description: 〈span〉Understanding the history of the response of coral reefs to past climate changes can provide valuable information for predicting the future response of modern reefs. However, dating such ancient biotic carbonate is still challenging because of its sensitivity to diagenetic alteration processes, scarcity of well-preserved fossils, and low magnetic mineral content. There have been a long debates about the origin and evolutionary history of coral reefs in the northern South China Sea, mainly due to the lack of direct and reliable age constraints. This provides us with a good opportunity to verify the practicability of different dating approaches, especially the strontium (Sr) isotope analysis of bulk carbonate. Here, we retrieved a 972.55-m-long core from the Xisha Islands to provide a credible chronologic constraint on the carbonate platform evolution. The lithostratigraphy, strontium isotope stratigraphy, and magnetostratigraphy were analyzed throughout the whole reef sequence. The lithostratigraphic results show that the 873.55 m reef sequence developed on an ancient volcaniclastic basement and experienced multiple evolutionary phases. The 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr results of all 100 bulk carbonate samples vary from 0.708506 to 0.709168 and show a monotonic increase with decreasing depth, except for a few outliers. Trace-element criteria and stable isotope (δ〈sup〉18〈/sup〉O and δ〈sup〉13〈/sup〉C) methods were applied to these bulk carbonate samples, and results imply that the primary or near-primary seawater 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr values were likely preserved, although different degrees of diagenetic alteration occurred. In addition, the paleomagnetic results indicate 10 normal polarity and eight reversed polarity magnetozones. Based on the 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr ratios of the selected 58 samples and paleomagnetic reconstruction of polarity reversals, the bottom of the reef sequence is dated to 19.6 Ma, and the observed polarity chronozones extend from chron C6 (19.722−18.748 Ma) at 866.60 m to present at the top. Based on the new data, we propose a new chronologic framework for the evolutionary history of the reef islands, where: (1) the reefs initiated in the early Miocene (19.6 Ma) and were drowned until 16.26 Ma; (2) during 16.26−10.66 Ma, lagoon to lagoon slope environments prevailed; (3) the lagoon environment progressively transformed into a reef crest environment from 10.66 to 4.36 Ma and 4.36 to 1.59 Ma; and (4) the reef started to be drowned again during 1.59−0 Ma. Our study provides a new and reliable chronologic constraint on the general evolutionary history of the reef islands in the northern South China Sea. Furthermore, the 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr results from bulk carbonate indicate that strontium isotope stratigraphy is a powerful dating tool only when rigid sample selection, sequential leaching procedures, and strict trace-element and isotopic criteria are applied.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We used a numerical model to investigate the formation processes of coast-parallel magnetite enrichments observed in the western Bay of Plenty, New Zealand. The model simulates differential transport of two separate grain fractions with different sizes and densities on an inclined surface, using the average hydrodynamic and geological conditions of the study area as input. In the observed and modeled structures, magnetite concentrations increase coastwards from the depth where quartz grains are first entrained until the critical shear stress of magnetite is surpassed. Beyond this point, the concentration decreases. The dominant process for structure formation is grain-size-selective entrainment. The degree of enrichment is further controlled by differential transport of the fractions. Additional tests show the influence of hydrodynamic (wave height and period, current velocities) and geological parameters (grain size and density, bed slope). Parameters that change the bed shear stress shift the enrichment into deeper or shallower water depths. The model shows the importance of cross-shelf sediment transport for effective enrichment, demonstrates the mechanisms for the formation of this type of structure, and should be able to predict the location of enrichment zones in other coastal environments for a variety of heavy minerals.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Middle–late Campanian strata of the Book Cliffs, Utah, USA, archive three clastic wedges of the North American Cordilleran foreland basin east of the Sevier fold and thrust belt. Variations in wedge geometries provide an opportunity to evaluate controls on foreland basin stratigraphic architecture. There is a significant increase in eastward progradation rate from the Lower to Upper Castlegate Sandstone, followed by a decreased progradation rate in the overlying Bluecastle Tongue and Price River Formation. Rapid progradation during deposition of the Upper Castlegate Sandstone has been attributed to increased sediment supply due to either rapid exhumation of the Sevier fold and thrust belt or introduction of a new sediment source. Rapid progradation has also been attributed to a reduction in basin subsidence due to intraforeland Laramide uplifts or a change in the locus of dynamic subsidence. We test these hypotheses by documenting temporal and spatial changes in the detrital zircon U-Pb age spectra attributable to changes in sediment dispersal patterns, changes in orogenic belt configuration, and/or major tectonic episodes. Mixture modeling of relative contributions from potential source areas indicates a mixed contribution of thrust-belt, northern, and southern sources during rapid progradation. The introduction of multiple sources is likely due to a northward propagation of exhumation in the thrust belt coeval with introduction of axially-sourced detritus in the medial–distal foreland basin. The northward propagation of exhumation culminated with exhumation in the Charleston-Nebo Salient and Uinta Uplift north of the Book Cliffs and may point to a common cause for these phenomena. Mixture modeling points to a change to a dominant southern source during slower progradation in the overlying strata. The return to slower progradation is coeval with a decrease in thrust-belt derived detritus and transition to a dominant southern-source contribution. Transition to a dominantly axial fluvial system during this interval is consistent with a northeastward migration of the regional depocenter. Migration of subsidence and exhumation can be attributed to dynamic subsidence and uplift associated with the northeastward subduction of the Conjugate Shatsky Rise oceanic plateau beneath the Book Cliffs area. Rapid progradation observed during deposition of the Upper Castlegate Sandstone was controlled by a combination of increased sediment supply due to increased exhumation in the Sevier thrust belt and introduction of multiple sources.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We used a suite of topographic metrics and cosmogenic 〈sup〉10〈/sup〉Be-derived catchment-averaged denudation rates from 18 watersheds to evaluate patterns of millennial-scale erosion of the Santa Lucia Mountains and test the explanatory power of the power-law incision rule for this landscape. Catchment-averaged denudation rates in the study area vary between ∼0.07 and 0.4 mm/yr, with a single drainage yielding a rate 〉0.45 mm/yr. Channel steepness ranges from ∼90 to 390 m for these catchments. We used these observations to test two forms of the power-law incision rule, one incorporating multiple, lithologically dependent erodibilities, and one containing a single erodibility term. Statistical analyses indicated the power-law incision rule provides an improved fit relative to a model that does not relate erosion rate to channel steepness. However, tests comparing both model forms indicate no significant improvement in model fit when using unique erodibility and channel steepness terms for each lithology. Estimates of erodibility coefficients were ∼10〈sup〉–6〈/sup〉–10〈sup〉–5〈/sup〉 m〈sup〉0.2〈/sup〉/yr for all rock types evaluated. Where range asymmetry was most pronounced, watershed divides were identified as mobile by the χ landscape metric, yet we resolved no cross-divide change in erosion rate. Conversely, divides predicted by χ to be close to an equilibrium configuration showed markedly different erosion rates in adjoining basins. These conflicting data sets, considered in conjunction with geomorphic indicators of river capture, may suggest that divide migration occurs in abrupt, punctuated episodes, rather than by gradual migration of divides. Finally, erosion rates measured here are broadly consistent with late Pleistocene uplift rates recorded by geomorphic markers and are a factor of 2–6 lower than previously reported rates of exhumation gauged by apatite helium thermochronology. This suggests that there has been a secular slowing of erosion, and perhaps rock uplift rates, between the Pliocene and today.〈/span〉

Description: 〈span〉A combination of detrital zircon U-Pb geochronology and rare earth element geochemistry was employed to investigate the “source to sink” pathways of Eocene−lower Miocene strata in the northern South China Sea (SCS) taking into account the evolution of the surrounding drainage systems in comparison to the basin sediments. Drastic provenance transformation on the SCS sedimentary patterns has been first discovered that source of Central Vietnam was replaced by Pearl River networks. Rapidly changing local provenances of different source rocks were discerned during Eocene and early Oligocene times. Briefly, the Pearl River drainage system gradually evolved by regional tectonic processes into the present scale and exerted its influence on the northern SCS basins. During early Oligocene, only limited areas received sediments from Pearl River tributaries on the South China continent, while the majority of the southern areas were controlled by the Central Vietnam provenance before the late Oligocene. After the early Miocene, significant amounts of sediment from the inner South China continent were transported into northern SCS basins by the western Pearl River tributaries. Meanwhile, an impressive “Kontum-Ying-Qiong” river system (K-Y-Q) had delivered huge amounts of sediments from Central Vietnam mafic-to-ultramafic source rocks to the regions farther east since the early Oligocene or even Eocene. This remarkable K-Y-Q traversing the whole Qiongdongnan Basin was gradually replaced by a transport system from the adjacent Hainan Island, in modern-day China, providing acidic detritus after the early Miocene. Admittedly, Tethyan subduction beneath Sundaland makes the paleo-reconstruction work tough and elusive. However, we laid the emphasis on possible basin depositions to unravel the mystery and our discovery of the impressive conduit from Central Vietnam of the Indochina Block, transferring abundant basic sediments derived from the Indosinian event, surely would be of remarkable significance on research of (1) paleo-geographical evolution by Tethys-southern Eurasia Plate collisional effects and (2) tectonic reconstruction for the entire southeastern margin of the Eurasian continent.〈/span〉

