ALBERT

Beschreibung: 〈span〉Sediment supplies are a fundamental component of alluvial river systems, but the importance of sustained supplies of externally derived sediments for the evolution of meandering planforms remains unclear. Here we demonstrate the importance of sediment supply in enhancing the growth of point bars that influence the rate of sinuosity increase through flow deflections in meander bends. We use an archive of Landsat images of 16 meandering reaches from across the Amazon Basin to show that rivers transporting larger sediment loads increase their sinuosity more rapidly than those carrying smaller loads. Sediment-rich rivers are dominated by downstream-rotating meanders that increase their sinuosity more rapidly than both extensional and upstream-rotating meanders. Downstream-rotating meanders appear to establish larger point bars that expand throughout the meander, in contrast to extensional meanders, which have smaller bars, and upstream rotating meanders, which are characterized by deposition over the bar head. These observations demonstrate that the size and position of point bars within meander bends influences flow routing and thus controls the dominant direction of meander growth. Rivers with low sediment supplies build smaller point bars, which reduces their capacity to increase meander curvature and the resulting sinuosity.〈/span〉

Beschreibung: 〈span〉The Sentinel Bluffs Member is the youngest unit of the Grande Ronde Basalt, having typical physical characteristics of most large-volume Columbia River Basalt Group (CRBG) sheet flows. These flows have been studied for over 50 years, resulting in numerous scientific publications from academic institutions, federal and state regional mapping projects, more detailed mapping associated with the U.S. Department of Energy’s (USDOE) Hanford Site (southeastern Washington State), and borehole investigations focused in part on petroleum exploration in the Columbia Basin. The compositional heterogeneity of individual flows, although small, has been interpreted by many workers as being the result of magma chamber processes (e.g., Wright et al., 1973; Wright and Swanson, 1980; Mangan et al., 1986; Hooper, 2000; Reidel, 2005) combined in places with surficial mixing as the flows are emplaced (e.g., Reidel and Tolan, 2013). Sawlan (2018) argues instead that flow heterogeneity in the Sentinel Bluffs Member is largely the result of previously unrecognized chemical alteration of basalt in contact with aquifers, accompanied by significant mass loss (up to 50%). The fact that nearly all CRBG lava flows are in contact with the confined aquifer system, even those in the anticlinal ridges, suggests that such alteration, if it existed, could be widespread.The ramifications of widespread, hitherto undocumented alteration are compelling, in part because of the ambiguity such alteration would place on previous interpretations that rely on high-quality chemical data. Such data could be queried as problematic or invalid in a variety of studies that focus, for example, on chemostratigraphic correlations (e.g., Reidel, 2005; Conrey and Wolff, 2010; Vye-Brown et al., 2013; Martin et al., 2013; Wells et al., 2009) or on petrogenesis and evolution of the basalts (e.g., Davis et al., 2017; Blake et al., 2010; Martin, 1989; Wolff et al., 2008; Wolff and Ramos, 2013; Ramos et al., 2005, 2009, 2013; Tollstrup et al., 2002; Hooper, 1984; Hooper and Hawkesworth, 1993; Rodriguez and Sen, 2013; Thordarson and Self, 1998), as well as in paleomagnetic studies where the results are extremely sensitive to alteration especially in the case of iron minerals (e.g., magnetite, hematite) and rely on compositional data for flow identification (e.g., Wells et al., 1989; Hagstrum et al., 2010).Consideration and scrutiny of the Sawlan (2018) study is both timely and relevant to workers on the CRBG, but it is also pertinent to workers on other basalt provinces where aquifers are present (e.g., Hawaii, the Snake River Plain [Idaho], and the Oregon High Lava Plains). We therefore embrace debate on the potential role of these aquifers in modifying primary magmatic compositions. It is our contention that careful sample collection of the CRBG has generated largely unaltered chemical compositions, and that the alteration hypothesis of Sawlan (2018) is incorrect due to flawed methodology. As a contribution to this debate, we (1) examine long-established procedures and sampling techniques that have allowed workers to avoid significant alteration when collecting and analyzing CRBG flows; (2) describe data collection of the Sentinel Bluffs Member and chemical variations that have been attributed to both magmatic processes and secondary alteration; and (3) discuss the concerns we have on the rationale and analytical methodology that form the basis for Sawlan’s (2018) conclusions that compositional variations in the Sentinel Bluff Member are the result of chemical alteration from basalt-water interaction.〈/span〉

Beschreibung: 〈span〉I welcome the opportunity to continue the discussion of my mass analysis methodology, and I would like to take this opportunity to acknowledge the workers who have contributed to the extensive literature on the mapping and geochemistry of Columbia River Basalt Group (CRBG) lava flows. Notably, the junior authors of the comment by Baker et al. (2019) have produced much of the formative work on CRBG chemostratigraphy over the past several decades. To each of them I express my appreciation for providing the foundation on which I have built my own research.In their comment, Baker et al. state that careful sampling by previous workers, and sample selection through examination of numerous thin sections, have resulted in a geochemical database for samples from the Sentinel Bluffs Member of the Grande Ronde Basalt that are mostly unaffected by secondary alteration (e.g., Reidel and Valenta, 2000). Moreover, they state that their geochemical methods are “rigorous,” “well established,” and have a “long history” of application in the study of CRBG lavas. They claim, however, that my mass analysis method (see Section 3) is “flawed,” “invalid,” and “arbitrary,” and that “it seems improbable that samples with fresh outcrop appearance and commonly glassy and unaltered mineralogy in thin section could simultaneously show chemical trends resulting from considerable mineral dissolution.”In reply, I will show that both anoxic alteration and oxic surface weathering of CRBG lavas are prevalent both east and west of the Cascade Range, that my methodology is based on quantitative techniques that have been used in the analysis of weathered rocks and soils for decades (Nesbitt, 1979; Brimhall et al., 1992; Anderson et al., 2002), and that my own careful field and laboratory sampling, as documented in Sawlan (2018), can result in an unweathered and relatively unaltered sample collection, whereas those samples previously collected in SB lavas (e.g., Reidel and Valenta, 2000) have not only experienced significant anoxic water-rock interaction with groundwater but have subsequently undergone appreciable oxic weathering at the ground surface.〈/span〉

Beschreibung: 〈span〉New analyses of U-Pb ages along with previously published analyses of detrital zircons from sandstones in the foreland of the Marathon orogen in west Texas have significant implications regarding provenance. The most prominent concentrations of U-Pb ages are at 1200–1000, 700–500, and 500–290 Ma. The accreted Coahuila terrane in the Marathon hinterland and nearby terranes with Gondwanan (Amazonia) affinity include Paleozoic volcanic and plutonic rocks, as well as Precambrian basement rocks. Late Paleozoic Las Delicias arc rocks have ages of 331–270 Ma. Detrital zircons from Upper Jurassic and Lower Cretaceous sandstones, which were deposited in local basins around the Coahuila terrane, provide a record of detritus available from proximal sources within Coahuila, including important peaks at 1040, 562, 422, 414, 373, and 282 Ma. Components of the detrital-zircon populations in the Marathon foreland have unique matches with primary and/or detrital sources in the Coahuila terrane. Although some components of the Marathon populations also have age matches in Laurentia (Appalachians), others do not; however, all components of the Marathon populations have potential sources in Coahuila. Analyses of ε〉Hf〈sub〉t〈/sub〉 show generally more negative values in Amazonia than in Laurentia, and εHf〈sub〉t〈/sub〉 values for Marathon sandstones have distributions similar to those in Amazonia. Therefore, the Coahuila terrane provides a provenance for all of the detrital-zircon ages in the Marathon foreland, requiring no mixing from other sources.〈/span〉

Beschreibung: 〈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〉

Beschreibung: 〈span〉The late Ludlow Lau Event was a severe biotic crisis in the Silurian, characterized by resurgent microbial facies and faunal turnover rates otherwise only documented during the "big five" mass extinctions. This asynchronous late Silurian marine extinction event preceded an associated positive carbon isotope excursion (CIE), the Lau CIE, although a mechanism for this temporal offset remains poorly constrained. Here, we report thallium isotope data from locally reducing late Ludlow strata within the Baltic Basin to document the earliest onset of global marine deoxygenation. The initial expansion of anoxia coincided with the onset of the extinction and therefore preceded the Lau CIE. Additionally, sulfur isotope data record a large positive excursion parallel to the Lau CIE, interpreted to indicate an increase in pyrite burial associated with the widely documented CIE. This suggests a possible global expansion of euxinia (anoxic and sulfidic water column) following deoxygenation. These data are the most direct proxy evidence of paleoredox conditions linking the known extinction to the Lau CIE through the progressive expansion of anoxia, and most likely euxinia, across portions of the late Silurian oceans.〈/span〉

Beschreibung: 〈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〉

Beschreibung: 〈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〉

Beschreibung: 〈span〉Oxidation of particulate organic carbon (POC) during fluvial transit releases CO〈sub〉2〈/sub〉 to the atmosphere and can influence global climate. Field data show large POC oxidation fluxes in lowland rivers; however, it is unclear if POC losses occur predominantly during in-river transport, where POC is in continual motion within an aerated environment, or during transient storage in floodplains, which may be anoxic. Determination of the locus of POC oxidation in lowland rivers is needed to develop process-based models to predict POC losses, constrain carbon budgets, and unravel links between climate and erosion. However, sediment exchange between rivers and floodplains makes differentiating POC oxidation during in-river transport from oxidation during floodplain storage difficult. Here, we isolated in-river POC oxidation using flume experiments transporting petrogenic and biospheric POC without floodplain storage. Our experiments showed solid phase POC losses of 0%–10% over ~10〈sup〉3〈/sup〉 km of fluvial transport, compared to ~7% to 〉50% losses observed in rivers over similar distances. The production of dissolved organic carbon (DOC) and dissolved rhenium (a proxy for petrogenic POC oxidation) was consistent with small POC losses, and replicate experiments in static water tanks gave similar results. Our results show that fluvial sediment transport, particle abrasion, and turbulent mixing have a minimal role on POC oxidation, and they suggest that POC losses may accrue primarily in floodplain storage.〈/span〉

Beschreibung: 〈span〉Plume-modified orogeny involves the interaction between a mantle plume and subducting oceanic lithosphere at accretionary margins. We propose that a plume can also be involved in collisional orogeny and accounts for the late Paleozoic geological relations in Central Asia. Continental collision between the Tarim and Central Tianshan–Yili blocks at the end Carboniferous resulted in an orogeny lacking continental-type (ultra)high-pressure [(U)HP] rocks and significant syncollision surface erosion and uplift, features normally characteristic of continent-continent interactions. Their absence from the Tianshan region corresponded with the arrival of a mantle plume beneath the northern Tarim. Elemental and isotopic data reveal an increasing influence of the mantle plume on magmatic petrogenesis from ca. 300 to 280 Ma, immediately after collision at 310–300 Ma. The rising mantle plume interrupted the normal succession of collisional orogenic events, destroying the deeply subducted continental crust and hence preventing slab break-off–induced continental rebound. Plume-modified continental collision thus limited continental (U)HP rock exhumation and associated surface uplift.〈/span〉

Beschreibung: 〈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〉

Beschreibung: 〈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〉

Beschreibung: 〈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〉

Beschreibung: 〈span〉Pre-existing structures in the crust such as shear zones, faults, and mobile belts are known to exert a significant control on the structural evolution of continental rifts. However, the influence of such features when the extension direction progressively changes over time remains uncertain. Here we present new results from three-dimensional lithospheric-scale laboratory experiments of rotational extension that provide key insights into the temporal evolution of propagating rifts. We specifically test and characterize how rifts propagate and interact with linear crustal rheological heterogeneities oriented at variable angles with respect to the extension direction. Results show that approxi­mately rift-parallel pre-existing heterogeneities favor the formation of long, linear faults that reach near-final lengths at early stages. Low angles between the heterogeneities and the propagating rift axis may result in strong strike-slip reactivation of the pre-existing structures if they are suitably oriented with respect to the stretching direction. When the linear heterogeneities are oriented at intermediate to high angles rift branches become laterally offset as they propagate, resulting in complex rhombic fault patterns. Rift-perpendic­ular crustal heterogeneities do not affect fault trends during rift propagation, but cause stalling and deepening of laterally growing rift basins. Similarities between the analogue experimental results and selected natural examples provide insights on how nature finds the preferential pathway to breakup in heterogeneous continental lithosphere.〈/span〉

Beschreibung: 〈span〉Can climate affect societies? This question, of both past and present importance, is encapsulated by the major socioeconomic crisis that affected the Mediterranean 3200 yr ago. The demise of the core civilizations of the Aegean and eastern Mediterranean during the Late Bronze Age and the early Iron Age (Dark Ages) is still controversial because it raises the question of climate-change impacts on ancient societies. Although evidence for this climate shift has gradually gained currency, recent attempts to quantify its magnitude remain equivocal. Here we focus on the northern Levant (coastal Syria) where the economic, political, and cultural changes were particularly acute. We quantify past climate changes and find that mean annual temperatures attained anomalies of –2.3 ± 0.3 °C to –4.8 ± 0.4 °C compared to present-day conditions. Rainfall regimes displayed an important shift in seasonality, with a 40% decrease in winter precipitation. A 300 yr period of dry and cool climate started ~3200 yr ago and was coeval with deep social changes in the eastern Mediterranean. These “Little Ice Age”–type conditions affected harvests, leading to severe food shortages that probably aggravated the sociopolitical tensions. This crisis highlights the fragility of societies, both past and present, to major climate-change episodes and their broader consequences.〈/span〉

Beschreibung: 〈span〉Dunes are the most common bedform in sand-bedded rivers. Small, high-angle dunes (HADs) dominate in shallow (〈2.5 m) flows and have lee sides with steep downstream-facing slipfaces (〉24°) and reversed upslope-oriented flow in the separation vortex. In contrast, large, deep (〉2.5 m) rivers have low-angle dunes (LADs) with gentle lee-side slipfaces (〈24°; often 〈10°), little or no flow separation, and more-pronounced downslope currents. Avalanches driven primarily by particle interaction maintain slipfaces of HADs. On LADs, excess pore pressures that occur during the failure of thick, loosely packed deposits can produce liquefied avalanches that flow and stop on gentle gradients. As lee-side angles decrease over LADs, downslope currents increase in strength, accelerating avalanches, transporting bedload, and creating smaller, superimposed dunes and uniform concave slopes on the lee side. Nearly a century of research on dunes in shallow laboratory flows has assumed that dune morphodynamics are scale invariant, which is not true.〈/span〉