Description: 〈span〉Modern global warming and its possible contribution to sea-level rise and flooding of low-lying coastal areas have moved both the Antarctic and Greenland ice sheets into the focus of public and scientific interest. Research has concentrated on reconstructing the dynamics of the ice sheets in order to understand their vulnerability to a changing climate by collecting multidisciplinary data and conducting numerical simulations. Synchronous changes in ice-sheet extent along its margin are assumed by a number of authors. The study presented here analyzes the regional and chronological appearance, distribution, and modification of sedimentary features and structures identified at the slope and rise off the West Antarctic Ice Sheet margin to test whether this assumption is correct.In general, a synchronous West Antarctic Ice Sheet dynamic is not supported by the analysis presented here; instead, this study documents a west-east trend with an early Miocene ice advance in the Amundsen Sea, while a glacial advance in the Bellingshausen Sea occurred only post–15 Ma. For the Bellingshausen Sea, stronger variability in sediment flux is observed, indicating stronger variability in ice extent. The dominance of downslope or along-slope sediment transport shows opposing trends between the two seas, which also reflects the advance/retreat of the local ice sheet and thus an increase/decrease in sediment input from the continent and a modification in intensity and relocation of the bottom current pathways. A possible reason for this west-east trend in ice-sheet dynamics may be the local geology (hinterland and basal), basal geomorphology, and the geom-etry of the local ice sheets.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The timing of and tectonic controls on Ordovician magmatism and deformation within accreted terranes are key elements in reconstructing the generation of the Caledonian-Appalachian orogen. This paper addresses the topic through integrated geochronological, structural, and multi-isotope geochemical investigation of magmatic arc–related peri-Gondwanan rocks of Ganderian affinity in the Leinster-Lakesman terrane in SE Ireland and the Isle of Man.A basaltic andesite from a previously unrecognized volcaniclastic sequence in the Manx Group at Port-e-Vullen on the Isle of Man yielded an age of 472.7 ± 2.8 Ma (secondary ion mass spectrometry [SIMS] zircon U-Pb), and an arc-related rhyolite from the Avoca volcanic sequence in SE Ireland was dated at 463.6 ± 2.6 Ma. Two granitoids from Graiguenamanagh (SE Ireland) yielded ages of 462.0 ± 2.7 Ma (augen gneiss) and 460.5 ± 3.2 Ma (equigranular granite), whereas the Dhoon granite (Isle of Man) was dated at 457.2 ± 1.2 Ma. Each of the granitic rocks from Graiguenamanagh and Dhoon was previously considered to be of Silurian–Devonian age. In addition, two sheets of the Croghan Kinshelagh granite (SE Ireland) yielded indistinguishable ages of 456.9 ± 2.4 Ma and 455.4 ± 2.8 Ma.Multi-isotopic analyses (SIMS zircon oxygen, laser ablation–multi-collector–inductively coupled plasma–mass spectrometry [LA-MC-ICP-MS] zircon Lu-Hf, whole-rock Sm-Nd) on the rocks of the Leinster-Lakesman terrane suggest a significant source contribution from the Ganderian microcontinent, represented by the Early Ordovician metasedimentary Ribband and Manx Groups.Structural relationships at Graiguenamanagh demonstrate a late Middle Ordovician deformation event at ca. 460 Ma. This deformation appears to be equivalent to the widespread D1 event that affected Early Ordovician rocks in SE Ireland and corresponds to the early structures within the East Carlow deformation zone, previously interpreted as an exclusively Devonian structure. Along strike, the early deformation is spatially associated with serpentinite emplacement. The East Carlow deformation zone is interpreted as an intra-Ganderian suture along which separate fragments of a peri-Gondwanan magmatic arc were juxtaposed at ca. 460 Ma. This deformation represents a Caledonian tectonic event that has not been recognized within Ganderia in the Appalachian sector of the orogen.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉New fossil data from two Early Triassic (Griesbachian to Dienerian) sections from South China show unusually high levels of both benthic and nektonic taxonomic richness occurring in the late Griesbachian. In total, 68 species (including 26 newly originated species) representing mollusks, brachiopods, foraminifers, conodonts, ostracods, and echinoderms occur in the late Griesbachian, indicating well-established and relatively complex marine communities. Furthermore, the nekton shows higher origination rates than the benthos. Analyses of the sedimentary facies and size distribution of pyrite framboids show that this high-diversity interval is associated with well-oxygenated environments. In contrast to the previously suggested scenario, which inferred that persistently harsh environmental conditions impeded the biotic recovery during the Early Triassic, our new findings, combined with recent work, indicate a fitful regional recovery pattern after the Permian-Triassic crisis, resulting in three main diversity highs: late Griesbachian–early Dienerian, early–middle Smithian, and Spathian. The transient rebound episodes were therefore influenced by both extrinsic local (e.g., redox condition, temperature) and intrinsic (e.g., biological tolerances, origination rates) parameters.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Sierras Pampeanas of Argentina have been used as a classic case study to understand the processes and mechanisms involved in thick-skinned deformation; however, the history of exhumation and uplift of these ranges remains enigmatic. This study presents new thermochronology and field observations from the Sierra de Velasco, one of the highest relief (〉4 km) and least studied mountains in the Sierras Pampeanas. Advances in the annealing and diffusion kinetics of the apatite fission track and (U-Th-Sm)/He systems provide the tools to interpret a data set with a complex Carboniferous to Miocene cooling history. Our results show that rocks sampled across 2 km of structural depth have experienced protracted exposure to temperatures at or above ∼60 °C from ca. 320 to 120 Ma. The well-constrained thermal history allows us to identify subsequent thermal perturbations including an elevated geothermal gradient during Cretaceous rifting, late Cretaceous–Paleocene cooling due to isothermal relaxation and/or exhumation, and accelerated Miocene exhumation. Quantitative estimates of the minimum rock overburden suggest that 1.0 ± 0.8 km of rock has been eroded from the Sierra de Velasco since the early Miocene. Although a low geothermal gradient (≤25 °C/km) may support the exhumation of Sierra de Velasco coincident with the onset of flat-slab subduction in the late Miocene, we suggest inherited paleotopography existed before the Miocene and possibly since the Paleozoic. Final cooling and exhumation beginning in the early to middle Miocene, concurrent with the onset of flat slab subduction, contributed to the topography observed today, but cannot explain the entirety of the modern relief. We propose that a history of long-lived topography may be extrapolated throughout the Sierras Pampeanas region.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We sought to understand the time scale, mechanisms, and extent of landscape modification in unglaciated central Pennsylvania by studying sediment moving through and stored in a sandstone headwater valley. In this landscape, the timing and extent of landscape modification are poorly constrained, and it is unclear whether, and how much, periglacial processes drive landscape evolution during cold glacial periods. Our investigation pairs geomorphic mapping with in situ cosmogenic 〈sup〉10〈/sup〉Be and 〈sup〉26〈/sup〉Al measurements of surface material and buried clasts to estimate the residence time and depositional history of colluvium within Garner Run, a 1 km〈sup〉2〈/sup〉 headwater valley in the central Appalachian Mountains containing relict Pleistocene periglacial features, including solifluction lobes, boulder fields, and thick colluvial toe-slope deposits. The preservation of periglacial landforms into the present interglacial suggests active hillslope sediment transport in cold climates followed by only limited modification in the Holocene. The 〈sup〉10〈/sup〉Be concentrations of stream sediment and hillslope regolith indicate slow erosion rates (6.6 ± 0.6 m m.y.〈sup〉–1〈/sup〉) over the past ∼100 k.y. From 〈sup〉26〈/sup〉Al/〈sup〉10〈/sup〉Be burial dating of valley-bottom deposits recovered from a 9 m drill core, we infer two pulses of deposition since 340 ± 80 ka, a record that spans at least three glacial terminations and implies limited removal of valley-bottom deposits during interglacial periods. The age of valley-bottom sediment is consistent with independent calculations determined from valley-fill volume estimates, total hillslope contributing area, and the catchment-average erosion rate integrated over multiple climate cycles. We conclude that outside of the Last Glacial Maximum ice margin, landscapes in the central Valley and Ridge physiographic province of eastern North America have experienced extensive periglacial landscape modification. Our study suggests that sandstone headwater valleys preserve soils and thick colluvial deposits that present opportunities for direct examination of the rates and dates of climate-modulated hillslope processes.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We developed a conceptual model and suspended sediment budget for a 38 km reach of the fifth-order South River, Virginia, for the past 75 yr. Bedrock, terraces, and alluvial fans confine 64% of the channel’s lateral boundaries, while bedrock exposures impose vertical confinement along 37% of the channel. Bedrock exposures in the bed separate pools and riffles developed in gravelly bed material, create unusual kilometer-long pools, and divide the study area into a gently sloping upstream reach and a steeply sloping downstream reach. Bedrock exposures upstream and downstream of an alluvial monitoring site limit changes in bed elevation (documented by scour chains and repeat surveys) by flows with up to 10 yr return periods. Fifty-seven islands (features rarely mentioned in previous studies), mostly created by avulsive floodplain incision, occur in the study reach. Rates of bank retreat, likely moderated by bedrock exposures, have modal values of only a few centimeters per year, while floodplain growth by lateral accretion is negligible. Overbank deposition dominates the sediment budget, but the areal of the extent of the floodplain is currently being reduced by bank erosion and channel widening. The South River stores 2.5% of its annual suspended sediment load per kilometer of downstream transport, demonstrating that suspended sediment storage along partly confined, mixed bedrock-alluvial rivers can be equivalent to storage along fully alluvial rivers. The future evolution of the South River will likely be controlled by bank stabilization designed to control mercury loading into the channel from erosion of contaminated floodplain sediments.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉High-magnitude flood events are among the world’s most widespread and significant natural hazards and play a key role in shaping river channel–floodplain morphology and riparian ecology. Development of conceptual and quantitative models for the response of bedrock-influenced dryland rivers to such floods is of growing scientific and practical importance, but in many instances, modeling efforts are hampered by a paucity of relevant field data. Here, we combined extensive aerial and field data with hydraulic modeling to document erosion, deposition, and vegetation changes that have occurred during two successive, cyclone-driven, extreme floods along a 50-km-long reach of the bedrock-influenced Sabie River in the Kruger National Park, eastern South Africa. Aerial light detection and ranging (LiDAR) data and photography obtained after extreme floods in 2000 and 2012 (discharges 〉4000 m〈sup〉3〈/sup〉 s〈sup〉–1〈/sup〉) were used to generate digital elevation models (DEMs) and provide the boundary conditions for hydraulic modeling (flow shear stresses for three discharges up to 5000 m〈sup〉3〈/sup〉 s〈sup〉–1〈/sup〉). For the Sabie River study reach as a whole, DEM differencing revealed that the 2012 floods resulted in net erosion of ∼1,219,000 m〈sup〉3〈/sup〉 (∼53 mm m〈sup〉–2〈/sup〉). At the subreach scale, however, more complex spatial patterns of erosion, deposition, and vegetation change occurred, as largely controlled by differences in channel type (e.g., degree of bedrock and alluvial exposure) and changing hydraulic conditions (shear stresses widely 〉1000 N m〈sup〉–2〈/sup〉 across the river around peak flow). The impact of flood sequencing and relative flood magnitude is also evident; in some subreaches, remnant islands and vegetation that survived the 2000 floods were removed during the smaller 2012 floods owing to their wider exposure to flow. These findings were synthesized to refine and extend a conceptual model of bedrock-influenced dryland river response that incorporates flood sequencing, channel type, and sediment supply influences. In particular, with some climate change projections indicating the potential for future increases in the frequency of cyclone-generated extreme floods in eastern southern Africa, the Sabie and other Kruger National Park rivers may experience additional sediment stripping and vegetation removal. Over time, such rivers may transition to a more bedrock-dominated state, with significant implications for ecological structure and function and associated ecosystem services. These findings contribute to an improved analysis of the Kruger National Park rivers in particular, but also to growing appreciation of the global diversity of dryland rivers and the relative and synergistic impacts of extreme floods.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Bimodal (mafic and felsic) rock suites are common in intracontinental orogens, but there is no consensus about their origin. This paper presents a combined study of zircon U-Pb ages and Hf-O isotopes, whole-rock major-trace elements and Sr-Nd-Hf isotopes, as well as phenocryst major elements for Mesozoic bimodal volcanics from the Middle-Lower Yangtze Valley in South China. The results indicate the presence of a common crustal component in the magma sources of both mafic and felsic volcanics, leading to proposition of a new mechanism for the origin of bimodal magmatism. Zircon U-Pb dating yields consistent ages of 134–125 Ma for the bimodal volcanism. Lithochemically, the mafic volcanics are mostly basaltic to trachybasaltic whereas the felsic volcanics are mostly trachytic to rhyolitic. Geochemically, the mafic rocks are characterized by: (1) arc-like trace element distribution patterns in the primitive mantle-normalized spidergram; (2) moderate enrichment in whole-rock Sr-Nd-Hf isotope compositions, with (〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr)〈sub〉i〈/sub〉 ratios around 0.7066, ε〈sub〉Nd〈/sub〉(t) values of –5.47 to –5.30 and ε〈sub〉Hf〈/sub〉(t) values of –3.01 to –2.73; and (3) highly variable zircon ε〈sub〉Hf〈/sub〉(t) values of –20.5 to 7.9. These geochemical features indicate that the mafic volcanics would originate from ultramafic metasomatites that were generated by reaction of the mantle wedge peridotite with altered oceanic basalt-derived aqueous solutions and terrigenous sediment-derived hydrous felsic melts. The felsic volcanics show peraluminous lithochemistry and systematically more enrichment in whole-rock Sr-Nd-Hf isotope compositions than the mafic volcanics. Zircons from the felsic volcanics exhibit similar ε〈sub〉Hf〈/sub〉(t) values to those of the mafic rocks and have variably high δ〈sup〉18〈/sup〉O values of 6.3–9.2‰. These felsic volcanics cannot be produced by differentiation of the mafic magmas or partial melting of the local crust. Instead, they would be derived from partial melting of terrigenous sediment at a lower lithosphere depth. There are similar zircon Hf isotope compositions between the mafic and felsic volcanics, suggesting that their magma sources contain a common crustal component, which is most likely terrigenous sediment. All these observations, together with the tectonic setting of the target region, indicate that the magma sources of both mafic and felsic volcanics were formed during subduction of the Cathaysia oceanic slab in the Precambrian and were stored in the fossil orogenic root for hundreds of millions of years. These two sources underwent reactivation in the Mesozoic for simultaneous partial melting to generate the bimodal magmatism. As such, this study reveals a new mechanism for bimodal magmatism in an intracontinental setting, emphasizing the presence of a common crustal component in the sources of bimodal rock suites.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Decorah structure, recently discovered in northeastern Iowa, now appears as an almost entirely subsurface, deeply eroded circular basin 5.6 km in diameter and ∼200 m deep, that truncates a near-horizontal series of Upper Cambrian to Lower Ordovician platform sediments. Initial analysis of geological and well-drilling data indicated characteristics suggestive of meteorite impact: a circular outline, a shallow basin shape, discordance with the surrounding geology, and a filling of anomalous sediments: (1) the organic-rich Winneshiek Shale, which hosts a distinctive fossil Lagerstätte, (2) an underlying breccia composed of fragments from the surrounding lithologies, and (3) a poorly known series of sediments that includes shale and possible breccia. Quartz grains in drill samples of the breccia unit contain abundant distinctive shock-deformation features in ∼1% of the individual quartz grains, chiefly 〈span〉planar fractures〈/span〉 (cleavage) and 〈span〉planar deformation features〈/span〉 (PDFs). These features provide convincing evidence that the Decorah structure originated by meteorite impact, and current models of meteorite crater formation indicate that it formed as a 〈span〉complex impact crater〈/span〉 originally ∼6 km in diameter. The subsurface characteristics of the lower portion of the structure are not well known; in particular, there is no evidence for the existence of a 〈span〉central uplift,〈/span〉 a feature generally observed in impact structures of comparable size. The current estimated age of the Decorah structure (460–483 Ma) suggests that it may be associated with a group of Middle Ordovician impact craters (a terrestrial “impact spike”) triggered by collisions in the asteroid belt at ca. 470 Ma.〈/span〉

Description: 〈span〉New studies of forearc tonalite-trondhjemite plutons in southern Alaska provide further support for an Early Cretaceous intra-oceanic ridge subduction along the northern Cordilleran margin. The geological setting together with the timing and geochemistry of the plutons strongly suggest the origin of these plutons can be related to a ridge subduction event in the Early Cretaceous. The plutons were emplaced along the Border Ranges fault system, the structural boundary between forearc and arc assemblages in southern Alaska. The plutons were emplaced late in the ductile deformational history of an amphibolite- to greenschist-facies shear zone that developed in a previously accreted mélange at this arc-forearc boundary. New U-Pb zircon dates reported here indicate the five main plutons are indistinguishable in age at ca. 123 Ma. The zircon hafnium (ε〈sub〉Hf〈/sub〉[〈span〉t〈/span〉] 〉 +12 to +20) and oxygen (δ〈sup〉18〈/sup〉O = 4.6‰−6.0‰) isotope compositions strongly suggest that these plutons were derived from a typical depleted mantle source. These Hf-O isotope compositions coupled with the absence of zircon inheritance indicate a minimal contribution from partial melting of either subducting slab or preexisting evolved Jurassic crust. The restricted timing (ca. 123 Ma), zircon Hf-O isotopic compositions, and the whole-rock major and trace-element geochemistry of these plutons strongly support their origin by anatexis of a garnet-free amphibolitic source derived from a mafic rock of mid-ocean-ridge basalt composition, unaffected by seawater interactions, and without significant contribution of continental crust (sediment) components. Therefore, we suggest that during a restricted time period from 126 to 123 Ma, a “slab window” opened up when a ridge segment was subducted underneath the forearc region. By analogy with younger sites of ridge subduction, we infer that decompressing asthenosphere melted lower-oceanic-crust metagabbros that presumably had been underplated by earlier phases of the ridge interaction. The magma thus generated was differentiated and emplaced as near-trench plutons. Our results suggest that these Alaskan forearc plutons represent a primary, relatively unmodified magma type that may be diagnostic of magmas generated during ridge subduction. Because ridge subduction represents a potential modern analog to Archean subduction, comparison to compositionally similar tonalite-trondhjemite suites and oceanic plagiogranites is informative for general models of granitoid petrogenesis. The mantle-like Hf-O isotope compositions of zircons from these forearc tonalite plutons in southern Alaska are comparable to those of oceanic plagiogranites, suggesting partial melting of similar sources such as depleted mantle and unaltered oceanic crust. These forearc tonalite plutons are systematically depleted in rare earth elements (REEs) compared to the suprasubduction-zone ophiolite-related plagiogranites produced by extreme fractional crystallization. However, the REE patterns of these Alaska tonalites exhibit near-complete overlap with those observed in ophiolite plagiogranites produced by partial melting of metamorphosed oceanic crust. Several Archean tonalite-trondhjemite-granodiorite (TTG) rocks have chondritic Hf and mantle-like oxygen isotopic compositions coupled with a REE pattern similar to that observed in experimental melts produced from garnet-absent amphibolite. These similarities suggest that some Archean TTG rocks may have been produced by partial melting of garnet-free amphibolite underplates, possibly in similar ridge subduction tectonic settings.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Changes in mammalian faunal composition and structure following the Cretaceous–Paleogene mass extinction are central to understanding not only how terrestrial communities recovered from this ecological perturbation but also the evolution of archaic groups leading to extant mammalian clades. Here, we analyzed changes in mammalian local faunas during the earliest Paleogene biotic recovery on a small spatiotemporal scale. We compiled samples of mammals from four localities in the Hell Creek Formation and Tullock Member of the Fort Union Formation, in the McGuire Creek area, McCone County, Montana, USA, and placed these localities into a high-precision chronostratigraphic framework using 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar tephra ages and magnetostratigraphy. Within this framework, we quantitatively compared faunal composition, heterogeneity, and richness among McGuire Creek local faunas and made broader comparisons to other earliest Paleogene faunas from throughout the Western Interior of North America. In the first ∼320 k.y. of the recovery, mammalian local faunas at McGuire Creek, all of which can be placed in the Puercan 1 North American Land Mammal Age (NALMA) interval zone, underwent modest increases in taxonomic richness and heterogeneity, indicating the beginning of biotic recovery; however, no McGuire Creek fauna reached fully recovered levels of taxonomic richness. Further, appearance of immigrant taxa such as 〈span〉Purgatorius〈/span〉 in younger McGuire Creek faunas demonstrates important compositional changes within the Pu1 of McGuire Creek. These results highlight the difficulties with describing the nuanced mammalian recovery process using the NALMA system and emphasize the increasing importance of high-precision dating, especially when comparing faunas across large geographic distances.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Greater Himalayan Sequence in India and Nepal records crustal thickening processes that took place during and following the onset of India-Asia collision (ca. 54–50 Ma). These resulted in late Eocene–early Miocene kyanite- and sillimanite-grade regional metamorphism, and Oligocene–Miocene crustal anatexis, which formed migmatites and leucogranites. We present new U-Pb monazite data for kyanite- and sillimanite-grade gneisses of the Neoproterozoic to Cambrian Tanawal Formation in the Pakistan Himalaya, which have metamorphic ages of 482.4 ± 7.9 Ma and 464.5 ± 4.0 Ma, respectively. These ages, together with along-strike equivalent rocks in the Lesser Himalaya of India and Nepal, help to define an Ordovician orogeny (the Bhimpedian orogeny, also called the Kurgiakh orogeny) spanning at least from 490 to 460 Ma, which followed Neoproterozoic–Cambrian sedimentation and preceded Late Ordovician–Silurian postorogenic molasse deposition and development of a stable shelf margin at 460–440 Ma. These new ages for peak metamorphism of Neoproterozoic rocks in the Pakistan Himalaya overlap with those of widespread S-type granites that exist along the length of the Lesser Himalaya (e.g., Mansehra, Mandi, and Kathmandu granites), Greater Himalaya (e.g., Nanga Parbat, Kinnaur Kailas, and Ama Drime), and in the North Himalayan domes (e.g., the Tso Morari, Kangmar, and Kampa domes). Thus, based on stratigraphic, metamorphic, and U-Pb geochronological criteria, we equate rocks of the Tanawal Formation gneisses to the Lesser Himalayan rocks to the east and rocks of the Alpurai Group to the Higher Himalaya. Our data also confirm that the Lesser, Greater, and Tethyan Himalaya are all part of a single contiguous plate. Staurolite-grade schists from a higher structural level of the Tanawal Formation record U-Pb monazite ages of ca. 272–267 Ma, indicating the occurrence of Permian regional metamorphism in the area concomitant with major rifting and extrusion of the Panjal Trap volcanics.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Late Pleistocene glaciers and lakes of the Great Basin have been studied extensively due to their excellent utility as paleoclimate proxies; such records are particularly useful where interpreted in combination. The Wasatch Range on the eastern edge of the Great Basin hosted extensive glaciers during the Last Glacial Maximum (LGM), in addition to Lake Bonneville, the region’s largest pluvial lake. However, the timing of range-wide glaciation in the Wasatch remains uncertain as compared to the well-established Lake Bonneville chronology. Here we present new cosmogenic exposure ages from the Wasatch Range from low altitude striated bedrock (132.2 ± 5.9 ka), lateral moraine (20.2 ± 1.1 ka), nested moraines (15.5 ± 0.8 ka), and high-altitude bedrock (14.8 ± 0.7 ka) sites. These ages broadly reveal the local timing of Termination II, LGM, and Lateglacial stadia in the Wasatch, as well as subsequent deglaciation. We then use coupled glacier energy-mass balance and ice-flow models to reconstruct LGM and Lateglacial ice extents in order to determine paleoclimate conditions necessary to produce glacier extents and estimate LGM ice extents in a catchment lacking clear terminus features. Our results provide robust limits on the extents of late Pleistocene glaciers in three drainages of the central Wasatch Mountains and suggest that LGM glaciers reached and abandoned their maxima prior to the Bonneville highstand, while later undergoing rapid deglaciation coeval with the lake’s fall from the Provo level. Combining modeled paleoclimate estimates with previously published pluvial lake reconstructions yields LGM temperature depressions ranging from –10 to –8 °C (with respect to modern) with 80–150% of modern precipitation, with a best matching Lateglacial temperature depression of –5 °C with 200% of modern precipitation. These results support the hypothesis of a relatively cold LGM climate in the Great Basin with little to no increase in precipitation, followed by a period of increased precipitation and warming coincident with Heinrich Stadial 1.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Long-term global carbon isotope records (δ〈sup〉13〈/sup〉C〈sub〉carb〈/sub〉 and δ〈sup〉13〈/sup〉C〈sub〉org〈/sub〉) for the Silurian have been largely derived from unrestricted open-marine carbonates and shales. Here, we demonstrate how organic carbon harvested from halite-dominated evaporite deposits in a restricted intracratonic basin can be used to produce a carbon isotope record. Inorganic and organic carbon isotope data were generated and compared from four subsurface cores from the Silurian Michigan Basin, representing unrestricted carbonate and restricted evaporite/carbonate deposition. The δ〈sup〉13〈/sup〉C〈sub〉carb〈/sub〉 and δ〈sup〉13〈/sup〉C〈sub〉org〈/sub〉 records exhibit a number of long-term trends and major carbon isotope excursions (CIE) that are correlated with the globally identified Ireviken, Mulde, and Linde events. These data provide temporal and stratigraphic constraints in rocks where paleontological data are sparse or absent. They also potentially highlight the effect of enhanced local evaporation on isotope fractionation. This new technique for generating a long-term organic carbon isotope profile from Silurian halite sequences, which can be correlated to the global curve, is of broad interest to the geoscience and paleoclimate science communities. These data not only provide a valuable tool for understanding the chronostratigraphic framework within an evaporative interior basin, but they also provide a rare temporal link between periods of prolonged evaporite deposition and events of known paleoclimate change.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Marine terminating ice streams are a major component of contemporary ice sheets and are likely to have a fundamental influence on their future evolution and concomitant contribution to sea-level rise. To accurately predict this evolution requires that modern day observations can be placed into a longer-term context and that numerical ice sheet models used for making predictions are validated against known evolution of former ice masses. New geochronological data document a stepped retreat of the paleo–Irish Sea Ice Stream from its Last Glacial Maximum limits, constraining changes in the time-averaged retreat rates between well-defined ice marginal positions. The timing and pace of this retreat is compatible with the sediment-landform record and suggests that ice marginal retreat was primarily conditioned by trough geometry and that its pacing was independent of ocean-climate forcing. We present and integrate new luminescence and cosmogenic exposure ages in a spatial Bayesian sequence model for a north-south (173km) transect of the largest marine-terminating ice stream draining the last British–Irish Ice Sheet. From the south and east coasts of Ireland, initial rates of ice margin retreat were as high as 300–600 m a〈sup〉–1〈/sup〉, but retreat slowed to 26 m a〈sup〉–1〈/sup〉 as the ice stream became topographically constricted within St George’s Channel, a sea channel between Ireland to the west and Great Britain to the east, and then stabilized (retreating at only 3 m a〈sup〉–1〈/sup〉) at the narrowest point of the trough during the climatic warming of Greenland Interstadial 2 (GI-2: 23.3–22.9 ka). Later retreat across a normal bed-slope during the cooler conditions of Greenland Stadial 2 was unexpectedly rapid (152 m a〈sup〉–1〈/sup〉). We demonstrate that trough geometry had a profound influence on ice margin retreat and suggest that the final rapid retreat was conditioned by ice sheet drawdown (dynamic thinning) during stabilization at the trough constriction, which was exacerbated by increased calving due to warmer ocean waters during GI-2.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Gibraltar Arc is a complex tectonic region, and several competing models have been proposed to explain its evolution. We studied the sedimentary fill of the Guadalquivir Basin to identify tectonic processes that were occurring when the reopening of the Strait of Gibraltar led to the reestablishment of Mediterranean outflow. We present a chronostratigraphic framework for the Lower Pliocene sediments from the lower Guadalquivir Basin (SW Spain). The updated chronology is based on magnetobiostratigraphic data from several boreholes. Our results show that the studied interval in the La Matilla core is in the early Pliocene section, providing better constraints on the sedimentary evolution of the basin during that period. Migrating depositional facies led to a younger onset of sandy deposition basinward. At the northwestern passive margin, a 0.7 m.y. period of sedimentary bypass related to a sharp decrease in sedimentation rates and lower sea levels resulted from the tectonic uplift of the forebulge. In contrast, high sedimentation rates with continuous deep-marine sedimentation are recorded at the basin center due to continuous tectonic subsidence and west-southwestward progradation of axial depositional systems. The marginal forebulge uplift, continuous tectonic basinal subsidence, and southward progradation of clinoforms in the early Pliocene can be explained by the pull of a lithospheric slab beneath the Gibraltar Arc as the Strait of Gibraltar opened. These findings are, to our knowledge, the first reported sedimentary expression of slab pull beneath the Betics related to the opening of the Strait of Gibraltar after the Messinian salinity crisis.〈/span〉