Beschreibung: 〈span〉The abundance of Precambrian organic-rich shales, heated beyond the oil and gas window, requires that enormous volumes of hydrocarbons were generated and transported through the ancient crust. However, the former passage of fluid hydrocarbons rarely leaves a trace, so evidence for this process in the early Precambrian rock record is sparse. Here, we report the widespread presence of solidified oil (pyrobitumen) in the iron formation of the 1.88 Ga Gunflint Formation, Ontario, Canada. Petrographic textures indicate at least two phases of oil migration, an early phase marked by pyrobitumen in granules and intergranular pores, synchronous with synsedimentary silica cementation, and a later phase restricted to crosscutting fractures. The paragenetic relationships between the pyrobitumen and iron oxides indicate that oil migration commenced before hematite and some magnetite growth. Our evidence for early oil migration can be explained by petroleum generation during the 1.86–1.80 Ga Penokean orogeny, expelling hydrocarbons generated in the core of the fold-and-thrust belt outwards and updip through the sediments of the Gunflint Formation.〈/span〉

Beschreibung: 〈span〉A co-investigation of mantle melting conditions and seismic structure revealed an evolutionary record of mantle dynamics accompanying the transition from subduction to collision along the Africa-Eurasia margin and the 〉1 km uplift of the Anatolian Plateau. New 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar dates of volcanic rocks from the Eastern Taurides (southeast Turkey) considerably expand the known spatial extent of Miocene-aged mafic volcanism following a magmatic lull over much of Anatolia that ended at ca. 20 Ma. Mantle equilibration depths for these chemically diverse basalts are interpreted to indicate that early to middle Miocene lithospheric thickness in the region varied from ~50 km or less near the Bitlis suture zone to ~80 km near the Inner Tauride suture zone. This southward-tapering lithospheric base could be a vestige of the former interface between the subducted (and now detached) portion of the Arabian plate and the overriding Eurasian plate, and/or a reflection of mantle weakening associated with greater mantle hydration trenchward prior to collision. Asthenospheric upwelling driven by slab tearing and foundering along this former interface, possibly accompanied by convective removal of the lithosphere, could have led to renewed volcanic activity after 20 Ma. Melt equilibration depths for late Miocene and Pliocene basalts together with seismic imaging of the present lithosphere indicate that relatively invariant lithospheric thicknesses of 60–70 km have persisted since the middle Miocene. Thus, no evidence is found for large-scale (tens of kilometers) Miocene delamination of the lower lithosphere from the overriding plate, which has been proposed elsewhere to account for late Miocene and younger uplift of Anatolia.〈/span〉

Beschreibung: 〈span〉Drainage reversals, an end-member case of drainage reorganization, often occur toward cliffs. Reversals are commonly identified by the presence of barbed tributaries, with a junction angle 〉90°, that preserve the antecedent drainage geometry. The processes that form reversed drainages are largely unknown. Particularly, barbed tributaries cannot form through a spatially uniform migration of the cliff and drainage divide, which would be expected to erase the antecedent drainage pattern, and tectonic tilting toward the cliff that could reverse the flow direction is inconsistent with geodynamic models of large-scale escarpment, where many reversals are documented. Here, we propose a new mechanism for drainage reversal, where the slope imbalance across a cliff, together with the high erodibility of sediments that fill cliff-truncated valleys, result in faster divide migration along valleys compared to interfluves. We demonstrate this mechanism along channels that drain toward the escarpment of the Arava Valley in Israel. Reversal is established by observations of barbed tributaries and opposite-grading terraces. We show that drainage reversal occurs when erodible valley fill exists, and that the reversal extent correlates with the thickness of this fill, in agreement with the predictions of the proposed mechanism. This new reversal mechanism demonstrates that valley fill could play an acute role in fluvial reorganization processes, and that reversals could occur independently of tectonic tilting.〈/span〉

Beschreibung: 〈span〉We present a reconstruction of episodic fluid flow over the past ~250 k.y. along the Malpais normal fault, which hosts the Beowawe hydrothermal system (Nevada, USA), using a novel combination of the apatite (U-Th)/He (AHe) thermochronometer and a model of the thermal effects of fluid flow. Samples show partial resetting of the AHe thermochronometer in a 40-m-wide zone around the fault. Numerical models using current fluid temperatures and discharge rates indicate that fluid flow events lasting 2 k.y. or more lead to fully reset samples. Episodic fluid pulses lasting 1 k.y. result in partially reset samples, with 30–40 individual fluid pulses required to match the data. Episodic fluid flow is also supported by an overturned geothermal gradient in a borehole that crosses the fault, and by breaks in stable isotope trends in hydrothermal sinter deposits that coincide with two independently dated earthquakes in the past 20 k.y. This suggests a system where fluid flow is triggered by repeated seismic activity, and that seals itself over ~1 k.y. due to the formation of clays and silicates in the fault damage zone. Hydrothermal activity is younger than the 6–10 Ma age of the fault, which means that deep (~5 km) fluid flow was initiated only after a large part of the 230 m of fault offset had taken place.〈/span〉

Beschreibung: 〈span〉Recent studies have debated the timing and spatial configuration of a possible intersection between the Pacific-Izanagi spreading ridge and the northeast Asian continental margin during Cretaceous or early Cenozoic times. Here we examine a newly compiled magmatic catalog of ~900 published Cretaceous to Miocene igneous rock radioisotopic values and ages from the northeast Asian margin for ridge subduction evidence. Our synthesis reveals that a nearsynchronous 56–46 Ma magmatic gap occurred across ~1500 km of the Eurasian continental margin between Japan and Sikhote-Alin, Russian Far East. The magmatic gap separated two distinct phases of igneous activity: (1) an older, Cretaceous to Paleocene pre–56 Ma episode that had relatively lower ε〈sub〉Nd(t)〈/sub〉 (–15 to + 2), elevated (〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr)〈sub〉0〈/sub〉 (initial ratio, 0.704–0.714), and relatively higher magmatic fluxes (~1090 km〈sup〉2〈/sup〉/m.y.); and (2) a younger, late Eocene to Miocene post–46 Ma phase that had relatively elevated ε〈sub〉Nd(t)〈/sub〉 (–2 to + 10), lower (〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr)〈sub〉0〈/sub〉 (0.702–0.707), and a lower 390 km〈sup〉2〈/sup〉/m.y. magmatic flux. The 56–46 Ma magmatic gap links other geological evidence across northeast Asia to constrain an early Cenozoic, low-angle ridge-trench intersection that had profound consequences for the Eurasian continental margin, and possibly led to the ca. 53–47 Ma Pacific plate reorganization.〈/span〉

Beschreibung: 〈span〉Crystal aggregates in igneous rocks have been variously ascribed to growth processes (e.g., twinning, heterogeneous nucleation, epitaxial growth, dendritic growth), or dynamical processes (e.g., synneusis, accumulation during settling). We tested these hypotheses by quantifying the relative orientation of adjacent crystals using electron backscatter diffraction. Both olivine aggregates from Kīlauea volcano (Hawaiʻi, USA) and chromite aggregates from the Bushveld Complex (South Africa) show diverse attachment geometries inconsistent with growth processes. Near-random attachments in chromite aggregates are consistent with accumulation by settling of individual crystals. Attachment geometries and prominent geochemical differences across grain boundaries in olivine aggregates are indicative of synneusis.〈/span〉

Beschreibung: 〈span〉Thermomechanical models of mantle convection and melting in an inferred hotter Archean Earth show the emergence of pressure-temperature (〈span〉P〈/span〉-〈span〉T〈/span〉) regimes that resemble present-day plate tectonic environments yet developed within a non–plate tectonics regime. The models’ 〈span〉P〈/span〉-〈span〉T〈/span〉 gradients are compatible with those inferred from evolving tonalite-trondhjemite-granodiorite series rocks and the paired metamorphic belt record, supporting the feasibility of divergent and convergent tectonics within a mobilized, yet laterally continuous, lithospheric lid. “Hot” 〈span〉P〈/span〉-〈span〉T〈/span〉 gradients of 10–20 °C km〈sup〉–1〈/sup〉 form along asymmetric lithospheric drips, then migrate to areas of deep lithospheric downwelling within ~300–500 m.y., where they are overprinted by high-pressure warm and, later, cold geothermal signatures, up to ~8 °C km〈sup〉–1〈/sup〉. Comparisons with the crustal production and reworking record suggest that this regime emerged in the Hadean.〈/span〉

Beschreibung: 〈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〉

Beschreibung: 〈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〉

Beschreibung: 〈span〉The concept of a 〈span〉critical moment〈/span〉 in a petroleum system (the time of highest probability of entrapment and preservation of oil and gas) has underlain petroleum exploration for over 25 years. However, one area where understanding the critical moment is challenging is the Faroe-Shetland Basin (FSB; offshore UK). Isotopic dating of oils suggests that petroleum generation began between ca. 68 and 90 Ma; however, most basin models invoke an earlier generation beginning in the mid-Cretaceous at ca. 100 Ma, predating deposition of Paleocene and Eocene reservoirs. This time discrepancy has previously been explained by remigration from intermediary accumulations (“motel” hypothesis) and/or overpressure retardation of kerogen maturation. The FSB is characterized by a thick Cretaceous stratigraphic package (up to 5 km) that includes a large net thickness (up to 2 km) of Paleogene igneous material. In our model, separating sedimentary and igneous material and adding the igneous material at the correct time between ca. 58 and 55 Ma shallows the modeled burial depth of the Upper Jurassic source rocks during the Cretaceous sufficiently to delay maturation by 17 m.y. in comparison to results of previous studies. Additionally, previous studies have invoked crustal radiogenic heat production (RHP) based on the Phanerozoic crust averaging ~2.8 μW/m〈sup〉3〈/sup〉 in the North Sea (300 km to the east). However, the FSB basement is composed of significantly older, colder Neoarchean orthogneisses (ca. 2.7–2.9 Ga), reducing RHP by up to 50% to ~1.6 μW/m〈sup〉3〈/sup〉 (σ = 0.74). For the first time, our model unifies geological, geochronological, and geochemical observations, delaying the onset of petroleum expulsion by up to 40 m.y. in comparison to previous models.〈/span〉

Beschreibung: 〈span〉Continental arc systems often show evidence of large-scale migration both toward and away from the incoming plate. In oceanic arc systems, however, while slab roll-back and the associated processes of backarc spreading and arc migration toward the incoming plate are commonplace, arc migration away from the incoming plate is rarely observed. We present a new compilation of marine magnetic anomaly and seismic data in order to propose a new tectonic model for the eastern Caribbean region that includes arc migration in both directions. We synthesized new evidence to show two phases of backarc spreading and eastward arc migration toward the incoming Atlantic. A third and final phase of arc migration to the west subdivided the earlier backarc basin on either side of the present-day Lesser Antilles arc. This is the first example of regional multidirectional arc migration in an intra-oceanic setting, and it has implications for along-arc structural and geochemical variations. The back and forth arc migrations were probably due to the constraints imposed by the neighboring American plates on this isolated subduction system, rather than variations in subducting slab buoyancy.〈/span〉

Beschreibung: 〈span〉Reducing the uncertainty in predictions of future climate change is one of today’s greatest scientific challenges, with many significant problems unsolved, including the relationship between 〈span〉p〈/span〉CO〈sub〉2〈/sub〉 and global temperature. To better constrain these forecasts, it is meaningful to study past time intervals of global warmth, such as the Eocene (56.0–33.9 Ma), serving as climatic analogues for the future. Here we reconstructed 〈span〉p〈/span〉CO〈sub〉2〈/sub〉 using the stomatal densities of a large fossil Lauraceae (laurel) leaf database from ten sites across the Eocene of Australia and New Zealand. We show that mostly moderate 〈span〉p〈/span〉CO〈sub〉2〈/sub〉 levels of ~450–600 ppm prevailed throughout the Eocene, levels that are considerably lower than the 〈span〉p〈/span〉CO forcing currently needed to recreate Eocene temperatures in climate models. Our data record significantly lower 〈span〉p〈/span〉CO〈sub〉2〈/sub〉 than inferred from marine isotopes, but concur with previously published Northern Hemisphere Eocene stomatal proxy 〈span〉p〈/span〉CO〈sub〉2〈/sub〉. We argue that the now globally consistent stomatal proxy 〈span〉p〈/span〉CO〈sub〉2〈/sub〉 record for the Eocene is robust and that climate sensitivity was elevated and/or that additional climate forcings operated more powerfully than previously assumed.〈/span〉