Description: 〈span〉New geomorphic mapping and cosmogenic 〈sup〉3〈/sup〉He geochronology on the west Klamath Lake fault zone in southern Oregon, USA, constrain the age of glacial and post-glacial landforms and the rate of normal-fault slip over the past ca. 150 k.y. We focus on geomorphic features that record progressive offset over multiple time intervals in order to detect potential secular changes in fault slip rate. Thirty-nine new cosmogenic 〈sup〉3〈/sup〉He surface exposure dates and a depth profile establish a late-Pleistocene glacial chronology in the southern Cascade Range of Oregon and constrain the timing of the last two major glacial advances in the region at 17.6 ± 2.1 ka (1σ) and 97.6 ± 12.1 ka. Additionally, these data provide evidence that an older glacial advance likely occurred during marine isotope stage (MIS) 6. Fault scarp profiles from high-resolution airborne lidar, in combination with the exposure age data, allow the calculation of fault slip rates over intervals of ca. 10〈sup〉4〈/sup〉−10〈sup〉5〈/sup〉 years. Our calculations indicate dip-slip rates of ∼0.3 mm/yr since ca.100 ka, which may represent an increase from 〈0.1 mm/yr prior to ca. 100 ka, depending on the absolute age of the older glacial moraines. These rates provide insights into the structure of the Klamath Basin and suggest that despite a complex surface expression, individual fault strands likely merge to a single fault at depth.〈/span〉

Description: 〈span〉The sedimentary record of the Paleoproterozoic Aravalli Supergroup in northwest India provides evidence for a progression from passive margin to active margin and back to passive margin that can be linked to the Precambrian crustal evolution of peninsular India. U-Pb detrital zircon age data from clastic sedimentary rocks constrain the depositional age of the Aravalli Supergroup to within the ca. 2.4–1.6 Ga age bracket. The 3.3–2.9 and 2.5 Ga basement granitoids of the Banded Gneiss Complex have been identified as the source rocks for the Delwara Formation from the Lower Aravalli Supergroup. The Delwara sandstones show enrichment of Zr and depletion of Ti and Sc, indicating a passive-margin depositional environment. Immature sandstones of the Middle Aravalli Udaipur Formation incorporate detritus from ca. 1.97–1.79 Ga magmatic arc rocks (characterized by zircon ε〈sub〉Hf[〈span〉t〈/span〉]〈/sub〉 values varying from +6.1 to -7.8) and continental basement (ca. 2.5 Ga). Tectonic discrimination characteristics of Middle Aravalli sandstones, such as K〈sub〉2〈/sub〉O/Na〈sub〉2〈/sub〉O-SiO〈sub〉2〈/sub〉, TiO〈sub〉2〈/sub〉-Fe〈sub〉2〈/sub〉O〈sub〉3〈/sub〉, La-Th-Sc, and Th-Sc-Zr/10, suggest a continental arc setting, in agreement with their average modal composition of Q〈sub〉23〈/sub〉F〈sub〉27〈/sub〉L〈sub〉50〈/sub〉 (where Q is quartz, F is feldspar, and L is lithics), which indicates a proximal source and transitional arc setting. The stratigraphically higher Bowa Formation of the Middle Aravalli Supergroup received additional detritus from 1.78 to 1.72 Ga extension-related magmatic arc rocks, and these strata are characterized by zircons with subchondritic ε〈sub〉Hf(〈span〉t〈/span〉)〈/sub〉 values, suggesting a back-arc setting, in agreement with tectonic discrimination plots. The Upper Aravalli Debari sandstones are dominated by 1.87–1.65 Ga detritus. Their high maturity (average: Q〈sub〉92〈/sub〉F〈sub〉4〈/sub〉L〈sub〉4〈/sub〉) and geochemical features are consistent with a passive-margin setting. Integration of the lithostratigraphic relationships and compositional data suggests that the rocks of Aravalli Supergroup archive the record for the development of early Paleoproterozoic passive-margin platform sedimentation, followed by a transition to continental arc and subsequently to an evolved rift-basin–passive-margin setting.〈/span〉

Description: 〈span〉Granulite occupies the root of orogenic belts, and understanding its formation and evolution may provide critical information on orogenic processes. Previous studies have mainly focused on garnet-bearing high-pressure and medium-pressure granulites, whereas the metamorphic evolution and pressure-temperature (〈span〉P〈/span〉-〈span〉T〈/span〉) paths of garnet-absent, low-pressure granulites are more difficult to constrain. Here, we present zircon U-Pb ages and mineral chemistry for a suite of newly discovered two-pyroxene granulites in the North Altyn Tagh area, southeastern Tarim craton, northwestern China. Conventional geothermobarometry and phase equilibrium modeling revealed that these rocks experienced a peak granulite-facies metamorphism at 〈span〉T〈/span〉 = 790−890 °C and 〈span〉P〈/span〉 = 8−11 kbar. The mineral compositions and retrograde symplectites record a clockwise cooling and exhumation path, possibly involving near-isothermal decompression followed by near-isobaric cooling. Zircon U-Pb dating yielded a ca. 1.97 Ga metamorphic age, which likely represents the initial cooling age, based on Ti-in-zircon thermometry. Combined with regional geological records, we interpret that these granulites originated from the basement rocks of a late Paleoproterozoic magmatic arc that was subsequently involved in a collisional orogen in the southern Tarim craton, presumably related to the assembly of the Columbia/Nuna supercontinent. The clockwise 〈span〉P〈/span〉-〈span〉T〈/span〉 paths of the granulites record crustal thickening and burial followed by crustal thinning and exhumation in the upper plate of the collisional orogen. Our data indicate that the initial exhumation of this orogen probably occurred no later than ca. 1.97 Ga, which is supported by widespread 1.93−1.85 Ga postorogenic magmatism in this area.〈/span〉

Description: 〈span〉The Bengal Fan provides a Neogene record of Eastern and Central Himalaya exhumation. We provide the first detrital thermochronological study (apatite and rutile U-Pb, mica Ar-Ar, zircon fission track) of sediment samples collected during International Ocean Discovery Program (IODP) Expedition 354 to the mid−Bengal Fan. Our data from rutile and zircon fission-track thermochronometry show a shift in lag times over the interval 5.59−3.47 Ma. The oldest sample with a lag time of 〈1 m.y. has a depositional age between 3.76 and 3.47 Ma, and these short lag times continue to be recorded upward in the core to the youngest sediments analyzed, deposited at 〈1 Ma. We interpret the earliest record of short lag times to represent the onset of extremely rapid exhumation of the Eastern Himalayan syntaxial massif, defined as the syntaxial region north of the Nam La Thrust. Below the interval characterized by short lag times, the youngest sample analyzed with long lag times (〉6 m.y.) has a depositional age of 5.59−4.50 Ma, and the zircon and rutile populations then show a static peak until 〉12 Ma. This interval, from 5.59−4.50 Ma to 〉12 Ma, is most easily interpreted as recording passive erosion of the Greater Himalaya. However, single grains with lag times of 〈4 m.y., but with high analytical uncertainty, are recorded over this interval. For sediments older than 10 Ma, these grains were derived from the Greater Himalaya, which was exhuming rapidly until ca. 14 Ma. In sediments younger than 10 Ma, these grains could represent slower, yet still rapid, exhumation of the syntaxial antiform to the south of the massif. Lag times 〈1 m.y. are again recorded from 14.5 Ma to the base of the studied section at 17 Ma, reflecting a period of Greater Himalayan rapid exhumation. Mica 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar and apatite U-Pb data are not sensitive to syntaxial exhumation: We ascribe this to the paucity of white mica in syntaxial lithologies, and to high levels of common Pb, resulting in U-Pb ages associated with unacceptably high uncertainties, respectively.〈/span〉

Description: 〈span〉Temperature-dependent clay mineral assemblages, vitrinite reflectance, and one-dimensional (1-D) thermal and three-dimensional (3-D) geological modeling of a Neogene wedge-top basin in the Sicilian fold-and-thrust belt and its pre-orogenic substratum allowed us to: (1) define the burial history of the sedimentary succession filling the wedge-top basin and its substratum, (2) reconstruct the wedge-top basin geometry, depocenter migration, and sediment provenance through time in the framework of a source-to-sink system, and (3) shed new light into the kinematic evolution of the Apennine-Maghrebian fold-and-thrust belt. The pre-orogenic substratum of the Scillato basin shows an increase in levels of thermal maturity as a function of stratigraphic age that is consistent with maximum burial to 3.5 km in deep diagenetic conditions. In detail, R〈sub〉o〈/sub〉% values range from 0.40% to 0.94%, and random ordered illite-smectite (I-S) first converts to short-range ordered structures and then evolves to long-range ordered structures at the base of the Imerese unit. The wedge-top basin fill experienced shallow burial (∼2 km) and levels of thermal maturity in the immature stage of hydrocarbon generation and early diagenesis. Vitrinite reflectance and mixed-layer I-S values show two populations of authigenic and inherited phases. The indigenous population corresponds to macerals with R〈sub〉o〈/sub〉% values of 0.33%−0.45% and I-S with no preferred sequence in stacking of layers, whereas the reworked group corresponds to macerals with R〈sub〉o〈/sub〉% values of 0.42%−0.47% and short-range ordered I-S with no correlation as a function of depth. Authigenic and reworked components of the Scillato basin fill allowed us to unravel sediment provenance during the Neogene, identifying two main source areas feeding the wedge-top basin (crystalline units of the European domain and sedimentary units of the African domain), and to detect an early phase of exhumation driven by low-angle extensional faults that predated Neogene compression. 〈/span〉

Description: 〈span〉The late Carboniferous Altenberg-Teplice caldera in the northwestern Bohemian Massif provides an intriguing example of rhyolite eruptions evolving to a trapdoor collapse and coeval ring dike emplacement. Geochemical data suggest that withdrawal of an underlying stratified magma chamber beneath the caldera took place in two steps. Eruptions of the reversely zoned Teplice rhyolite drained the chamber, followed by a trapdoor collapse of the caldera floor and emplacement of voluminous microgranite ring dike system at 312 Ma. The mechanism proposed here is that the subsiding caldera floor increased vertical load on the magma chamber and triggered remobilization of residual, cumulate-like and otherwise non-eruptible magma mush. The magnetic fabrics (determined using the anisotropy of magnetic susceptibility [AMS]), carried by paramagnetic ferrosilicates and titanomagnetite, indicate that the ring dike magma first flowed upwards via four main feeder domains from which the magma was then distributed laterally. The asymmetric trapdoor collapse generated domains of dilation and contraction along the caldera limit, evidenced by different shapes of fabric ellipsoids and uneven apparent width of the individual ring dike segments. Based on this case example, we develop a general kinematic model for polyphase caldera collapse and ring dike emplacement, invoking a combination of multiple space-making processes: piston and trapdoor collapses together with downsag and faulting of the caldera floor, regional tectonic extension, reactivation of the preexisting structures, and volume changes in the caldera floor due to thermal stresses.〈/span〉

Description: 〈span〉Reconstructing the paleofluid evolution in mature fault zones, which typically have complex structural architectures, is a challenging task because reactivation of pre-existing deformation structures and dissolution-reprecipitation processes are very abundant. Understanding why specific structural elements are preferentially mineralized and what are the factors leading to rapid fluid migration and accumulation, bears geological and economic implications, especially in seismically active fault zones. We studied the Compione Fault on the Tyrrhenian Sea side of the Northern Apennines orogenic wedge, Italy, which is a segment of the 30-km-long Northern Lunigiana high-angle extensional fault system still active today. The Compione Fault propagated from the metamorphic basement and accumulated about 1.5 km of displacement. We used structural, petrographic, isotopic, microthermometric, compositional, and organic matter analyses to constrain fluid and host rock properties during fault zone evolution. This approach allowed us to quantify the thermal anomaly in the fault zone and to infer the processes responsible for such a disequilibrium. Specifically, we show that in the fault process zone ahead of the upper fault tip, which is twice as wide as the damage zone, seismic pumping caused suprahydrostatic fluid pressures and that local dilation promoted the nucleation of a highly permeable mesh of conjugate extensional shear fractures hosting calc-silicate mineralization. The thermal difference between hydrothermal minerals in the conjugate fracture mesh and the host rock is 60−90 °C. The mineralizing fluids were deeply sourced from metamorphic reactions. Propagation of the upper fault tip caused process zone folding and incorporation into the fault damage zones. As the upper fault tip breached through shallower structural levels, it favored mixing between deep and meteoric fluids.〈/span〉

Description: 〈span〉Sunset Crater volcano, located ∼25 km northeast of downtown Flagstaff (Arizona, USA) in the San Francisco volcanic field, represents an interesting case of highly explosive monogenetic volcanism. Sunset Crater is a young (ca. 1085 CE) scoria cone, ∼290 m high, that produced 0.52 km〈sup〉3〈/sup〉 dense rock equivalent (DRE) of basaltic magma (0.16, 0.12, and 0.24 km〈sup〉3〈/sup〉 DRE for the scoria cone, lava flows, and tephra deposit, respectively). The activity developed in three distinct phases: an initial fissure phase, followed by a highly explosive phase, and a final low-explosivity waning phase. The first phase was characterized by the opening of a 10-km-long fissure, which produced the Gyp Crater spatter mounds and the Vent 512 lava flow ∼10 km SE of the main scoria cone, followed by lava fountaining activity that produced the first tephra layer (unit 1, 0.01 km〈sup〉3〈/sup〉 DRE). During the second highly explosive phase, the activity migrated to the northwest to evolve into a single-vent eruption, which formed the main scoria cone. The central vent activity was initially characterized by variable eruptive styles, which started the process of cone building, including deposition of a second tephra layer (unit 2, 0.01 km〈sup〉3〈/sup〉 DRE), and the effusion of the two main lava flows (Bonito and Kana’a). Following the initiation of effusive activity, the eruption increased in explosivity to produce three subplinian tephra layers from the central vent (units 3, 4, and 5), which emitted more than 0.22 km〈sup〉3〈/sup〉 DRE of basalt, with associated eruption columns 〉20 km high. By the end of this phase, the Kana’a lava flow was almost completely emplaced, but the Bonito flow continued to grow, forming a 3-km-diameter flow field. A final waning phase produced several tephra units, commonly discontinuous and reworked, with 〈0.01 km〈sup〉3〈/sup〉 DRE cumulative volume. The comparatively large volume of tephra, the high eruptive columns, and high mass eruption rates make Sunset Crater the most explosive monogenetic eruption studied to date (volcano explosivity index [VEI] 3−4). Sunset Crater volcano represents an interesting case of monogenetic volcanism in that its activity was characterized by highly explosive eruptive phases up to subplinian in scale and intermittent episodes of lava effusion that heavily affected prehistoric communities in the area. Explosive basaltic volcanism should be considered when assessing volcanic hazards in continental settings, such as in the western United States.〈/span〉