Beschreibung: 〈span〉The nature of metamorphism, magma compositions, the spatial distribution of plutons, and foreland sediments reflect, in part, the character and thickness of continental crust. We utilized metamorphic pressure-temperature-time (〈span〉P-T-t〈/span〉) paths, garnet Sm-Nd ages, zircon U-Pb ages, and pluton compositions to estimate paleocrustal thickness and temporal changes in crustal magma sources in the Blue Ridge of the southernmost Appalachians. Garnet Sm-Nd ages for amphibolite-facies metamorphic rocks range from 331 ± 4 to 320 ± 3 Ma. Low and high-Sr/Y plutons that intruded these metamorphic rocks have zircon U-Pb ages of 390 ± 1 to 365 ± 1 Ma and 349 ± 2 to 335 ± 1 Ma, respectively. Therefore, garnet growth began during regional metamorphism synchronous with or shortly after intrusion of the youngest high-Sr/Y trondhjemite plutons. Phase diagram sections and thermobarometry indicate that garnet growth initiated at ∼5.8 kbar and 540 °C and grew during temperature increases of 60–100 °C and pressure increases of 2–3 kbar. The older, low-Sr/Y magmas are inferred to have been sourced in the crust at depths 〈∼30 km, insufficient for garnet to be stable. However, the younger, high-Sr/Y magmas are inferred to have been sourced at 〉30 km depths where garnet was stable. Hafnium isotopic compositions for all the plutons, but one, exhibit a range from negative initial ε〈sub〉Hf(i)〈/sub〉 to weakly positive initial ε〈sub〉Hf(i)〈/sub〉, indicating incomplete mixing of dominantly crustal sources. Our data require minimum crustal thicknesses of ∼33 km at 331 Ma; however, Alleghanian crustal thicknesses must have locally reached 39 km, based on crustal reconstruction adding the Alleghanian thrust sheet beneath the eastern Blue Ridge. We infer the presence of hot, tectonically thickened crust during intrusion of the early Alleghanian high-Sr/Y plutons and conclude that garnet growth and plutonism reflect a progressive increase in crustal thickness and depth of magma generation. The crustal thickening was synchronous with deposition of Mississippian to early Pennsylvanian sediments in the foreland basin of the Appalachian orogen between 350 and 320 Ma. This crustal thickening may have preceded emplacement of the Alleghanian thrust sheets onto the North American craton.〈/span〉

Beschreibung: 〈span〉In the southern Gulf of California, the generation of new oceanic crust has resulted in linear magnetic anomalies and seafloor bathymetry that are characteristic of active seafloor-spreading systems. In the northern Gulf of California and the onshore (southeastern California, USA) Salton Trough region, a thick sedimentary package overlies the crystalline crust, masking its nature, and linear magnetic anomalies are absent. We use potential-field data and a geotherm analysis to constrain the composition of the crust (oceanic or continental) and develop a conceptual model for rifting. Gravity anomalies in the northern Gulf of California and Salton Trough are best fit with crustal densities that correspond to continental crust, and the fit is not as good if densities representative of mafic rocks, i.e., oceanic crust or mafic underplating, are assumed. Because extensive mafic underplated bodies would produce gravity anomalies that are not in agreement with observed gravity data, we propose, following earlier work, that the anomalies might be due to serpentinized peridotite bodies such as found at magma-poor rifted margins. The density and seismic velocities of such serpentinized peridotite bodies are in agreement with observed gravity and seismic velocities. Our conceptual model for the Salton Trough and northern Gulf of California shows that net crustal thinning here is limited because new crust is formed rapidly from sediment deposition. As a result, continental breakup may be delayed.〈/span〉

Beschreibung: 〈span〉The southwestern margin of the Late Cretaceous Sonora Pass Intrusive Suite, northern Sierra Nevada, California (USA), preserves a densely populated zone of magmatic structures that record dynamic magmatic layer forma­tion and deformation (faulting and folding) within a solidifying upper-crustal magma mush. This zone consists largely of coupled melanocratic (or schlieren) and leucocratic bands hosted within the 95.6 ± 1.5 Ma Kinney Lakes granodio­rite (Leopold, 2016), with orientations approximately parallel to the intrusive margin and with inward younging directions. Schlieren consist of a high modal abundance of medium-grained ferromagnesian minerals (hornblende + biotite), zircon, sphene, apatite, opaque minerals, and minor plagioclase and interstitial quartz. Leucocratic bands are dominated by coarse-grained feldspar + quartz with minor ferromagnesian and accessory minerals. Whole-rock geochemical and Sr and Nd isotopic data indicate that the schlieren are derived from the Kinney Lakes granodiorite by effective mechanical separation of mafic minerals and accessory phases. We interpret that the schlieren zone at the margin of the Kinney Lakes granodiorite formed by large-scale collapse of crystal mush by “magma avalanching,” facilitated by gravity, local convection, and possibly by host-rock stoping at the margin. This process eroded a significant portion of the solidifying margin of the chamber and resulted in the formation of magmatically deformed layered structures, which experienced further mingling, re-intrusion, magmatic erosion, and recycling processes. We envisage that magma avalanching of magma mushes in plutons can be achieved by any unstable process (e.g., tectonic, fluid-assisted, stoping, or gravity-driven) in large, long-lived magma-mush chambers.〈/span〉

Beschreibung: 〈span〉Understanding the tectonic and landscape evolution of the Colorado Plateau−southern Rocky Mountains area requires knowledge of the Laramide stratigraphic development of the San Juan Basin. Laramide sediment-transport vectors within the San Juan Basin are relatively well understood, except for those of the Nacimiento and Animas formations. Throughout most of the San Juan Basin of northwestern New Mexico and adjacent Colorado, these Paleocene units are mudstone-dominated fluvial successions intercalated between the lowermost Paleocene Kimbeto Member of the Ojo Alamo Sandstone and the basal strata of the lower Eocene San Jose Formation, both sandstone-dominated fluvial deposits. For the Nacimiento and Animas formations, we present a new lithostratigraphy that provides a basis for basin-scale interpretation of the Paleocene fluvial architecture using facies analysis, paleocurrent measurements, and 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar sanidine age data.In contrast to the dominantly southerly or southeasterly paleoflow exhib­ited by the underlying Kimbeto Member and the overlying San Jose Formation, the Nacimiento and Animas formations exhibit evidence of diverse paleoflow. In the southern and western part of the basin during the Puercan, the lower part of the Nacimiento Formation was deposited by south- or southeast- flowing streams, similar to those of the underlying Kimbeto Member. This pattern of southeasterly paleoflow continued during the Torrejonian in the western part of the basin, within a southeast-prograding distributive fluvial system. By Torrejonian time, a major east-northeast–flowing fluvial system, herein termed the Tsosie paleoriver, had entered the southwestern part of the basin, and a switch to northerly paleoflow had occurred in the southern San Juan Basin. The reversal of paleoslope in the southern part of the San Juan Basin probably resulted from rapid subsidence in the northeast part of the basin during the early Paleocene. Continued Tiffanian-age southeastward progradation of the distributive fluvial system that headed in the western part of the basin pushed the Tsosie paleoriver beyond the present outcrop extent of the basin.In the eastern and northern parts of the San Juan Basin, paleoflow was gen­erally toward the south throughout deposition of the Nacimiento and the Animas formations. An important exception is a newly discovered paleodrainage that exited the northeastern part of the basin, ~15 km south of Dulce, New Mexico. There, an ~130-m-thick Paleocene sandstone (herein informally termed the Wirt member of the Animas Formation) records a major east-flowing paleoriver system that aggraded within a broad paleovalley carved deeply into the Upper Cretaceous Lewis Shale. 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar dating of detrital sanidine documents a maximum depositional age of 65.58 ± 0.10 Ma for the Wirt member. The detrital sanidine grains are indistinguishable in age and K/Ca values from sanidines of the Horseshoe ash (65.49 ± 0.06 Ma), which is exposed 10.5 m above the base of the Nacimiento Formation in the southwestern part of the basin. The Wirt member may represent the deposits of the Tsosie paleoriver where it exited eastward from the basin.Our study shows that the evolution of Paleocene fluvial systems in the San Juan Basin was complex and primarily responded to variations in subsidence-related sedimentary accommodation within the basin.〈/span〉

Beschreibung: 〈span〉Within the upper Ediacaran Esmeralda Member of the Deep Spring Formation in southeastern California, USA, an ~3 m stratigraphic interval contains multiple clastic bedding surfaces with enigmatic, three-dimensionally preserved corrugated tubes (〈60 cm in length and 6 cm in width). When viewed as fragments and 〈span〉in situ〈/span〉 on bedding planes, these resemble larger versions of annulated, tubular soft-bodied macrofossils that are common in late Ediacaran biotic assemblages regionally and globally. Despite superficial similarities to casts and molds of body fossils preserved in correlative strata, we suggest these tubes are instead previously undescribed organosedimentary structures that developed through differential compaction of rippled heterolithic interbeds bound by pyritized microbial mat layers. These distinctive structures formed within peritidal settings in the latest Ediacaran Period as the result of specific ecological and environmental conditions marked by flourishing microbial mat communities and dysoxic sediments. This interpretation may inform the biogenicity of other structures previously reported as macroscopic body or trace fossils.〈/span〉

Beschreibung: 〈span〉The Turkana Depression of northern Kenya and southern Ethiopia contains voluminous plume-related basalts that mark the onset of the Paleogene–recent East African Rift System (EARS) at ca. 45 Ma. Thus, the Turkana Depression is crucial to understanding the inception of intracontinental rifting. However, the precise chronology of early rift-basin formation in Turkana is poorly constrained. We present apatite fission-track and (U-Th-Sm)/He thermochronology data from basement rocks from the margins of the north-south–trending Lokichar Basin that constrain the onset of rift-related cooling. Thermal history modeling of these data documents pronounced Eocene to Miocene denudational cooling of the basin-bounding Lokichar fault footwall. These results, along with ~7 km of Paleogene to middle Miocene syn-rift strata preserved in the Lokichar fault hanging wall, suggest that formation of the Lokichar Basin began as early as ca. 45–40 Ma. Preexisting lithospheric heterogeneities inherited from earlier Mesozoic rifting and Eocene plume magmatism likely facilitated the broadly concurrent nucleation of strain in the Turkana Depression, up to ~15 m.y. earlier than EARS initiation elsewhere. Late Paleogene extension in the Lokichar Basin and other parts of Turkana significantly predate the Miocene creation of pronounced plume-related topography in East Africa, suggesting that other mechanism(s), such as far-field stresses or mantle basal drag, likely played a critical role during EARS inception.〈/span〉

Beschreibung: 〈span〉The Upper Devonian Ignacio Formation (as stratigraphically revised) comprises a transgressive, tide-dominated estuarine depositional system in the San Juan Mountains (Colorado, USA). The unit backfills at least three bedrock paleovalleys (10–30 km wide and ≥42 m deep) with a consistent stratigraphy of tidally influenced fluvial, bayhead-delta, central estuarine-basin, mixed tidal-flat, and estuarine-mouth tidal sandbar deposits. Paleovalleys were oriented northwest while longshore transport was to the north. The deposits represent Upper Devonian lowstand and transgressive systems tracts. The overlying Upper Devonian Elbert Formation (upper member) consists of geographically extensive tidal-flat deposits and is interpreted as mixed siliciclastic-carbonate bay-fill facies that represents an early highstand systems tract. Stratigraphic revision of the Ignacio Formation includes reassigning the basal conglomerate to the East Lime Creek Conglomerate, recognizing an unconformity separating these two units, and incorporating strata previously mapped as the McCracken Sandstone Member (Elbert Formation) into the Ignacio Formation. The Ignacio Formation was previously interpreted as Cambrian, but evidence that it is Devonian includes reexamined fossil data and detrital zircon U-Pb geochronology. The Ignacio Formation has a stratigraphic trend of detrital zircon ages shifting from a single ca. 1.7 Ga age peak to bimodal ca. 1.4 Ga and ca. 1.7 Ga age peaks, which represents local source-area unroofing history. Specifically, the upper plate of a Proterozoic thrust system (ca. 1.7 Ga Twilight Gneiss) was eroded prior to exposure of the lower plate (ca. 1.4 Ga Uncompahgre Formation). These results are a significant alternative interpretation of the geologic history of the southern Rocky Mountains.〈/span〉

Beschreibung: 〈span〉The present-day topography in Iberia is related to geodynamic processes dealing with lithospheric-scale deformation. However, little attention has been paid to the role of inherited crustal- or lithospheric-scale structures involved in the recent observed large-scale topographic patterns. Whereas the analysis of brittle structures focuses on the evolution of Mesozoic sedimentary basins and their subsequent response to tectonic inversion, their contribution to mountain building has been underestimated. Large numbers of structures, from ductile to brittle, which affected the whole lithosphere, were developed during the evolution of the Cantabrian orocline (ca. 310–300 Ma). The contribution of these Paleozoic post-Variscan structures, together with lithospheric mantle evolution and replacement during orocline development in the Mesozoic and Cenozoic geological evolution of Iberia, remains unexplored. To explore the role of these inherited structures on the final configuration of topography during N-S Pyrenean shortening, we carried out a series of analogue experiments complemented by surface velocity field analyses. Our experiments indicate that strain was concentrated along preexisting crustal- to lithospheric-scale discontinuities, and they show several reactivation events marked by differences in the velocity vector field. Differences in fault displacement were also observed in the models depending upon preexisting fault trends. The obtained results may explain the different amount of displacement observed during the reactivation of some of the post-orocline structures in Iberia during the Cenozoic, indicating the key role of unveiled structures, which probably have accommodated most of the Alpine shortening.〈/span〉

Beschreibung: 〈span〉Landscapes developed over heterogeneous stratigraphy exhibit a spectrum of landforms from dramatic cliffbands to hogbacks, depending on the dip and spacing of the layers. In deeply incised landscapes, a single cliffband may consist of multiple resistant layers, whereas similar stratigraphy elsewhere is separated by strike valleys into individual cuesta benches or hogbacks. This paper presents a geometric analysis, informed by a numerical landscape model, to explain the conditions for development of a strike valley floored by erodible rocks. The results define a threshold incision rate below which strike valleys are more likely to form; this threshold incision rate is proportional to the stratigraphic spacing of cliff-forming layers and a trigonometric function of dip angle. The analysis also yields a time scale for the adjustment of structural landforms to changes in regional incision rate, which is a function of dip angle and the coupling between cliff retreat rate and escarpment height. In example landscapes of the Colorado Plateau, this time scale is likely much longer than that of documented variations of incision rates due to late Quaternary climate and land-use changes. The transitional state of escarpments in layered rock may therefore contain information about regional downcutting rates over time scales different from those recorded by the fluvial network. The utility of such features will require better understanding of the coupling between incision of a foot slope and the retreat rate of the cliff above in different kinds of rocks.〈/span〉

Beschreibung: 〈span〉Plate tectonics, the principal vehicle for dissipation of planetary energy, is believed to buffer the δ〈sup〉18〈/sup〉O of seawater at its near-modern value of 0‰ SMOW (Standard Mean Ocean Water) because the hot and cold cells of hydrothermal circulation at oceanic ridges cancel each other. The persistence of plate tectonics over eons apparently favors attribution of the well-documented oxygen isotope secular trends for carbonates (cherts, phosphates) to progressively warmer oceans, from 40–70 °C in the early Paleozoic to 60–100 °C in the Archean. We argue that these oceanic hydrothermal systems are dominated by low-temperature (〈350 °C) cells that deplete the percolating water in 〈sup〉18〈/sup〉O. Seawater δ〈sup〉18〈/sup〉O is therefore a proxy for, rather than being buffered by, the intensity of plate tectonics. Detrending the Phanerozoic carbonate δ〈sup〉18〈/sup〉O〈sub〉c〈/sub〉 secular trend for its “tectonic” component yields a stationary time series that, interpreted as a proxy for Phanerozoic climate, indicates low-latitude shallow ocean temperatures oscillating between 10 and 30 °C around a baseline of 17 °C, attributes comparable to modern temperature values.〈/span〉

Beschreibung: 〈span〉〈div〉Abstract〈/div〉Micropaleontological faunal studies coupled with foraminiferal geochemical analyses from the Bass River Site (Ocean Drilling Program [ODP] Leg 174AX; New Jersey, USA) reveal rapid changes in relative sea level due to million-year-scale glaciations during the early to middle Eocene, a time previously thought to have been mainly ice free. We examine benthic foraminiferal assemblages, stable isotopes (δ〈sup〉18〈/sup〉O and δ〈sup〉13〈/sup〉C), Mg/Ca, planktonic foraminiferal abundances, and ostracod abundances in eight lower to middle Eocene sequences at Bass River to reconstruct paleo–water depth and paleoceanographic changes within a sequence stratigraphic framework on the New Jersey paleo–continental shelf. Distinct benthic foraminiferal biofacies are identified and interpreted for paleodepth and environmental changes. Certain dominant species (e.g., 〈span〉Uvigerina〈/span〉 spp., 〈span〉Cibicidoides eocaenus〈/span〉, 〈span〉Spiroplectammina alabamensis〈/span〉, 〈span〉Siphonina claibornensis〈/span〉, and 〈span〉Cibicidoides pippeni〈/span〉) indicate changes in water depth and/or environmental conditions. We estimate middle to outer neritic (50–100+ m) paleodepths for much of the early to middle Eocene, with maximum water depths (∼150 ± 25 m) occurring in the early Eocene. We integrate these results with ostracod abundances and diversity, planktonic foraminiferal abundances, lithofacies, downhole logs, and core erosional surfaces to create a sequence framework for the early Eocene to early late Eocene of the New Jersey coastal plain. We compare the relationships among these sequences to foraminiferal biofacies of coreholes of the New Jersey Coastal Plain Drilling Project (Island Beach, Atlantic City, and ACGS#4), showing coeval hiatuses associated with regional base-level lowerings. Benthic and planktonic foraminifera δ〈sup〉18〈/sup〉O coupled with low-resolution Mg/Ca measurements provide a first-order correlation of sequence boundaries and δ〈sup〉18〈/sup〉O〈sub〉seawater〈/sub〉 variations, indicating glacioeustatic changes associated with the growth and decay of small ice sheets on the order of 20–30 m sea-level equivalent during the Eocene.〈/span〉

Beschreibung: 〈span〉Trace element (TE) ratios of convergent-margin magmas have been found to vary systematically with arc crustal thicknesses. Here we use statistical smoothing techniques along with Sr/Y and La/Yb trace element Moho depth proxies to determine crustal thickness along the volcanic front for three arc segments: the Central Volcanic Zone of the Andes arc, the Central America arc at Nicaragua and Costa Rica, and segments of the Alaska–Aleutian Islands arc (northwesternmost USA). The results are comparable to those from seismic surveys. TE depth proxies give ~70 km crust thickness beneath the Central Volcanic Zone’s Altiplano region and show thinner crust (60 km for La/Yb, 43 km for Sr/Y) as the volcanic line crosses into the Puna region. In Central America, the proxy analyses show crustal thickness changes between the Chorotega block and the Nicaragua depression, with both proxies agreeing for Nicaragua (~27 km) but with La/Yb giving considerable thicker (~45 km) crust than Sr/Y (~30 km) for Chorotega. For these two arc segments, the La/Yb proxy approximated the seismically inferred Moho depth to within 10 km for the entire profile, but the Sr/Y proxy–estimated crustal thicknesses diverge from those of the La/Yb proxy and seismic methods in the thin-crust regions. For the Alaska-Aleutian arc, both TE proxies indicate that crust varies from thick (~35 km) for the western Aleutian segment (175°E to 175°W), to thin (~22 km) for the transitional segment (175°W to 158°W), to thick (35+ km) for the eastern Alaska Peninsula (158°W to 150°W). Geophysical estimates favor a crustal thickness of 30–40 km for the same region. We propose that statistically treated geochemistry-based proxies can provide useful estimates of crustal thickness when estimates from Sr/Y and La/Yb agree. We investigated the disagreement in the Alaska-Aleutian case in more detail. Alaska-Aleutian crustal thickness was found to correlate with calc-alkaline (CA) versus tholeiitic (TH) segments of the arc, as represented by along-arc smoothing of the volcanoes’ CA-TH indices. The thin crust of the transitional segment trends TH while the thicker crust of the flanking segments trend CA. We find that crustal thickness also plays a role in inferred magma flux (here approximated by volcano volume), with greater flux associated with thinner crust. Thin crust beneath the Alaska-Aleutian transitional segment may reflect continuing loss of cumulates from the lower crust and/or lithospheric mantle into the asthenosphere, leading to enhanced melting beneath this region.〈/span〉

Beschreibung: 〈span〉The upper Homerian Mulde Event was an extinction event that devastated graptolite diversity and occurred before and during the onset of a major perturbation to the global carbon cycle, which is recorded as a double-peaked positive carbon isotope excursion (CIE). Whereas the Mulde Extinction and associated CIE are well documented globally, changes in global sea level associated with the Mulde Event have only been investigated in detail in the West Midlands, England, and Gotland, Sweden. A critical step toward understanding both the drivers and results of global climatic change during the Mulde Event is to constrain changes in eustasy. This study integrates carbon isotope chemostratigraphy and conodont biostratigraphy of Homerian strata in Tennessee, Indiana, and Ohio in an effort to determine if a global sequence boundary is recorded within the ascending limb of the Mulde CIE and to produce a high-resolution chronostratigraphic framework for Homerian strata in the midcontinent United States. Six sections, two from each state, were measured and described. Five were sampled for carbon isotope chemostratigraphy, and one was sampled for conodont biostratigraphy. All sections from Tennessee and Indiana evidently contain the Mulde CIE, whereas the sections from Ohio are less clear due to the truncation of upper Homerian strata. These data demonstrate that a sequence boundary identified herein in Indiana and Tennessee is the same sequence boundary that formed during the ascending limb of the Mulde Excursion in the West Midlands and Gotland.〈/span〉

Beschreibung: 〈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〉

Beschreibung: 〈span〉ORIGINAL ARTICLE: 2019, v. 15, no. 2, p. 〈strong〉〈a href="https://pubs.geoscienceworld.org/article.aspx?volume=&page="〉425–453〈span〉〈/span〉〈/a〉〈/strong〉, 〈a href="https://doi.org/10.1130/GES01698.1"〉https://doi.org/10.1130/GES01698.1〈/a〉. Final version published 5 March 2019.〈/span〉

Beschreibung: 〈span〉〈div〉Abstract〈/div〉Revised stratigraphy and new U-Pb geochronological data are reported for the Nacientes del Teno Formation and lowermost strata of the Río Damas Formation in Maule, Chile (∼35.15°S), a succession of sedimentary and volcanic rocks deposited along the western margin of the Neuquén backarc basin during the Middle to Late Jurassic (174–145 Ma). These results provide insights into the location, composition, and timing of coeval arc volcanic systems, and thus represent an important contribution to our understanding of the tectonostratigraphic evolution of the Andean orogen.New U-Pb geochronological data from detrital and primary volcanic zircon grains provide the first absolute ages (ca. 172–161 Ma) reported for the Nacientes del Teno Formation. Detrital zircon ages include a tightly clustered population ca. 165 Ma that coincides with a Middle Jurassic trough that exists in frequency histogram and relative probability plots of published U-Pb detrital zircon age data sets, challenging the notion of a lull in volcanic arc activity at this time.Along with the tight distribution of U-Pb detrital zircon ages, sedimentological evidence for proximal volcaniclastic deposition of Nacientes del Teno Formation strata in alternating marine-subaerial environments suggests the nearby presence of andesitic stratovolcanoes located along the western margin of the Neuquén Basin during the Middle Jurassic. These hypothesized volcanoes would have been located farther east than defined in traditional models for the location of the “main magmatic arc” in the Jurassic, suggesting that arc volcanism at this time occurred across a greater width of the continental margin than previously recognized.A maximum depositional age of ca. 151 Ma calculated from new U-Pb detrital zircon ages agrees with previously published maximum depositional ages for the Río Damas Formation. Sedimentological and geochronological evidence suggest that the Late Jurassic regression represented by the Río Damas Formation resulted from tectonic uplift.〈/span〉

Beschreibung: 〈span〉〈div〉Abstract〈/div〉Subsurface mapping and core analyses of upper crustal intrusions and mineralization at the Questa porphyry molybdenum deposit, New Mexico, reveal that Mo-mineralization occurred through episodic emplacement of at least six intrusive units. The structure of intrusions associated with the Questa deposit is documented in a series of detailed cross sections and visualized with a 3D animation. Mineralizing intrusions are underlain by two post-mineralization intrusions and cut by late-stage barren dikes. The plutonic complex was structurally focused along a system of preexisting flat-lying faults and their associated fractures. Mineralization is spatially associated with specific intrusive units in the subsurface, and the highest Mo ore grades within established ore blocks are structurally associated with the smallest intrusions. Existing U/Pb thermal ionization mass spectrometry (TIMS) zircon geochronology in conjunction with new relative chronology presented herein indicate that mineralization began before 24.91 Ma. We present three new chemical abrasion U/Pb TIMS zircon ages—one from an amphibole-bearing intrusion associated with high-grade mineralization (dark-matrix porphyry, 24.74 ± 0.37 Ma), a rhyolite dike that cuts ore-grade rocks (24.50 ± 0.02 Ma), and an equigranular granite discovered during deep drilling (23.67 ± 0.02 Ma). The dark-matrix porphyry contains clasts of an earlier amphibole-free intrusion that is spatially associated with low-grade mineralization. Thus, mineralizing intrusions were, in part, intruded into slightly older porphyries, confirming that episodic mineralization continued after 24.91 Ma. The age of the barren dike (24.50 ± 0.02 Ma) is indistinguishable from that of a previously dated granite porphyry that is associated with low-grade mineralization (〈0.05 wt% MoS〈sub〉2〈/sub〉; Questa granite porphyry). These data suggest that mineralization waned by 24.5 Ma and that ore deposition occurred over ∼500 ka. The new 23.67 Ma age of the deep equigranular granite, which underlies the Questa granite porphyry, further suggests that intrusions underlying the deposit were not related to mineralization. Detailed subsurface mapping and exploratory drilling indicate that intrusions associated with mineralization were small in volume and cooled rapidly, as evidenced by multiple internal contacts within sheets and rebrecciation textures. On the basis of observed cross-section reconstructions, petrology, alteration, and mineralization, the porphyritic rhyolite intrusions associated with mineralization in one of the largest orebodies in the deposit (the deep northeast) are less than 20-m-thick sheets that are separated by andesite wall rock. Thus, there is no evidence that this orebody formed above a cylindrical magma conduit that facilitated rapid convection, as is often modeled in these systems. We hypothesize that a set of similarly small-volume intrusions were responsible for the bulk of the ore in the southwest ore zone. Our interpretation that the mineralizing intrusions are small, thin, and subhorizontal distinguishes the Questa deposit from other Climax-type molybdenum deposits.〈/span〉

Beschreibung: 〈span〉〈div〉Abstract〈/div〉We merge structural results from the ST. Elias Erosion/tectonics Project (STEEP), other studies, and seismicity data to build a comprehensive, three-dimensional model of the lithosphere of the subduction corner in southern Alaska. The model is defined by three surfaces: (1) a top of the subducting lithosphere surface, (2) Moho surfaces, and (3) a base of subducting lithosphere surface. We model the eastern edge of the subducting lithosphere using the southern tip of the Yakutat microplate as an anchor. Kinematic reconstructions using that anchor suggest the modern Fairweather fault is likely inherited from motion of the margin in the 6–10 Ma period. We constructed a 4D kinematic model of crustal deformation in the vicinity of Mount St. Elias. We call this model the middlebuster model because the geometry is similar to a two-sided plow with that name. The west side of the plow is the eastern limit of the Aleutian megathrust constructed from the union of constraints from STEEP seismic results and slip models of the 1979 St. Elias earthquake. The east side is inferred from geologic mapping and slip models of the 1899 Yakutat Bay earthquake sequence. The top of the plow is near the Seward Glacier, where previous studies showed near world-record exhumation rates. GPS velocity vectors show a large rotation across the syntaxis at Mount St. Elias. West of the syntaxis, faults inferred from inversion of the GPS data are above the megathrust inferred from seismic imaging. That and other evidence suggest the presence of a wedge of ductile crust that partially decouples the subducting mantle lithosphere from the upper crust in the area near the suture with the Yakutat microplate.〈/span〉