Description: 〈span〉Rapid changes in geochemical and isotopic signatures of arc-related magmatic products can be used to trace magmatic processes in subduction zones across many scales, from the regional response of magmatism to large-scale geodynamic changes in the subduction system, to the emplacement of single intrusions. In this contribution, we use the south Patagonian subduction system as a natural laboratory to investigate magmatic processes in continental arcs. We use diverse intrusions and dikes from the retro-arc region at 49−51°S to investigate these processes both at the subduction zone scale, and within the exceptionally well-exposed Torres del Paine sheeted intrusion. We present Hf isotope data for zircon from 30 to 12 Ma magmatic units that were emplaced ∼50 km inboard of the main subduction-related batholith. These samples record an ∼18 m.y. period during which the region experienced profound geodynamic changes, resulting in transient migration of arc magmatism into the retro-arc, which then vanished to be replaced by more alkaline retro-arc magmatism. Integrating published whole rock geochemistry, we show that the Hf isotope signatures of these magmatic units directly record their mantle sources, with negligible assimilation of continental crust into magmas during transport through and storage in the crust. This allows us to trace the appearance and disappearance of the subduction component in the retro-arc mantle. Our data show that migration of calc-alkaline magmatism into the retro-arc produced magmas with a more enriched Hf isotope composition that was remarkably consistent over ∼200 km and 〉4 m.y. (ε〈sub〉Hf〈/sub〉〈span〉(i)〈/span〉 of −1 to +2.5). These signatures record addition of subducted continental crust to the mantle wedge during a period of subduction erosion that was associated with arc migration, and show that only a few m.y. of fluxing by a subduction component is sufficient to leave a distinct Hf isotope imprint on a mantle wedge previously unmodified by subduction. The Torres del Paine laccolith was built up by discrete pulses of magmatism in 〈200 k.y. Our data show isotopic differences between magmatic batches, and an abrupt shift to more juvenile Hf isotope compositions during the buildup of the youngest part of this magmatic complex, recording the rapid input of new mantle-derived melts during its formation. This rapid rejuvenation occurred within 20 ± 10 k.y., demonstrating that different batches of magmatism within a single intrusive complex can tap geochemically distinct mantle reservoirs on very short timescales of 〈200 k.y.〈/span〉

Description: 〈span〉Integrated analysis of sedimentological, stratigraphic, paleontological, and chemostratigraphic data of the Ordovician−Silurian Pin Formation exposed in the Parahio Valley in north India reveals the nature and history of depositional systems in the Himalayan region at that time. Previous faunal data indicate that much of the unit is Katian in age, and that the Ordovician−Silurian boundary could exist near the base of its uppermost member, although no definitive Silurian fossils were found. We recovered a lower Silurian ozarkodinid fauna within the upper member that includes 〈span〉Ozarkodina〈/span〉 sp. cf. 〈span〉hassi〈/span〉, 〈span〉Oulodus〈/span〉 sp., 〈span〉Panderodus unicostatus〈/span〉, and 〈span〉Distamodus〈/span〉 sp., the latter of which is a lower Silurian index fossil, thus confirming a Silurian, specifically Llandovery, age for the uppermost Pin Formation. Here we present the first high-resolution (〈0.5 m spacing) δ〈sup〉13〈/sup〉C〈sub〉carb〈/sub〉 profile for rocks of this age in the northern Himalaya. In addition, our profile is potentially the highest-resolution δ〈sup〉13〈/sup〉C〈sub〉carb〈/sub〉 curve to date from any location through the latest Katian Stage of the Ordovician. Numerous late Katian published curves exist, but they have variable values and geometries, in part due to low-resolution sampling and diagenetic problems. Our Gondwanan Pin Formation curve is nearly identical to one from the western U.S., indicating that the curves likely represent a remarkably faithful representation of the isotopic signature of latest Ordovician to earliest Silurian seawater during a transition into a major icehouse episode. Our data include a long-term positive excursion that is capped by a conspicuous positive spike, recorded within a thin calcareous-algae-rich shale unit. Directly overlying strata show a very abrupt negative shift (〉1.5‰) and then further negative drift of ∼1.3‰. Our correlations to other sections globally indicate that the pattern represents the Paroveja ( = BC-14, Elkhorn) positive excursion, the last major excursion prior to the Katian−Hirnantian boundary interval. Further correlations indicate that the Pin Formation also records an excursion in that boundary interval, herein named the KaH excursion ( = lower HICE), which is a record of initial Hirnantian global cooling. The HICE itself is removed under an unconformity at the base or top of a sandstone unit. More generally, our data cover a late Katian faunal and climatic shift, known as the Boda event, which is generally interpreted to represent a pre-Hirnantian warming interval. The large positive Paroveja excursion in our data is a signal of part of the event, and possibly a mid-Boda cooling episode. Overall, our data provide a definitive isotopic curve for the upper Katian to lower Hirnantian and the resulting intercontinental chemostratigraphic framework of a biostratigraphically contentious interval exposes previous miscorrelations of isotopic excursions and provides a standard for chronostratigraphic studies of this critical interval in Earth history. 〈/span〉

Description: 〈span〉Collision-related magmatism in accretionary-to-collisional orogens records a tectonic transition from early subduction-accretionary processes to collisional orogenesis, and also plays a significant role in continental growth. Here, we present an integrated study of field observations, geochemistry, whole-rock Rb-Sr and Sm-Nd isotopes, and zircon U-Pb ages and Lu-Hf isotopes for the Laohushan mafic to felsic magmatic rocks related to initial collision between the Alxa terrane and the Central Qilian block along the North Qilian orogenic belt, northeastern Tibet. The Laohushan magmatic rocks are dominated by quartz diorites (ca. 426 Ma), with minor tonalites enclosing dioritic enclaves (ca. 430 Ma) and hornblendite xenoliths (ca. 448 Ma), and some coeval dolerite dikes (ca. 427 Ma) intruded into the accretionary complex. The quartz diorites are characterized by light rare earth element (LREE)- and large ion lithophile element (LILE)-enrichment but have high field strength element (HFSE)-depleted trace element patterns and negative initial ε〈sub〉Nd〈/sub〉 (−1.6 to −2.9) and positive zircon initial ε〈sub〉Hf〈/sub〉 (+3.0 to +6.2) values. The dioritic enclaves are also characterized by LREE-enriched and HFSE-depleted patterns and have mostly negative initial ε〈sub〉Nd〈/sub〉 (−9.2 to +0.03) but positive zircon initial ε〈sub〉Hf〈/sub〉 (+3.0 to +5.9) values. These geochemical and isotopic features, together with isotopic mixing calculations, suggest that the quartz diorites were likely derived from partial melting of the lower crust dominated by accreted mafic oceanic rocks with minor sediments, whereas the dioritic enclaves originated from underplated mantle-derived magmas mixed with crust-derived melts. The hornblendite xenoliths have high MgO, Cr, and Ni contents, positive Th, U, and Pb anomalies, and negative Nb, Ta, and Ti anomalies. They have negative initial ε〈sub〉Nd〈/sub〉 (−2.8), near chondritic zircon initial ε〈sub〉Hf〈/sub〉 (−0.4 to +1.4) values and an Archean Nd model age (T〈sub〉DM〈/sub〉 = 2.74 Ga), suggesting that the hornblendites were likely produced by partial melting of subcontinental lithospheric mantle peridotite that was metasomatized by subduction-related melts beneath the Archean−Proterozoic Alxa terrane. We propose that partial melting of the lower crust of the early Paleozoic North Qilian orogenic belt was in response to slab breakoff and asthenospheric upwelling during the initial stage of collisional orogenesis. This study demonstrates that heterogeneous magma sources, involving accretionary materials (i.e., accreted oceanic crust and sediments) and various mantle-derived components, were mixed to form the collision-related magmatic rocks. It also highlights the significance of collision-related magmatism in continental growth and stabilization of newly-assembled crust in accretionary-to-collisional orogens.〈/span〉

Description: 〈span〉Identification of arc magmatic rock associations in a subduction zone has important implications for specifically revealing the geodynamic evolution of the subduction system. The closure time of the Bangong-Nujiang Tethyan Ocean and the detailed subduction processes have been hotly debated, hindering our understanding of the tectonic evolution of central Tibet. Here, we investigated the ca. 110–104 Ma Gerze lavas (basalts, basaltic andesites, andesites, dacites, and rhyolites) in southern Qiangtang. Fusion of slab fluid-metasomatized mantle wedge could yield the basalts, and such basaltic magmas, if contaminated with ancient basement orthogneisses, could have formed the andesites. The basaltic andesites with high Nb and Nb/La are similar to the Nb-enriched arc basalts and probably originated from slab melt-metasomatized mantle. The dacites were generated by fractional crystallization of the subducted mélange-derived intermediate magmas. The rhyolites have geochemical characteristics (high SiO〈sub〉2〈/sub〉 and La/Yb; low MgO and Sr/Y) similar to those of Jamaican-type adakites and were possibly sourced from the subducted oceanic plateau at low pressures. The Gerze Jamaican-type adakites and Nb-enriched basalt association could imply intense slab-mantle interactions. The Gerze lava suites show clear arc affinities, indicating that oceanic subduction may have lasted until 100 Ma. Based on previous studies and a noticeable ca. 145–125 Ma magmatic lull in southern Qiangtang, we suggest that the Bangong-Nujiang oceanic subduction geodynamics involved normal subduction (170–145 Ma), flat subduction (145–125 Ma), and slab roll-back (125–101 Ma). Moreover, the flat subduction was most likely caused by subduction of the oceanic plateau. Therefore, we propose, for the first time, that Tethyan oceanic plateau subduction during the Early Cretaceous could explain the tectonic evolution of the Bangong-Nujiang Ocean and distinctive magmatism in southern Qiangtang, central Tibet.〈/span〉

Description: 〈span〉The Honghai volcanogenic massive sulfide (VMS) deposit occurs in the Kalatage inlier of the Dananhu-Haerlik arc, Eastern Tianshan, in the southern Altaids. The deformed deposit, hosted in a suite of early Paleozoic basic to intermediate volcanic, volcaniclastic, and sedimentary rocks, consists of lenticular massive to semimassive, and sulfide-silicate stockwork orebodies. The orebodies are characterized by zoning of metallogenic elements and sulfide minerals outward from a central stockwork zone as follows: (Fe ± Cu), (Fe + Cu + Zn + Au + Ag; Cu 〉 Zn), (Fe + Zn + Cu + Au + Ag; Zn 〉 Cu), and (Fe ± [Cu/Zn]). A typical VMS hydrothermal alteration zone that formed around the foot of the massive orebodies is several times larger than the lenticular massive orebody. The hydrothermally altered rocks contain sericite, chlorite, and epidote zones from the core outward. Re-Os and argon dating of pyrite and sericite shows that the Honghai VMS deposit formed in the Early Silurian (436 ± 2 Ma) and was deformed in the Early Devonian (410 ± 4 Ma). This is the first documented early Paleozoic VMS deposit in the Eastern Tianshan. Our multidisciplinary data indicate that the Honghai Cu-Zn deposit formed in an immature/nascent island arc, where early tholeiitic lavas evolved into transitional basic-andesitic volcanic rocks and calc-alkaline intrusions with relatively high ε〈sub〉Nd〈/sub〉(〈span〉t〈/span〉) values (+6.2−+8.4) and low (〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr)〈sub〉i〈/sub〉 ratios (0.70412−0.70541). The Sr-Nd-Pb-S isotopic and geochemical data suggest that the ore-forming melts started in mantle-derived volcanic rocks and deep subvolcanic intrusions, and some alteration was influenced by seawater. The deposit formed in a high-level subvolcanic intrusion driven by sub-seafloor CO〈sub〉2〈/sub〉-rich, NaCl-CaCl〈sub〉2〈/sub〉-H〈sub〉2〈/sub〉O hydrothermal fluids (temperatures: 99−377 °C, salinities: 6%−18% NaCl equivalent) circulating in active extensional faults in a submarine caldera. Extension of the immature (nascent) intra-oceanic arc was one of the important factors that controlled the development and preservation of the VMS deposit. Future work will benefit from this improved understanding of the VMS deposits formed in immature (nascent) intra-oceanic arcs, especially in early Paleozoic island arcs in the Eastern Tianshan in the southern Altaids. 〈/span〉