Beschreibung: 〈span〉〈div〉Abstract〈/div〉Permian–Triassic strata in the Transantarctic Mountains and West Antarctica carry a significant detrital component derived from a contemporaneous magmatic belt along the Gondwana margin. Hf- and O-isotope characteristics were determined for near-contemporaneous (as shown by U-Pb zircon geochronology) detrital igneous zircons in Upper Permian and Triassic sandstones. Zircons from six granitoids in the contemporaneous magmatic belt were also analyzed for Hf and O isotopes in order to gain insight into the potential detrital zircon sources. Although the ages of these granitoids only loosely correspond with the depositional ages of the sandstones, the initial ε〈sub〉Hf〈/sub〉 and δ〈sup〉18〈/sup〉O isotope compositions for these igneous zircon grains, in general, overlap those recorded for the detrital igneous zircon grains. Results demonstrate a range of ε〈sub〉Hf〈/sub〉 and δ〈sup〉18〈/sup〉O values. Features of particular interest are the very low δ〈sup〉18〈/sup〉O values in two of the granitoids, and similar low values also recorded in the detrital igneous zircons in two sandstones. The distribution of Permian–Triassic granitoids must be much greater than is apparent from the existing outcrops in the extensively ice-covered region. The Permian and one of the Triassic granitoids have Hf-isotope characteristics similar to the Cretaceous granites and Devonian–Carboniferous plutons of West Antarctica, whereas the other Triassic granite differs from both. Importantly, the zircon isotopic data from the Permian–Triassic rocks suggest that an Hf-defined Upper Mesoproterozoic lithosphere underlies much of the magmatic belt.〈/span〉

Beschreibung: 〈span〉〈div〉Abstract〈/div〉To investigate the effect of crustal heterogeneities inherited from previous tectonic phases on magma-poor rifting processes, we performed numerical experiments of lithospheric extension with initial conditions that included strength variations from inherited crustal fabrics. Crustal fabrics were introduced in the model by using an element-wise bimineralic composition in which mineral phases were distributed in a way that was compatible with the orientation and distribution of kilometric-scale heterogeneities observed in seismic reflection data. Our numerical models show that strength variations from inherited crustal fabrics strongly influence the mechanisms of deformation in the stretching and thinning phases of rifting. The strength variations also generate alternative models for the evolution of faulting during distributed stretching and localized thinning phases that are usually associated with detachment or sequential faulting models. During the stretching phase, inherited strength variations control the distribution and the processes of deformation. Vertical fabrics favor the formation of horst-and-graben structures. Horizontal and dipping fabrics favor the formation of detachment faults and core complexes. During the thinning phase, processes differ depending on the orientation of the crustal fabrics and involve either a combination of detachment faults and sequential normal faults or an alternative model in which deformation remains decoupled between the upper crust and lithospheric mantle, with the formation of high-angle faults in the upper crust and a low-angle detachment fault in the upper mantle. As a consequence, strength variations inherited from crustal fabrics also control the resulting geometry of the margin and the width of the necking and hyperextended domains. Finally, our models demonstrate that inherited crustal fabrics do not control breakup and mantle exhumation. These processes are ubiquitously associated with the development of new detachment faults exhuming mantle to the seafloor.〈/span〉

Beschreibung: 〈span〉〈div〉Abstract〈/div〉We present results from integrated field, microstructural, and textural analysis of the Burlington mylonite zone (BMZ) in eastern Massachusetts (northeastern USA) to establish a unified microkinematic framework for vorticity analysis in heterogeneous shear zones. Specifically, we develop a methodology for the structural analysis of polyphase lithologies that defines the vorticity-normal surface based on lattice-scale rotation axes calculated from electron backscatter diffraction data using orientation statistics. In doing so, we objectively identify a suitable reference frame for rigid grain methods of vorticity analysis that can be used in concert with field and microstructural methods of strain analysis and textural studies to constrain field- to plate-scale kinematics and deformation geometries without assumptions that may bias tectonic interpretations, such as relationships between kinematic axes and fabric-forming elements or the nature of the deforming zone (e.g., monoclinic versus triclinic shear zones).Rocks within the BMZ comprise a heterogeneous mix of quartzofeldspathic ± hornblende-bearing mylonitic gneisses and quartzites. Vorticity axes inferred from lattice rotations lie within the plane of mylonitic foliation perpendicular to lineation—a pattern consistent with monoclinic deformation geometries involving simple shear and/or wrench-dominated transpression. The mean kinematic vorticity number (〈span〉W〈/span〉〈sub〉m〈/sub〉) is calculated using rigid grain net analysis and ranges from 0.25 to 0.55, indicating dominant general shear. Using the calculated vorticity values and the dominant geographic fabric orientation, we constrain the angle of paleotectonic convergence between the Nashoba and Avalon terranes to ∼56°–75° with the convergence vector trending ∼142°–160° and plunging ∼3°–10°. Application of the quartz recrystallized grain size piezometer suggests differential stresses in the BMZ mylonites ranging from ∼44 to 92 MPa; patterns of quartz crystallographic preferred orientation are consistent with deformation at greenschist- to amphibolite-facies conditions. We conclude that crustal strain localization in the BMZ involved a combination of pure and simple shear in a sinistral reverse transpressional shear zone that was active at or near the brittle-ductile transition under relatively high stress conditions. Moreover, we demonstrate the utility of combined crystallographic and rigid grain methods of vorticity analysis for deducing deformation geometries, kinematics, and tectonic histories in polyphase shear zones.〈/span〉

Beschreibung: 〈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〉

Beschreibung: 〈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〉

Beschreibung: 〈span〉Permian volcanic deposits are widespread throughout southwestern Gondwana and record voluminous silicic continental arc volcanism (e.g., Choiyoi Province) that may have contributed to Permian global warming and environmental degradation. Many Permian volcanic deposits of southwestern Gondwana (southern South America, southern Africa, West Antarctica and eastern Australia), however, remain to be accurately correlated to magmatic source regions along the active paleo-Pacific margin of Gondwana, and this lack of correlation limits our understanding of the timing and distribution of voluminous volcanism. Here we present detrital zircon U-Pb and Hf isotope data for Permian volcaniclastic sedimentary rocks from the Ellsworth Mountains, Pensacola Mountains, and the Ohio Range of central Antarctica in southwestern Gondwana used to determine their volcanic source along the paleo-Pacific margin of Gondwana. Rocks in central Antarctica record Permian (ca. 268 Ma) volcanism with a mean zircon εHf〈sub〉〈span〉i〈/span〉〈/sub〉 of -0.04 ± 4.8 (2 standard deviation). Comparison of these zircon age and Hf data with compilations for adjacent regions along the Gondwana margin suggest derivation of the Antarctic zircons from a major episode of Permian explosive arc volcanism that is broadly synchronous with, and geochemically similar to, the voluminous Choiyoi Province in South America. This correlation also relates the source of synchronous volcaniclastic deposits in the Karoo Basin, South Africa, to the same major Permian volcanic episode associated with the Choiyoi Province. In aggregate, geochemical data from Permian zircon in central Antarctica support an along-arc variation in geochemistry, with isotopically enriched high-flux magmatism associated with thicker crust and lithospheric mantle in South America, and isotopically depleted magmatism and thinner crust and lithospheric mantle in Australia. The timing of inferred Choiyoi-related explosive arc volcanism recorded in the Antarctic sector, South African sector, and South American sector is contemporaneous with a documented increase in global arc flux, an increase in atmospheric CO〈sub〉2〈/sub〉, a decrease in δ〈sup〉13〈/sup〉C of benthic marine fossils, and mass extinction events. We suggest that the Choiyoi Province and correlated arc volcanism along the Gondwana margin contributed to increased global arc flux in the Permian leading to elevated background levels of atmospheric CO〈sub〉2〈/sub〉 conducive to producing an environmental crisis during mafic large igneous province emplacement, and may serve as an example of continental arc outgassing exerting a first order control on climate.〈/span〉

Beschreibung: 〈span〉Over four decades of research on the Semail ophiolite (Oman–U.A.E.) has greatly influenced our understanding of processes occurring at fast-spreading ocean ridges. While the well-developed sheeted dike complex and comagmatic lower pillow lavas indicate that the early Semail crust formed at a spreading axis, the precise tectonic setting of this axis—whether true mid-ocean ridge, back-arc or “proto”‑arc— is contentious. This is largely because the tectonic implications of the geochemistry of the main axial volcanic unit (Geotimes/V1) are disputed. We bypass this hurdle by focusing on intercalations of primitive lavas that are depleted relative to mid-ocean-ridge basalt and that are deeply intercalated within the early Geotimes axial volcanostratigraphy throughout the northern ophiolite. Our analyses of these intercalations show a clear trace-element influence from a subducting slab. We interpret the depleted axial melts to have formed by localized, high-degree partial melting assisted by a high-Th/Nb slab fluid. These results confirm a deep subduction influence on the entire axial spreading phase of the world’s largest ophiolite. Considered in the context of later hydrous and boninitic Alley volcanism and of insight from modern tectonic environments, our observations support a proto-arc, subduction-initiation setting for the origin of the Semail ophiolite.〈/span〉

Beschreibung: 〈span〉Abundant Early Paleozoic magmatism is preserved in northern Tibet and has important implications for continental crustal growth in response to continental collisions. To better constrain the evolution of the northern margin of the Tibetan Plateau and the resulting closure of the North Qilian Ocean, we conducted an integrated investigation involving U-Th-Pb zircon geochronology, whole-rock geochemistry, and syntheses of existing data sets across the North Qilian orogenic belt. Zircon U–Pb dating indicates that the Early Paleozoic intrusive rocks in the study area can be divided into two stages: 460–480 Ma and 440 Ma. The 478–480 Ma meta gabbro-diorite samples belong to the calc-alkaline series and show a fractional crystallization trend in the Harker diagrams. These samples have relatively high La/Nb (2.08–3.26) and low La/Ba (0.05–0.06) ratios, indicating a subduction-modified continental-lithospheric mantle source. The 460–480 Ma meta-granite samples are I-type and classified as high-K calc-alkaline series. Both the meta gabbro-diorite and meta-granite samples are characterized by negative Nb, Ta, and Ti anomalies and enriched LILEs, and showing arc signatures that may be related to the southward subduction of the North Qilian oceanic crust. The 441 Ma hornblende syenite samples have high contents of alkalis (K〈sub〉2〈/sub〉O + Na〈sub〉2〈/sub〉O = 7.80%–12.07%) and belong to the alkaline series, suggesting an extensional geological setting. The 440 Ma syenogranite samples are classified as high Ba-Sr granites and belong to the shoshonitic series. The syenogranite samples were most likely generated by partial melting of lithospheric mantle that had been metasomatized by slab-derived fluids, and mixed by the crust derived granitic magmas during the ascent of the magma. Both the hornblende syenite (441 Ma) and the syenogranite (440 Ma) samples were emplaced in a postcollisional tectonic setting associated with the closure process of the North Qilian Ocean.〈/span〉

Beschreibung: 〈span〉〈div〉Abstract〈/div〉Earth systems thinking (EST), or thinking of the Earth as a complex system made up of interworking subsystems, has been shown to reflect the highest level of knowing and understanding in the geosciences. Previous work has found four frameworks of EST that repeatedly appear in the geoscience education literature. This study aims to quantitatively build on this work by employing structural equation modeling to understand the current state of EST teaching as shown by the 2016 iteration of the National Geoscience Faculty Survey (United States; 〈span〉n〈/span〉 = 2615). Exploratory and confirmatory factor analyses were conducted on survey items to understand and develop three models, one for EST teaching practices, one for course changes, and one for active-learning teaching practices. Analyses revealed that reported EST teaching practices relate back to the four EST frameworks proposed in the literature. The three models explored in this study were used to build a full structural model, where it was hypothesized that active-learning teaching practices would predict EST course changes and EST teaching. However, the model revealed that EST course changes mediate, or bring about, the relationship between active-learning teaching practices and EST teaching. In other words, the relationship between active-learning and EST teaching practices is not direct. This implies the need for continued efforts to provide professional development opportunities in both active-learning teaching practices and EST, as active-learning practices are not sufficient to implicitly teach EST skills. Results also revealed that the teaching approaches that emphasize modeling and complexity sciences had the weakest relationship to the broader EST teaching practices, suggesting a need for more professional development opportunities as they relate to systems modeling, quantitative reasoning, and complexity sciences in the context of the Earth sciences.〈/span〉

Beschreibung: 〈span〉〈div〉Abstract〈/div〉The Mesozoic–Cenozoic convergent margin history of southern Alaska has been dominated by arc magmatism, terrane accretion, strike-slip fault systems, and possible spreading-ridge subduction. We apply 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar, apatite fission-track (AFT), and apatite (U-Th)/He (AHe) geochronology and thermochronology to plutonic and volcanic rocks in the southern Talkeetna Mountains of Alaska to document regional magmatism, rock cooling, and inferred exhumation patterns as proxies for the region’s deformation history and to better delineate the overall tectonic history of southern Alaska. High-temperature 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar thermochronology on muscovite, biotite, and K-feldspar from Jurassic granitoids indicates postemplacement (ca. 158–125 Ma) cooling and Paleocene (ca. 61 Ma) thermal resetting. 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar whole-rock volcanic ages and 45 AFT cooling ages in the southern Talkeetna Mountains are predominantly Paleocene–Eocene, suggesting that the mountain range has a component of paleotopography that formed during an earlier tectonic setting. Miocene AHe cooling ages within ∼10 km of the Castle Mountain fault suggest ∼2–3 km of vertical displacement and that the Castle Mountain fault also contributed to topographic development in the Talkeetna Mountains, likely in response to the flat-slab subduction of the Yakutat microplate. Paleocene–Eocene volcanic and exhumation-related cooling ages across southern Alaska north of the Border Ranges fault system are similar and show no S-N or W-E progressions, suggesting a broadly synchronous and widespread volcanic and exhumation event that conflicts with the proposed diachronous subduction of an active west-east–sweeping spreading ridge beneath south-central Alaska. To reconcile this, we propose a new model for the Cenozoic tectonic evolution of southern Alaska. We infer that subparallel to the trench slab breakoff initiated at ca. 60 Ma and led to exhumation, and rock cooling synchronously across south-central Alaska, played a primary role in the development of the southern Talkeetna Mountains, and was potentially followed by a period of southern Alaska transform margin tectonics.〈/span〉

Beschreibung: 〈span〉〈div〉Abstract〈/div〉Within extended orogens, records that reflect the driving processes and dynamics of early extension are often overprinted by subsequent orogenic collapse. The Copper Mountains of northeastern Nevada preserve an exceptional record of hinterland extensional deformation and high-elevation basin formation, but current geochronology and thermochronology are insufficient to relate this to broader structural trends in the region. This extension occurred concurrent with volcanism commonly attributed to Farallon slab removal. We combine thermochronology of both synextensional hanging-wall strata and footwall rocks to comprehensively evaluate the precise timing and style of this deformation. Specifically, we apply (U-Th)/(He-Pb) double dating of minerals extracted from Eocene–Oligocene Copper Basin strata with multi-mineral (U-Th)/He and 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar thermochronology of rocks sampled across an ∼20 km transect of the Copper Mountains. We integrate basement and detrital thermochronology records to comprehensively evaluate the timing and rates of hinterland extension and basin sedimentation. Cooling and U-Pb crystallization ages show the Coffeepot Stock, which spans the width of the Copper Mountains, was emplaced at ca. 109–108 Ma, and then cooled through the 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar muscovite and biotite closure temperatures by ca. 90 Ma, the zircon (U-Th)/He closure temperature between ca. 90 and 70 Ma, and the apatite (U-Th)/He closure temperature between 43 and 40 Ma. Detrital apatite and zircon (U-Th)/(He-Pb) double dating of late Eocene fluvial and lacustrine strata of the Dead Horse Formation and early Oligocene fluvial strata of the Meadow Fork Formation, both deposited in Copper Basin, shows that Early Cretaceous age detrital grains have a cooling history that is analogous to proximal intrusive rocks of the Coffeepot Stock. At ca. 38 Ma, cooling and depositional ages for Copper Basin strata reveal rapid exhumation of proximal source terranes (cooling rate of ∼37 °C/m.y.); in these terranes, 8–12 km of slip along the low-angle Copper Creek normal fault exhumed the Coffeepot Stock in the footwall. Late Eocene–early Oligocene slip along this fault and an upper fault splay, the Meadow Fork fault, created a half graben that accommodated ∼1.4 km of volcaniclastic strata, including ∼20 m of lacustrine strata that preserve the renowned Copper Basin flora. Single-crystal sanidine 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar geochronology of interbedded tuffs in Copper Basin constrains the onset of rapid exhumation to 38.0 ± 0.9 Ma, indicating that surface-breaching extensional deformation was coincident with intense proximal volcanism. Coarse-grained syndeformational sediments of the Oligocene Meadow Fork Formation were deposited just prior to formation of an extensive regional Oligocene–Miocene unconformity and represent one of the most complete hinterland stratigraphic records of this time. We interpret this history of rapid late Eocene exhumation across the Copper Mountains, coeval volcanism, and subsequent unconformity formation to reflect dynamic and thermal effects associated with Farallon slab removal. The final phase of extension is recorded by late, high-angle normal faults that cut and rotate the early middle Miocene Jarbidge Rhyolite sequence, deposited unconformably in the hanging wall. These results provide an independent record of episodic Paleogene to Miocene exhumation documented in Cordilleran metamorphic core complexes and establish that substantial extension occurred locally in the hinterland prior to province-wide Miocene extensional break-up.〈/span〉

Beschreibung: 〈span〉〈div〉Abstract〈/div〉Understanding the tectonic and landscape evolution of the Colorado Plateau−southern Rocky Mountains area requires knowledge of the Laramide stratigraphic development of the San Juan Basin. Laramide sediment-transport vectors within the San Juan Basin are relatively well understood, except for those of the Nacimiento and Animas formations. Throughout most of the San Juan Basin of northwestern New Mexico and adjacent Colorado, these Paleocene units are mudstone-dominated fluvial successions intercalated between the lowermost Paleocene Kimbeto Member of the Ojo Alamo Sandstone and the basal strata of the lower Eocene San Jose Formation, both sandstone-dominated fluvial deposits. For the Nacimiento and Animas formations, we present a new lithostratigraphy that provides a basis for basin-scale interpretation of the Paleocene fluvial architecture using facies analysis, paleocurrent measurements, and 〈sup〉40〈/sup〉Ar/ 〈sup〉39〈/sup〉Ar sanidine age data.In contrast to the dominantly southerly or southeasterly paleoflow exhibited by the underlying Kimbeto Member and the overlying San Jose Formation, the Nacimiento and Animas formations exhibit evidence of diverse paleoflow. In the southern and western part of the basin during the Puercan, the lower part of the Nacimiento Formation was deposited by south- or southeast-flowing streams, similar to those of the underlying Kimbeto Member. This pattern of southeasterly paleoflow continued during the Torrejonian in the western part of the basin, within a southeast-prograding distributive fluvial system. By Torrejonian time, a major east-northeast–flowing fluvial system, herein termed the Tsosie paleoriver, had entered the southwestern part of the basin, and a switch to northerly paleoflow had occurred in the southern San Juan Basin. The reversal of paleoslope in the southern part of the San Juan Basin probably resulted from rapid subsidence in the northeast part of the basin during the early Paleocene. Continued Tiffanian-age southeastward progradation of the distributive fluvial system that headed in the western part of the basin pushed the Tsosie paleoriver beyond the present outcrop extent of the basin.In the eastern and northern parts of the San Juan Basin, paleoflow was generally toward the south throughout deposition of the Nacimiento and the Animas formations. An important exception is a newly discovered paleodrainage that exited the northeastern part of the basin, ∼15 km south of Dulce, New Mexico. There, an ∼130-m-thick Paleocene sandstone (herein informally termed the Wirt member of the Animas Formation) records a major east-flowing paleoriver system that aggraded within a broad paleovalley carved deeply into the Upper Cretaceous Lewis Shale. 〈sup〉40〈/sup〉Ar/ 〈sup〉39〈/sup〉Ar dating of detrital sanidine documents a maximum depositional age of 65.58 ± 0.10 Ma for the Wirt member. The detrital sanidine grains are indistinguishable in age and K/Ca values from sanidines of the Horseshoe ash (65.49 ± 0.06 Ma), which is exposed 10.5 m above the base of the Nacimiento Formation in the southwestern part of the basin. The Wirt member may represent the deposits of the Tsosie paleoriver where it exited eastward from the basin.Our study shows that the evolution of Paleocene fluvial systems in the San Juan Basin was complex and primarily responded to variations in subsidence-related sedimentary accommodation within the basin.〈/span〉

Beschreibung: 〈span〉Sapropels are organic-rich sediment layers deposited in the eastern Mediterranean Sea during precession minima, resulting from an increase in export productivity and/or preservation. Increased freshwater delivery from the African continent resulted in stratification, causing deepwater anoxia, while nutrient input stimulated productivity, presumably at the deep chlorophyll maximum. Previous studies have suggested that during sapropel deposition, nitrogen fixation was widespread in the highly stratified surface waters, and that cyanobacteria symbiotic with diatoms (diatom-diazotroph associations, DDAs) were responsible. Here we analyzed sapropel S5 sediments for heterocyst glycolipids (HGs) from three locations in the eastern Mediterranean. HG biomarkers can differentiate between those heterocystous cyanobacteria that are free living (found predominately in freshwater or brackish environments) and those that are from DDAs (found in marine settings). In our primary core, from a location which would have been influenced by the Nile River outflow, we detected a HG with a pentose (C〈sub〉5〈/sub〉) head group specific for DDAs. However, HGs with a hexose (C〈sub〉6〈/sub〉) head group, specific to free-living cyanobacteria, were present in substantially (up to 60×) higher concentration. These data suggest that at our study location, free-living cyanobacteria were the dominant diazotrophs, rather than DDAs. The C〈sub〉6〈/sub〉 HGs increased substantially at the onset of sapropel S5 deposition, suggesting that substantial seasonal cyanobacterial blooms were associated with a brackish surface layer flowing from the Nile into the eastern Mediterranean. Two additional S5 sapropels were analyzed, one also from the Nile delta region and one from the region between Libya and southwestern Crete. Overall, comparison of the HG distribution in the three S5 sapropels provides evidence that all three locations were initially influenced by surface salinities that were sufficiently low to support free-living heterocystous cyanobacteria. While free-living heterocystous cyanobacteria continued to outnumber DDAs during sapropel deposition at the two Nile-influenced sites, DDAs, indicators of persistent marine salinities, were the dominant diazotrophs in the upper part of the sapropel at the more westerly site. These results indicate that N〈sub〉2〈/sub〉 fixation by free-living cyanobacteria offers an important additional mechanism to stimulate productivity in regions with strong river discharge during sapropel deposition.〈/span〉

Beschreibung: 〈span〉The remarkable preservation of carbonaceous microfossils in Precambrian chert has been attributed to rapid silica precipitation enclosing original cellular carbon. However, while silicification of bacteria can preserve cellular morphology, in modern silicifying environments, organic molecules are rapidly destroyed after death, raising questions about the origin of carbon in ancient microfossils. Here, we show that carbonaceous matter in filamentous microfossils in chert from the Carboniferous Red Dog Zn-Pb deposit, northern Alaska, represents relicts of migrated oil. Black, carbonaceous microfossils are associated with chert stained brown by solidified oil surrounding hairline fractures. The black filaments contain thin carbon films (〈50 nm) lining minute quartz crystals in intracellular cavities. Our observations indicate that silica nucleation on bacterial cell walls was followed by the decomposition of cellular carbon, producing silica-encased, bacterial molds that were later infiltrated by oil. This process represents a new mode of fossilization that raises questions about the origin of carbonaceous matter in some of Earth’s oldest microfossils.〈/span〉

Beschreibung: 〈span〉Knowledge of the age and compositional architecture of Archean cratonic lithosphere is critical for models of geodynamics and continental growth on early Earth, but can be difficult to unravel from the exposed geology. We report the occurrence of numerous 〉3.7 Ga zircon crystals in 3.45 Ga rhyolites of the eastern Pilbara Craton (Western Australia), which preserve evidence for an Eoarchean meta-igneous component in the deep Pilbara crust. This inherited zircon population shares similar and distinctive age and Hf-O isotope characteristics with the oldest gneissic components of the Yilgarn Craton ~500 km farther south, suggesting a common ca. 3.75 Ga felsic crustal nucleus to these two Archean granite-greenstone terranes. We infer a pivotal role for such ‘seeds’ in facilitating the growth and persistence of Archean continental lithosphere.〈/span〉

Beschreibung: 〈span〉Despite extensive outcrop and previous sedimentologic study, the role of tidal processes along sandy, wave- and river-dominated shorelines of the North American Cretaceous Western Interior Seaway remains uncertain, particularly for the extensive mid-Campanian (ca. 75–77.5 Ma) tidal deposits of Utah and Colorado, USA. Herein, paleotidal modeling, paleogeographic reconstructions, and interpretations of depositional process regimes are combined to evaluate the regional-scale (hundreds to thousands of kilometers) basin physiographic controls on tidal range and currents along these regressive shorelines in the “Utah Bight”, southwestern Western Interior Seaway. Paleotidal modeling using a global and astronomically forced tidal model, combined with paleobathymetric sensitivity tests, indicates the location of stratigraphic units preserving pronounced tidal influence only when the seaway had a deep center (~400 m) and southern entrance (〉100 m). Maximum tidal velocity vectors under these conditions suggest a dominant southeasterly ebb tide within the Utah Bight, consistent with the location and orientation of paleocurrent measurements in regressive, tide-influenced deltaic units. The modeled deep paleobathymetry increased tidal inflow into the basin and enhanced local-scale (tens to hundreds of kilometers) resonance effects in the Utah Bight, where an amphidromic cell was located. However, the preservation of bidirectional, mudstone-draped cross-stratification in fine- to medium-grained sandstones requires tides in combination with fluvial currents and/or local tidal amplification below the maximum resolution of model meshes (~10 km). These findings suggest that while regional-scale controls govern tidal potential within basins, localized physiography exerts an important control on the preservation of tidal signatures in the geologic record.〈/span〉

Beschreibung: 〈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〉

Beschreibung: 〈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〉

Beschreibung: 〈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〉

Beschreibung: 〈span〉Active fold-and-thrust belts can potentially accommodate large-magnitude earthquakes, so understanding the structure in such regions has both societal and scientific importance. Recent studies have provided evidence for large earthquakes in the Western Transverse Ranges of California, USA. However, the diverse set of conflicting structural models for this region highlights the lack of understanding of the subsurface geometry of faults. A more robust structural model is required to assess the seismic hazard of the Western Transverse Ranges. Toward this goal, we developed a forward structural model using Trishear in MOVE® to match the first-order structure of the Western Transverse Ranges, as inferred from surface geology, subsurface well control, and seismic stratigraphy. We incorporated the full range of geologic observations, including vertical motions from uplifted fluvial and marine terraces, as constraints on our kinematic forward modeling. Using fault-related folding methods, we predicted the geometry and sense of slip of the major faults at depth, and we used these structures to model the evolution of the Western Transverse Ranges since the late Pliocene. The model predictions are in good agreement with the observed geology. Our results suggest that the Western Transverse Ranges comprises a southward-verging imbricate thrust system, with the dominant faults dipping as a ramp to the north and steepening as they shoal from ∼16°–30° at depth to ∼45°–60° near the surface. We estimate ∼21 km of total shortening since the Pliocene in the eastern part of the region, and a decrease of total shortening west of Santa Barbara down to 7 km near Point Conception. The potential surface area of the inferred deep thrust ramp is up to 6000 km〈sup〉2〈/sup〉, which is of sufficient size to host the large earthquakes inferred from paleoseismic studies in this region.〈/span〉