Description: 〈span〉The sedimentary record of the Late Cretaceous North American Western Interior Seaway (WIS) is characterized by periods of enhanced organic carbon burial, including an interval that has been identified as Oceanic Anoxic Event (OAE) 3. The timing and duration of this event has proved difficult to constrain, calling into question whether a widespread organic carbon burial event actually occurred. In order to evaluate the nature of OAE 3 better, different localities are studied within the WIS to investigate whether a single anoxic, carbon burial event exists within Upper Cretaceous strata above the global OAE 2. New trace metal and Fe speciation records from the Niobrara Formation within the Sweetgrass Arch region in southern Alberta, Canada, highlight that there were several intervals of deoxygenation. Comparison of these new results with similar records from the southern portion of the seaway (Colorado, USA), allows seaway wide versus local changes in biogeochemical cycling to be differentiated. These results suggest that while enhanced organic carbon burial is favored in the WIS during the Niobrara transgression, there is no evidence for a single, discrete, seaway-wide anoxic event during the Coniacian−Santonian. In the Sweetgrass Arch region, enhanced organic carbon accumulation and the development of anoxia is controlled by the incursion of Tethyan waters into the region and may have been influenced by significant nutrient addition from ash deposition. A sea level control on the incursion of southern sourced water masses and, subsequent deoxygenation and enhanced organic carbon burial in the WIS, may explain why “OAE 3” is only identified in shallow and restricted marine environments that would be sensitive to changes in sea level. A combination of local and eustatic controls on sea level may also explain the diachronous timing of enhanced organic carbon burial in different regions.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Basin and Range Province is a classic locality of continental extension, and it is ideal for analyzing factors that control the collapse of thickened orogenic crust. However, the magnitude and distribution of extension, which are critical parameters for this analysis, remain poorly constrained in many areas. To address this problem, a cross section spanning the province at ∼39°N is presented. Retrodeformation yields 230 ± 42 km of cumulative extension (46% ± 8%), and an average pre-extensional thickness of 54 ± 6 km. When viewed at the scale of multiple ranges, two high-magnitude (∼60%–66%) and two low-magnitude (∼11%) domains of extension are apparent, and each can be related spatially to portions of the Cordilleran orogen that have high and low predicted crustal thickness, respectively. The eastern high-magnitude domain restores to a 60 ± 11 km thickness and corresponds to the western portion of the Sevier thrust belt and the estimated spatial extent of thick, underthrusted North American crust. The western high-magnitude domain restores to a 66 ± 5 km thickness and corresponds to the eastern part of the Sierran magmatic arc. Thickness variations inherited from Cordilleran orogenesis are interpreted as the primary control on extensional strain distribution. The eastern domain underwent a protracted, Late Cretaceous–Miocene transition to an extensional regime, while widespread extension in the western domain did not start until the Miocene, which is attributed to upper-crustal rheological differences between the granitic arc and the sedimentary section in the retroarc. Most extension can be temporally related to geodynamic driving events, including delamination, slab rollback, and plate-boundary reorganization, which caused gravitational collapse to proceed in distinct episodes.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Wasatch fault zone defines the eastern boundary of the actively extending Basin and Range Province (Utah, western United States) and poses a significant seismic hazard to the metropolitan areas along the Wasatch Range. A wealth of paleoseismological data documents ∼24 surface-rupturing M〈sub〉w〈/sub〉 ≥ 7 earthquakes along the Wasatch fault during the past 6400 yr. Here, we simulated the Holocene earthquake sequence on the Wasatch, Oquirrh–Great Salt Lake, and West Valley faults using three-dimensional finite-element forward modeling with the goal to calculate coseismic and postseismic Coulomb stress changes and to evaluate the slip and magnitude of hypothetical present-day and future earthquakes. Our results show that a good fit between modeled and observed paleoevents and time-integrated slip rates can be achieved within the uncertainties of the paleoseismological record and model parameters like the fault geometry. The Coulomb stress change analysis for selected paleoearthquakes showed that maximum positive stress changes are induced on faults located along strike of the source fault, while faults parallel to the source fault are generally located in stress shadow zones. Postseismic viscoelastic relaxation considerably modifies the coseismic stress changes; the resulting transient stress changes are recognizable for more than 100 yr after an earthquake. The modeled present-day state of Coulomb stress changes shows that the Brigham City, Salt Lake City, and Provo segments of the Wasatch fault are prone to failure in a M〈sub〉w〈/sub〉 ≥ 6.8 earthquake. Our study shows that simulation of an entire earthquake sequence based on a paleoseismological record is feasible and facilitates identification of possible gaps and inconsistencies in the paleoseismological record. Therefore, forward modeling of earthquake sequences may ultimately contribute to improved seismic hazard estimates.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Vincent fault in the San Gabriel Mountains, southern California, which forms the contact between the Proterozoic gneisses and Mesozoic magmatic arc in the upper plate and the subduction-associated Pelona Schist in the lower plate, has long been regarded as the Laramide subduction zone megathrust. Our investigation of the structural and thermal evolution of the Vincent fault and the associated mylonite zone requires a reevaluation of its tectonic significance. The base of the upper plate and the uppermost section of the lower plate are involved in a mylonite zone up to 1 km thick. Two mylonitic subzones are recognized within the upper-plate mylonites. The lower mylonite zone records pressure-temperature conditions of 0.46–0.52 GPa and 362–409 °C. Estimated shear stresses for the lower mylonite zone based on the recrystallized quartz grain size lie between 17.0 and 29.5 MPa. The upper mylonite zone records pressure-temperature conditions of 0.19–0.22 GPa and 323–341 °C, and shear stresses between 46.9 and 71.4 MPa. The microstructures show that part of the lower mylonite zone was overprinted by the upper mylonite zone and that the mylonites were formed during retrogressive metamorphism and exhumation. Sense of shear indicators preserved in the mylonitized Pelona Schist and the mylonitic gneisses consistently show that the upper plate moved southeastward relative to the lower plate. Zircon fission-track analysis suggests that the upper plate of the Vincent fault is not cut by any major faults, except for late Miocene to recent strike-slip faults of the San Andreas system. These observations do not support the idea that the Vincent fault is the Laramide subduction zone megathrust; instead, we conclude that it is a normal fault that contributed to the exhumation of the Pelona Schist.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Submerged paleoshorelines and terraces surrounding Santa Catalina Island and the Pilgrim/Kidney Banks in the California Continental Borderland demonstrate late Quaternary tectonic subsidence, in contrast to the other islands of the California Continental Borderland that are experiencing tectonic uplift. We used high-resolution seismic-reflection profiles to map a terrace package containing 16 successive parasequences surrounding Santa Catalina Island, preserved at depths from 30 to 470 m below modern mean sea level. The Pilgrim/Kidney Banks are surrounded by a terrace package containing 13 successive parasequences preserved at 90–310 m depth. The presence of marine terrace (beach) deposits at 〉400 m depth, far below the lowest estimates of Quaternary lowstand sea level (90–130 m), requires significant tectonic subsidence. Within each terrace, we identified the transgressive surface separating subaerial deltaic and shallow-marine deposits originating during sea-level lowstand from overlying subaqueous deltaic deposits emplaced after the lowstand. Remotely operated vehicle samples of sediment recovered from submerged terrace deposits offshore Santa Catalina Island contain faunal assemblages typical of submerged insular terraces in southern California. The distribution of equivalent extant mollusks and benthic foraminifera indicates deposition in water depths between 25 and 45 m. Extinct taxa present within the samples provide coarse (Late Pleistocene) age constraints on Santa Catalina’s deepest subsided terraces. We identified the transgressive surface corresponding to the Last Glacial Maximum and its paleo–sea-level marker at modern depths between –85 and –95 m surrounding Santa Catalina Island. Terraces surrounding Santa Catalina Island and Pilgrim Banks were correlated to lowstands and interstadials on a glacio-isostatic–adjusted, ice-volume–equivalent sea-level curve in order to evaluate subsidence rates. Santa Catalina Island has been tilting north and subsiding together with its surrounding platform at 0.08–0.27 mm/yr since at least 1.15 Ma (marine oxygen isotope stage [MIS] 34). Pilgrim Banks has been subsiding at 0.3 mm/yr for at least 0.35 m.y. but must have subsided no faster than 0.12 mm/yr between 0.35 and 1.15 Ma. We interpret the subsidence and 1.5° northward tilt of Santa Catalina Island as showing continued, although reduced, activity of the Catalina fault system simultaneous with increasing activity on the southern San Pedro Basin–San Diego Trough fault zone.〈/span〉

Description: 〈span〉Despite the worldwide ubiquity of carbonate terrains, quantification and understanding of rates and patterns of carbonate erosion are still lacking. Carbonate landscapes are prone to chemical weathering (dissolution) and should therefore be strongly influenced by climatic variables such as precipitation. However, isolating the impact of these variables is difficult, as denudation rates are also influenced by tectonic processes and lithology. This study focused on deciphering 10〈sup〉3〈/sup〉−10〈sup〉5〈/sup〉 yr denudation patterns across Mount Hermon, Israel, exploiting its climatic gradient (1500−600 mm/yr) and homogeneous lithology (Jurassic limestone). The concentrations of in situ cosmogenic 〈sup〉36〈/sup〉Cl in bedrock and sediment samples were utilized to characterize the spatial distribution of denudation and examine its potential drivers. Our results indicate differential denudation rates of Mount Hermon hilltops. The subalpine region (1700−2200 m, ∼1300−1500 mm yr〈sup〉−1〈/sup〉, mean annual temperature of 7 °C) yielded an average hilltop denudation rate of 19 ± 5 mm k.y.〈sup〉−1〈/sup〉 (〈span〉n〈/span〉 = 5), while hilltops at intermediate altitudes (1000−1600 m, ∼900−1200 mm yr〈sup〉−1〈/sup〉) seem to erode faster at 42 ± 8 mm k.y.〈sup〉−1〈/sup〉 (〈span〉n〈/span〉 = 7). In addition, soil cover seems to enhance denudation rates relative to bare bedrock conditions. This highlights a major difference between carbonates and silicates, where numerous silicates data sets suggest an exponential decrease in bedrock weathering and soil production as the soil thickness increases. Our new 〈sup〉36〈/sup〉Cl results, in conjunction with previously measured average hilltop lowering rates within the subhumid Mediterranean climate across the Judean mountain range (500−600 mm yr〈sup〉−1〈/sup〉, 19 ± 7 mm k.y.〈sup〉−1〈/sup〉), suggest that optimum conditions for carbonate denudation and chemical weathering occur below the tree line, where soil is abundant, 〈span〉p〈/span〉CO〈sub〉2〈/sub〉 is higher, and snow cover is scarce. This observation has intriguing implications in terms of long-term landscape evolution—an increase in the elevation of a mountain range above the tree line may provide negative feedback, which will decrease its hilltop erosion rate.〈/span〉