Beschreibung: 〈span〉Lake sediments are increasingly explored as reliable paleoflood archives. In addition to established flood proxies including detrital layer thickness, chemical composition, and grain size, we explore stable oxygen and carbon isotope data as paleoflood proxies for lakes in catchments with carbonate bedrock geology. In a case study from Lake Mondsee (Austria), we integrate high-resolution sediment trapping at a proximal and a distal location and stable isotope analyses of varved lake sediments to investigate flood-triggered detrital sediment flux. First, we demonstrate a relation between runoff, detrital sediment flux, and isotope values in the sediment trap record covering the period 2011–2013 CE including 22 events with daily (hourly) peak runoff ranging from 10 (24) m〈sup〉3〈/sup〉 s〈sup〉–1〈/sup〉 to 79 (110) m〈sup〉3〈/sup〉 s〈sup〉–1〈/sup〉. The three- to ten-fold lower flood-triggered detrital sediment deposition in the distal trap is well reflected by attenuated peaks in the stable isotope values of trapped sediments. Next, we show that all nine flood-triggered detrital layers deposited in a sediment record from 1988 to 2013 have elevated isotope values compared with endogenic calcite. In addition, even two runoff events that did not cause the deposition of visible detrital layers are distinguished by higher isotope values. Empirical thresholds in the isotope data allow estimation of magnitudes of the majority of floods, although in some cases flood magnitudes are overestimated because local effects can result in too-high isotope values. Hence we present a proof of concept for stable isotopes as reliable tool for reconstructing flood frequency and, although with some limitations, even for flood magnitudes.〈/span〉

Beschreibung: 〈span〉Friction-generated heat and the subsequent thermal evolution control fault material properties and thus strength during the earthquake cycle. We document evidence for transient, nanoscale fault rheology on a high-gloss, light-reflective hematite fault mirror (FM). The FM cuts specularite with minor quartz from the Pleistocene El Laco Fe-ore deposit, northern Chile. Scanning and transmission electron microscopy data reveal that the FM volume comprises a 〈50-μm-thick zone of polygonal hematite nanocrystals with spherical silica inclusions, rhombohedral twins, no shape or crystallographic preferred orientation, decreasing grain size away from the FM surface, and FM surface magnetite nanoparticles and Fe〈sup〉2+〈/sup〉 suboxides. Sub–5-nm-thick silica films encase hematite grains and connect to amorphous interstitial silica. Observations imply that coseismic shear heating (temperature 〉1000 °C) generated transiently amorphous, intermixed but immiscible, and rheologically weak Fe-oxide and silica. Hematite regrowth in a fault-perpendicular thermal gradient, sintering, twinning, and a topographic network of nanometer-scale ridges from crystals interlocking across the FM surface collectively restrengthened fault material. Results reveal how temperature-induced weakening preconditions fault healing. Nanoscale transformations may promote subsequent strain delocalization and development of off-fault damage.〈/span〉

Beschreibung: 〈span〉A drastic environmental change occurred during the Middle to Late Jurassic as much of East Asia transitioned from a wet seasonal to an extremely arid climate. The timing, scope, and especially mechanism for this aridification are contentious. In this study, we report paleomagnetic data and ages from Jurassic volcanic rocks in North China and for the first time reveal a large-scale southward displacement of ~25° for the Eastern Asian blocks (EABs) sometime between 174 ± 6 Ma and 157 ± 4 Ma. We suggest that the rapid motion documented by our paleomagnetic studies resulted from large-scale true polar wander (TPW). The TPW rotation displaced the EABs from the Northern Hemisphere humid-temperate belt into the subtropical/tropical arid zone. The resultant latitudinal motion coincided with a remarkable environmental change recorded over 10,000,000 km〈sup〉2〈/sup〉 in East Asia between ca. 165 Ma and 155 Ma. We call the climate transition the “Great Jurassic East Asian Aridification” and argue that TPW-induced climatic shifts were also responsible for the demise of the Yanliao Biota and subsequent radiation of the Jehol Biota during the Late Jurassic and Early Cretaceous.〈/span〉

Beschreibung: 〈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〉

Beschreibung: 〈span〉This paper reports the discovery of glacial deposits of likely Siderian–Rhyacian age (2.58–2.06 Ga) in South America (Carajás Basin, Brazil), thereby expanding the potential reach of Paleoproterozoic glaciations to the Amazonian craton for the first time. Glacially derived diamictites are stacked within a hitherto unrecognized ~600-m-thick siliciclastic succession, here named the Serra Sul Formation. Well-preserved textures, with evidence of glaciotectonism and ice rafting, indicate deposition in a coastal subglacial to glacial-fed submarine fan system, in which the immediately underlying units (banded iron formation and volcanic rock) were the main source and bedrock. The Serra Sul diamictite may be correlated with any of the known Paleoproterozoic glaciations, or with none of them.〈/span〉

Beschreibung: 〈span〉Fractures within the earth control rock strength and fluid flow, but their dynamic nature is not well understood. As part of a series of underground chemical explosions in granite in Nevada, we collected and analyzed microfracture density data sets prior to, and following, individual explosions. Our work shows an ~4-fold increase in both open and filled microfractures following the explosions. Based on the timing of core retrieval, filling of some new fractures occurs in as little as 6 wk after fracture opening under shallow (〈100 m) crustal conditions. These results suggest that near-surface fractures may fill quite rapidly, potentially changing permeability on time scales relevant to oil, gas, and geothermal energy production; carbon sequestration; seismic cycles; and radionuclide migration from nuclear waste storage and underground nuclear explosions.〈/span〉

Beschreibung: 〈span〉Recognition of seismic unconformities is crucial for interpreting basin history from seismic reflection data sets in both siliciclastic and carbonate settings. While it is well established that non-erosional changes in sedimentary facies can create seismic reflections that mimic seismic unconformities (i.e., pseudo-unconformities), these features are generally considered to be localized and uncommon, and, therefore, are largely overlooked during interpretation. Diagenetic alteration of strata can also affect the morphology of seismic reflectors and mislead seismic interpreters. This study is based on a three-dimensional (3-D) seismic data set and documents a 400 km〈sup〉2〈/sup〉 honeycomb structure (HS) masquerading as a regional erosional unconformity in the Oligocene–Miocene carbonate strata of Australia’s North West Shelf. This HS is located at the transition between the topsets and the foresets of clinoforms of carbonate to marly composition. The HS expression in 3-D seismic data cross sections is irregular, giving the HS the appearance of a truncated surface that could erroneously be interpreted as a regional seismic unconformity. Closer examination reveals that the HS crosscuts chronostratigraphic clinoform reflectors, and frequency extraction processing shows that the HS dominantly falls within a lower-frequency band than the clinoform reflectors. The morphology of the HS (i.e., continuous with densely packed cells) and its time-transgressive nature suggest that it has a burial diagenetic origin. This suggests that creation of pseudo-unconformities at basin scale by burial diagenesis may lead to surface misidentification, with negative consequences for paleoenvironmental studies and petroleum exploration activities.〈/span〉

Beschreibung: 〈span〉The presence and sources of pre-Columbian (before 1492 CE) lead (Pb) pollution in the midcontinental United States were investigated using geochemical and Pb isotope analyses on sediment cores recovered from Avery Lake, a floodplain lake located directly adjacent to the Kincaid Mounds archaeological site on the lower Ohio River, Illinois. Geochemical results indicate the presence of Pb pollution during the Baumer (300 BCE to 300 CE) and Mississippian (1150–1450 CE) occupations, and since the 1800s. Pb isotope results link Mississippian Pb pollution to the processing and use of galena primarily from southeastern and/or central Missouri, and to a lesser extent the upper Mississippi River valley, with ~1.5 t (metric tons) of galena-derived Pb deposited in Avery Lake during this time. Pb pollution during the Baumer phase, equating to ~0.4 t of Pb, was not accompanied by a Pb isotope excursion and most likely originated from local biomass burning. These results provide new information about the environmental impacts associated with pre-Columbian Native Americans’ interaction with and utilization of their landscape and its resources.〈/span〉

Beschreibung: 〈span〉The Geode of Pulpí (Almería, Spain) is an ~11 m〈sup〉3〈/sup〉 ovoid cavity, the walls of which are covered with meter-sized idiomorphic and highly transparent gypsum (CaSO〈sub〉4〈/sub〉·2H〈sub〉2〈/sub〉O) crystals. We performed a thorough study based on field work, and petrographic and geochemical data collection, which aimed to reconstruct the geological history leading to the formation of this geode. The geode is hosted in mineralized Triassic carbonate rocks with a discontinuous mineral sequence from iron-carbonates and barite to celestine and finally gypsum (microcrystalline and selenite). Data from fluid inclusions show that barite precipitated above 100 °C, celestine at ~70 °C, and gypsum below 25 °C. All δ〈sup〉34〈/sup〉S sulfate phases fall between Triassic and Tertiary evaporite values. Barite and gypsum, either microcrystalline or large selenite crystals, show variable δ〈sup〉34〈/sup〉S and δ〈sup〉18〈/sup〉O compositions, whereas celestine and centimetric selenite gypsum have homogeneous values. We propose that the growth of the large selenite crystals in the Geode of Pulpí was the result of a self-feeding mechanism consisting of isovolumetric anhydrite replacement by gypsum at a temperature of 20 ± 5 °C, episodically contributed by a ripening process enhanced by temperature oscillations due to climatic change.〈/span〉

Beschreibung: 〈span〉The Bayan Obo deposit in China is endowed with the largest rare earth element (REE) resource in the world. The mechanism resulting in this REE enrichment has been the focus of many studies. Carbonatite is known globally as the most favorable carrier of REE ores. In the Bayan Obo deposit, REE ores are hosted in dolomites (including coarse-grained and fine-grained varieties), and many carbonatite dikes (ferroan, magnesian, and calcic) have been identified. All of the dolomites and carbonatite dikes appear to be broadly coeval and possess similar geochemical characteristics. The Sm-Nd isochron age of apatite (1317 ± 140 Ma) from coarse-grained dolomite is consistent with the Th-Pb age of monazite (1321 ± 14 Ma) from a calciocarbonatite dike. The ε〈sub〉Nd(t)〈/sub〉 values and initial 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr ratios at 1.3 Ga of apatite from coarse-grained dolomite show a tight cluster between –2.5 and +1.0 and between 0.70266 and 0.70293, respectively. The δ〈sup〉18〈/sup〉O〈sub〉VSMOW〈/sub〉 values (relative to Vienna standard mean ocean water) of apatite also vary narrowly from 5.0‰ to 6.2‰. These results are consistent with primary mantle-derived carbonatite and prove a magmatic origin for the ore-hosting dolomite. Furthermore, the rim and core texture of dolomite and calcite in the magnesian and calcic carbonatite dikes shows that carbonatite at Bayan Obo has an evolutionary sequence from ferroan through magnesian to calcic in nature. There is a clear negative correlation between the iron content and REE concentration in different stages of carbonatite. Intense magmatic differentiation of carbonatite is likely the critical factor for the giant REE accumulation.〈/span〉

Beschreibung: 〈span〉The Paleotethys Ailaoshan Ocean separated the South China and Indochina blocks during the late Paleozoic. Uncertainty remains regarding subduction of this ocean—whether it was subducted eastward beneath the South China block or westward beneath the Indochina block. In this study, we present new detrital zircon U-Pb age, and Hf and O isotope data from the Longtan Formation, which was recognized to be deposited before the ocean closed. Our results show that the formation can be divided into three units: Unit 1 is distributed west of the suture and dominates the area; it contains major age peaks at 290−250 Ma and minor multiple old age peaks. Unit 2 consists of a minor distribution west of the suture, and it shows a dominant 250 Ma age peak; old zircons are very few or not present. Their Hf and O isotopic signatures are similar to those of unit 1. Unit 3 is distributed east of the suture and is characterized by a single distinct ca. 240 Ma age peak with almost no Precambrian zircons. We interpret that units 1 and 2 were likely deposited in a back-arc and forearc basin, respectively, and a volcanic arc developed on the eastern margin of the Indochina block, similar to the present-day northeastern Japan arc. Meanwhile, unit 3 was likely deposited in a forearc basin on the western margin of the South China block. Therefore, the Ailaoshan Ocean may undergone bipolar subduction both westward and eastward beneath the Indochina and South China blocks, respectively.〈/span〉

Beschreibung: 〈span〉Recycled crustal components have been widely identified in the source of continental basalts with geochemical features similar to oceanic island basalts (OIBs). However, the mechanism of how these recycled materials are involved remains highly debatable. Here we conduct comprehensive geochemical analyses (including whole-rock, olivine, and melt inclusion) and numerical modeling on Late Cretaceous Ji’an basalts from South China interior, aiming to investigate the possible role of recycled crustal components in basalt petrogenesis driven by the subducted paleo-Pacific oceanic plate. The Ji’an basalts show geochemical characteristics akin to OIBs and have depleted asthenospheric mantle-like Sr-Nd-Pb-Hf isotopic compositions with moderately radiogenic Os. Their olivine-hosted melt inclusions have low H〈sub〉2〈/sub〉O and highly negative δD values and olivine phenocrysts are mainly characterized by depletion of 〈sup〉18〈/sup〉O with δ〈sup〉18〈/sup〉O values lowering to 3.9‰. These features are consistent with positive Sr and Eu anomalies in some whole-rock samples. The combined geochemical data suggest that the primary magmas were derived from an asthenospheric mantle enriched by melts from an altered gabbroic oceanic crust, which had experienced intensive dehydration. Further numerical modeling shows that melting of the dehydrated oceanic crust can occur along the torn flank of the subducting lithosphere, in the case that the slab is strongly thinned and fractured. The low δ〈sup〉18〈/sup〉O preserved in olivine and the estimated slab age (〈300 Ma) from the radiogenic whole-rock Os and Pb compositions also require the involvement of a recently recycled slab, probably represented by the subducted paleo-Pacific oceanic plate. Rollback of the subducting paleo-Pacific slab might create a slab window, in which melt from the torn/fractured slab reacted with the upwelling asthenosphere to form an enriched mantle source for the Ji’an basalts and similar counterparts.〈/span〉