Description: 〈span〉The Santa Fe structure in northern New Mexico is one of the few confirmed impact craters in the western USA. The history of the impact structure is obscure as it is tectonized and eroded to the extent that an intact crater is not preserved, and what remains is located in a complex geological setting. Shatter cones and shocked quartz were previously cited to confirm an impact origin; however, estimates for both impact age (350−1200 Ma) and crater diameter (6−13 km) remain poorly constrained. To further evaluate the extent of shock deformation, we investigated ∼6600 detrital zircon grains for shock features, using material collected from 15 drainages and other sites within an ∼5 km radius of known shatter cone outcrops. Six detrital shocked zircon grains were found at three locations, including two near shatter cones and one near brecciated granitoid. Follow-up studies of bedrock at two sites proximal to detrital shocked zircon occurrences led to the discovery of shocked zircon in situ in a shatter cone-bearing sample of biotite schist; shocked grains were not found in brecciated granitoid at the second site. Electron backscatter diffraction confirms the presence of {112} shock-twin lamellae in five shocked zircon grains, and secondary ion mass spectrometry U-Pb data for three detrital shocked grains yielded 〈sup〉207〈/sup〉Pb/〈sup〉206〈/sup〉Pb crystallization ages from 1715 ± 22 to 1472 ± 35 Ma. Laser ablation−inductively coupled plasma−mass spectrometry U-Pb ages for detrital zircon grains at five of the investigated sites provide the first broad constraints on the local distribution of Paleo- to Mesoproterozoic bedrock in the area. The presence of shock-twinned zircon indicates that some exposed rocks at the Santa Fe structure may record impact pressures up to ∼20 GPa based on empirical studies, which is higher than previous reports of ∼10 GPa based on planar deformation features in shocked quartz. The 1472 ± 35 Ma date from a shock-twinned zircon yields the first direct radiometric maximum age constraint on the Santa Fe impact event, and expands the possible time period for impact to the Mesoproterozoic. Identification of shocked zircon in modern sediment led to the first discovery of shocked zircon in bedrock at this site, which is notable, as shocked zircon is otherwise not abundant in the studied rock samples. This study thus illustrates that detrital zircon surveys are an efficient way to search for diagnostic evidence of shock deformation at putative impact structures where shocked minerals may be present, but are not abundant in exposed bedrock.〈/span〉

Description: 〈span〉Harrat Rahat, one of several large, basalt-dominated volcanic fields in western Saudi Arabia, is a prime example of continental, intraplate volcanism. Excellent exposure makes this an outstanding site to investigate changing volcanic flux and composition through time. We present 93 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar ages and six 〈sup〉36〈/sup〉Cl surface-exposure ages for volcanic deposits throughout northern Harrat Rahat that, when integrated with a new geologic map, define 12 eruptive stages. Exposed volcanic deposits in the study area erupted 〈1.2 Ma, and 214 of 234 eruptions occurred 〈570 ka. Two eruptions occurred in the Holocene, including a historically described basalt eruption in 1256 C.E. and a trachyte eruption newly recognized as Holocene (4.2 ± 5.2 ka). An estimated ∼82 km〈sup〉3〈/sup〉 (dense rock equivalent) of volcanic product have erupted since 1.2 Ma, though this is a lower limit due to concealment of deposits 〉570 ka. Over the past 570 k.y., the average eruption rate was 0.14 km〈sup〉3〈/sup〉/k.y., but volcanism was episodic with periods alternating between low (0.04−0.06 km〈sup〉3〈/sup〉/k.y.) and high (0.1−0.3 km〈sup〉3〈/sup〉/k.y.) effusion rates. Before 180 ka, eruptions vented from the volcanic field’s dominant eastern vent axis and from a subsidiary, diffuse, western vent axis. After 180 ka, volcanism focused along the eastern vent axis, and the composition of volcanism varied systematically along its length from basalt dominated in the north to trachyte dominated in the south. We hypothesize that these compositional variations 〈180 ka reflect the growth of a mafic intrusive complex beneath the southern portion of the vent axis, which led to the development of evolved magmas.〈/span〉

Description: 〈span〉Global gold deposit classes are enigmatic in relation to first-order tectonic scale, leading to controversial genetic models and exploration strategies. Traditionally, hydrothermal gold deposits that formed through transport and deposition from auriferous ore fluids are grouped into specific deposit types such as porphyry, skarn, high- and low-sulfidation−type epithermal, gold-rich volcanogenic massive sulfide (VMS), Carlin-type, orogenic, and iron-oxide copper-gold (IOCG), and intrusion-related gold deposits (IRGDs). District-scale mineral system approaches propose interrelated groups such as porphyry Cu-Au, skarn Cu-Au-Ag, and high-sulfidation Au-Ag. In this study, the temporal evolution of subduction-related processes in convergent margins was evaluated to propose a continuum of genetic models that unify the various types of gold deposits. At the tectonic scale of mineral systems, all hydrothermal gold deposits are interrelated in that they formed progressively during the evolution of direct or indirect subduction-related processes along convergent margins. Porphyry-related systems formed initially from magmatic-hydrothermal fluids related to melting of fertile mantle to initiate calc-alkaline to high-K felsic magmatism in volcanic arcs directly related to subduction. Formation of gold-rich VMS systems was related to hydrothermal circulation driven by magmatic activity during rifting of oceanic arcs. Orogenic gold deposits formed largely through fluids derived from devolatilization of the downgoing slab and overlying sediment wedge during late transpression in the orogenic cycle. Carlin-type deposits, IRGDs, and some continental-arc porphyry systems formed during the early stages of orogenic collapse via fluids directly or indirectly related to hybrid magmatism from melting of lithosphere that was metasomatized and gold-fertilized by earlier fluid release from subduction zones near margins of continental blocks. The IOCGs were formed during postorogenic asthenosphere upwelling beneath such subduction-related metasomatized and fertilized lithospheric blocks via fluid release and explosive emplacement of volatile-rich melts. Thus, importantly, subduction is clearly recognized as the key unifying dynamic factor in gold metallogenesis, with subduction-related fluids or melts providing the critical ore components for a wide variety of gold-rich deposit types.〈/span〉

Description: 〈span〉Halite precipitates in the Dead Sea during winter but re-dissolves above the thermocline upon summer warming, “focusing” halite deposition below the thermocline (Sirota et al., 2016, 2017, 2018). Here we develop an “evaporite focusing” model for evaporites (nahcolite + halite) preserved in a restricted area of the Eocene Green River Formation in the Piceance Creek Basin of Colorado, USA. Nahcolite solubility is dependent on partial pressure of carbon dioxide (〈span〉p〈/span〉CO〈sub〉2〈/sub〉) as well as temperature (T), so these models covary with both T and 〈span〉p〈/span〉CO〈sub〉2〈/sub〉. In the lake that filled the Piceance Creek Basin, halite, nahcolite or mixtures of both could have precipitated during winter cooling, depending on the CO〈sub〉2〈/sub〉 content in different parts of the lake. Preservation of these minerals occurs below the thermocline (〉∼25 m) in deeper portions of the basin. Our modeling addresses both: (1) the restriction of evaporites in the Piceance Creek Basin to the center of the basin without recourse to later dissolution and (2) the variable mineralogy of the evaporites without recourse to changes in lake water chemistry. T from 20 to 30 °C and 〈span〉p〈/span〉CO〈sub〉2〈/sub〉 between 1800 and 2800 ppm are reasonable estimates for the conditions in the Piceance Creek Basin paleolake. Other evaporites occur in the center of basins but do not extend out to the edges of the basin. Evaporite focusing caused by summer-winter T changes in the solubility of the minerals should be considered for such deposits and variable 〈span〉p〈/span〉CO〈sub〉2〈/sub〉 within the evaporating brines also needs to be considered if 〈span〉p〈/span〉CO〈sub〉2〈/sub〉 sensitive minerals are found.〈/span〉

Description: 〈span〉How new subduction zones form is an ongoing scientific question with key implications for our understanding of how this process influences the behavior of the overriding plate. Here we focus on the effects of a Late Cretaceous subduction-initiation (SI) event in Iran and show how SI caused enough extension to open a back-arc basin in NE Iran. The Late Cretaceous Torbat-e-Heydarieh ophiolite (THO) is well exposed as part of the Sabzevar-Torbat-e-Heydarieh ophiolite belt. It is dominated by mantle peridotite, with a thin crustal sequence. The THO mantle sequence consists of harzburgite, clinopyroxene-harzburgite, plagioclase lherzolite, impregnated lherzolite, and dunite. Spinel in THO mantle peridotites show variable Cr# (10−63), similar to both abyssal and fore-arc peridotites. The igneous rocks (gabbros and dikes intruding mantle peridotite, pillowed and massive lavas, amphibole gabbros, plagiogranites and associated diorites, and diabase dikes) display rare earth element patterns similar to MORB, arc tholeiite and back-arc basin basalt. Zircons from six samples, including plagiogranites and dikes within mantle peridotite, yield U-Pb ages of ca. 99−92 Ma, indicating that the THO formed during the Late Cretaceous and was magmatically active for ∼7 m.y. THO igneous rocks have variable εNd(t) of +5.7 to +8.2 and εHf(t) ranging from +14.9 to +21.5; zircons have εHf(t) of +8.1 to +18.5. These isotopic compositions indicate that the THO rocks were derived from an isotopically depleted mantle source similar to that of the Indian Ocean, which was slightly affected by the recycling of subducted sediments. We conclude that the THO and other Sabzevar-Torbat-e-Heydarieh ophiolites formed in a back-arc basin well to the north of the Late Cretaceous fore-arc, now represented by the Zagros ophiolites, testifying that a broad region of Iran was affected by upper-plate extension accompanying Late Cretaceous subduction initiation.〈/span〉