Beschreibung: 〈span〉Although postcollisional adakitic rocks are widely distributed in the southern Lhasa subterrane, their petrogenesis remains controversial. Complex petrogenesis models, mainly including partial melting of subducted oceanic crust, partial melting of the Indian lower continental crust, and magma mixing, are pivotal in reconstruction of the postcollisional dynamic processes in south Tibet. In order to constrain the geodynamic processes, we present systemic geochronological and geochemical data for newly discovered adakitic dikes in the Xigaze area, southern Lhasa subterrane. Based on the K〈sub〉2〈/sub〉O and Na〈sub〉2〈/sub〉O contents, the Xigaze dikes can be divided into K-rich and Na-rich dikes. Zircon U-Pb dating for the Xigaze K- and Na-rich dikes yielded ages of ca. 10.31 Ma and 14.78−12.75 Ma, respectively. The K-rich dikes show porphyritic texture and are characterized by high SiO〈sub〉2〈/sub〉 (68.91−69.59 wt%) and K〈sub〉2〈/sub〉O (5.53−5.68 wt%) contents and low Na〈sub〉2〈/sub〉O/K〈sub〉2〈/sub〉O (0.48−0.60) ratios, with Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉/(CaO + Na〈sub〉2〈/sub〉O + K〈sub〉2〈/sub〉O) (=A/CNK) ratios of 1.07−1.23. They have lower MgO (0.63−0.64 wt%), Mg# (37−39), and Cr (18.56−26.62 ppm) and Ni (4.37−4.62) contents. In addition, the K-rich dikes display enriched ([La/Yb]〈sub〉N〈/sub〉 = 65−68) light rare earth elements (LREEs), low concentrations of heavy rare earth elements (HREEs) and Y (e.g., Yb = 0.83−0.86 ppm; Y = 10.56−11.55 ppm), and high Sr (841−923 ppm), with high Sr/Y (74−84) ratios, indicating geochemical characteristics of typical adakitic rocks. Compared with the K-rich dikes, the Na-rich dikes also display porphyritic texture, but they have lower SiO〈sub〉2〈/sub〉 (59.14−64.87 wt%) and K〈sub〉2〈/sub〉O (1.98−3.25 wt%) contents, and higher Na〈sub〉2〈/sub〉O (4.43−5.64 wt%) and MgO (1.40−3.08 wt%) contents, Mg# (46−59), and Cr (22.62−82.93 ppm) and Ni (8.91−39.76 ppm) contents. The HREE abundances (e.g., Yb = 0.36−0.81 ppm; Y = 5.30−10.56 ppm) of the Na-rich dikes are generally lower than the K-rich dikes. These Na-rich dikes are also characterized by adakitic geochemical features with high Sr/Y (60−223) but low (La/Yb)〈sub〉N〈/sub〉 (15−40) ratios. Both the K-rich and Na-rich dikes display distinct whole-rock-element geochemistry and Sr-Nd isotopic composition, with (〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr)〈sub〉i〈/sub〉 = 0.7121, ε〈sub〉Nd〈/sub〉(〈span〉t〈/span〉) = −8.62 to −8.11 and (〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr)〈sub〉i〈/sub〉 = 0.7054−0.7086, ε〈sub〉Nd〈/sub〉(〈span〉t〈/span〉) = −7.55 to −1.23 for K-rich and Na-rich dikes, respectively, which indicate different magma sources for the two types of dikes. The K-rich dikes were most likely derived from partial melts of Lhasa juvenile mafic lower crust with significant involvement of Indian continental crust compositions, whereas the Na-rich dikes were generated in the same way with less input of Indian continental crust compositions. Moreover, the postcollisional adakites in the southern Lhasa subterrane display distinctive spatial variations in geochemistry along the strike of this subterrane, indicating that the magma sources were heterogeneous. In combination with previously published data, we therefore suggest that all these late Oligocene to Miocene adakitic rocks were most likely generated dominantly by partial melting of the Lhasa mafic lower crust with involvement of Indian continental crust components, which was probably triggered by the tearing of the subducting Indian plate.〈/span〉

Beschreibung: 〈span〉Early Jurassic granitoids are widespread in the Lesser Xing’an–Zhangguangcai Ranges, providing excellent targets to understand the late Paleozoic to early Mesozoic tectonic framework and evolution of Northeast China, especially the Jiamusi block and its related structural belts. In this paper, we present new geochronological, geochemical, and isotopic data from the granitoids in the Lesser Xing’an–Zhangguangcai Ranges to constrain the early Mesozoic tectonic evolution of the Mudanjiang Ocean between the Jiamusi and Songnen blocks. Our results show that the granitic intrusions in the Lesser Xing’an–Zhangguangcai Ranges are mainly composed of syenogranite, monzogranite, granodiorite, and tonalite, which have crystallization ages from 196 to 181 Ma. Their geochemical features indicate that these Jurassic intrusions are all high-K calc-alkaline I-type granites with metaluminous to weakly peraluminous compositions. These granitoids are characterized by enrichments in large ion lithophile elements (e.g., Ba, Th, U) and light rare earth elements and depletions in high field strength elements (e.g., Nb and Ta) and heavy rare earth elements, which are typical for continental arc–type granites. The sources of these granitoids were likely derived from juvenile Mesoproterozoic to Neoproterozoic crustal materials (e.g., metabasaltic rocks). Integrated with data from regional coeval magmatism, metamorphism, metallogeny, and structure, our new data suggest that the granitoids in the Lesser Xing’an–Zhangguangcai Ranges were probably formed in an active continental margin setting, which fits well in our previous model of Early Jurassic westward subduction of the Mudanjiang Ocean between the Jiamusi and Songnen blocks.〈/span〉

Beschreibung: 〈span〉This work deals with scarce chlorite schists scattered through the Ronda peridotites (Betic Cordilleras, Spain). These schists have unusually high zircon contents, which contrast with the usual lack of this mineral in ultramafic rocks. From field data and detailed petrographic, geochemical, and geothermometric studies, we focused on the origin of the zircon, a relevant issue for the interpretation of geochronological results. The chlorite schists appear as concordant sheets with granite dikes and as blackwall zones between dikes and serpentinized peridotites. As the intrusion age of the dikes and chlorite schist zircon crystallization (ca. 22 Ma) is slightly older than the age of serpentinization and related chlorite schist formation (ca. 19 Ma), we propose that the chlorite schists are tied to the intrusion of the granite dikes and the subsequent serpentinization of peridotites. Trace and rare earth elements alone are not indicative of the magmatic or hydrothermal origin of the zircon, but the combination of information about zircon morphology, melt inclusions, geothermometry, and the structural relationships between granite dikes and chlorite schists points to late magmatic melts for the zircon origin. We suggest that high-temperature melts saturated in F and Cl acted as Zr carriers under low-pH conditions. A change of the pH conditions, due to hydrothermal alkaline fluids incoming for the concomitant peridotite serpentinization, would have led to zircon crystallization and concentration at the apical zones of the dikes, and to rodingitization before the extensive observed chloritization.〈/span〉

Beschreibung: 〈span〉Kimberlite magmas are volatile-rich, potassic, and ultramafic, and they are host to most of the world’s diamond deposits. A continental-scale kimberlite magmatic belt (the central Cretaceous kimberlite corridor [CCKC]) is found in the interior of the North American continent. Parallel to and coeval with the CCKC, the Cretaceous Omineca magmatic belt (OMB) is located in the Cordilleran orogen. Cordilleran magmatism, including the OMB, is commonly explained through long-lived east-dipping subduction beneath the western margin of the continent. However, this does not explain the temporal and spatial relationships between the OMB and CCKC. We suggest that west-dipping subduction of North American lithosphere beneath the eastern side of the Cordillera explains both. In this model, subduction resulted in arc magmatism of the OMB. The contemporaneous CCKC was formed by extensional stress acting on the continent as it flexed upon entry into the trench. Using a semi-infinite elastic beam model, we show that flexure of a subducting continental plate (elastic thickness = 120 km) produces tensile stresses in the deep continental lithosphere, coincident with the location of the CCKC.〈/span〉

Beschreibung: 〈span〉Bathymetric highs on the ocean floor ultimately sink into highly seismic subduction zones, raising vigorous debates on their potential to trigger or arrest large earthquakes (M〈sub〉w〈/sub〉 〉 7.5). Many geophysical and seismological studies addressing this problem meet penetration and/or resolution issues and deal with only the most recent earthquakes. We herein present the missing piece of the puzzle with the time-integrated field and petrographic record of a unique, almost intact subducted seamount cropping out along a fossil subduction interface. We document seamount buildup and subduction down to ~30 km, and we show that this seamount did not behave as a large earthquake asperity and may have acted as a barrier.〈/span〉

Beschreibung: 〈span〉We present the first spatiotemporal surface suspended sediment concentration (SSSC) analysis of the lower Amazon River, by combining geomorphological mapping, hydrosedimentological field investigations, and 15 years (2001–2015) of weekly averaged wash-load (fine sediment) fluxes at gauging stations estimated from field-calibrated remote-sensing models of SSSC. We found a downstream increase in the magnitude of sediment sinks, where the floodplain geomorphic style controls sediment trapping, and we conclude that the sediment net loss in the floodplain happens during the rising phase through seasonal hydrosedimentological connectivity. On average, 120 million tons per year of fine sediments are accumulated in the floodplain along 765 km of the lower Amazon River (from the confluence with the Madeira River to Monte Alegre city), making the lower Amazon River one of the most important fluvial sediment sinks among the world’s axial rivers.〈/span〉

Beschreibung: 〈span〉Subsurface mapping and core analyses of upper crustal intrusions and mineralization at the Questa porphyry molybdenum deposit, New Mexico, reveal that Mo-mineralization occurred through episodic emplacement of at least six intrusive units. The structure of intrusions associated with the Questa deposit is documented in a series of detailed cross sections and visualized with a 3D animation. Mineralizing intrusions are underlain by two post-mineralization intrusions and cut by late-stage barren dikes. The plutonic complex was structurally focused along a system of preexisting flat-lying faults and their associated fractures. Mineralization is spatially associated with specific intrusive units in the subsurface, and the highest Mo ore grades within established ore blocks are structurally associated with the smallest intrusions. Existing U/Pb thermal ionization mass spectrometry (TIMS) zircon geochronology in conjunction with new relative chronology presented herein indicate that mineralization began before 24.91 Ma. We present three new chemical abrasion U/Pb TIMS zircon ages—one from an amphibole-bearing intrusion associated with high-grade mineralization (dark-matrix porphyry, 24.74 ± 0.37 Ma), a rhyolite dike that cuts ore-grade rocks (24.50 ± 0.02 Ma), and an equigranular granite discovered during deep drilling (23.67 ± 0.02 Ma). The dark-matrix porphyry contains clasts of an earlier amphibole-free intrusion that is spatially associated with low-grade mineralization. Thus, mineralizing intrusions were, in part, intruded into slightly older porphyries, confirming that episodic mineralization continued after 24.91 Ma. The age of the barren dike (24.50 ± 0.02 Ma) is indistinguishable from that of a previously dated granite porphyry that is associated with low-grade mineralization (〈0.05 wt% MoS〈sub〉2〈/sub〉; Questa granite porphyry). These data suggest that mineralization waned by 24.5 Ma and that ore deposition occurred over ~500 ka. The new 23.67 Ma age of the deep equigranular granite, which underlies the Questa granite porphyry, further suggests that intrusions underlying the deposit were not related to mineralization. Detailed subsurface mapping and exploratory drilling indicate that intrusions associated with mineralization were small in volume and cooled rapidly, as evidenced by multiple internal contacts within sheets and rebrecciation textures. On the basis of observed cross-section reconstructions, petrology, alteration, and mineralization, the porphyritic rhyolite intrusions associated with mineralization in one of the largest orebodies in the deposit (the deep northeast) are less than 20-m-thick sheets that are separated by andesite wall rock. Thus, there is no evidence that this orebody formed above a cylindrical magma conduit that facilitated rapid convection, as is often modeled in these systems. We hypothesize that a set of similarly small-volume intrusions were responsible for the bulk of the ore in the southwest ore zone. Our interpretation that the mineralizing intrusions are small, thin, and subhorizontal distinguishes the Questa deposit from other Climax-type molybdenum deposits.〈/span〉

Beschreibung: 〈span〉Fluid migration and pore fluid pressure have been implicated in generating the transitional fault zone properties thought to be important for slow slip events (SSEs) in subduction zones. At the northern Hikurangi margin, New Zealand, overpressurized sediments spatially correlated with shallow SSEs are imaged downdip of subducted seamounts, providing an excellent environment to study the relationship between subducted oceanic relief, fluids, shallow SSEs, and related microseismicity. Using data from ocean-bottom seismometers, we detect “burst-type” repeating earthquakes coincident with tremor on an upper-plate fracture network above a subducted seamount. This activity occurred at the edge of a large, shallow SSE recorded offshore of Gisborne, New Zealand, in September–October 2014, but began days to weeks after the SSE and continued for nearly two months. We propose that during the large plate-boundary SSE, fluids migrated from the downdip overpressurized sediments into the fracture network, diverting aseismic slip to multiple faults in the upper plate. Thus, seamount subduction appears to play a key role in controlling the mechanics of shallow slow slip and microseismicity at the northern Hikurangi margin.〈/span〉

Beschreibung: 〈span〉New ages and strain analysis of syntectonic calcite from the west-central part of the North Anatolian fault zone (NAFZ; northern Turkey) shed light on the debate over the origin and reactivation behavior of this major active fault system. Contrasting views suggest that the west-central part of the NAFZ was already active during the mid-Eocene, mid-Miocene, or early Pliocene. The deformation style is also controversial: early stages of activity are associated with strike-slip, thrust, or normal faulting. Strike-slip deformation of the NAFZ marks the onset of tectonic escape of the Anatolian block westwards and is therefore a key player in tectonic reconstructions. We sampled syntectonic calcite from polished fault surfaces with horizontal slickenlines at four key sites along a 190-km-long sector of the NAFZ. All sites still show active faulting during the Holocene. Combined U-Pb ages (laser ablation–multicollector–inductively coupled plasma–mass spectrometry) and strain analyses of mechanically twinned calcite indicate that the west-central part of the NAFZ was associated with an extensional normal fault system at 42 Ma and reactivated as a dextral strike-slip fault at least 11 m.y. ago. The results indicate that the NAFZ evolved through the reactivation of major preexisting fault structures rather than by a gradual east (13 Ma) to west (5 Ma) propagation. The onset of westward translation of the Anatolian block occurred at least 11 m.y. ago, possibly as a result of the Miocene collision between the Arabian plate and Anatolian block.〈/span〉

Beschreibung: 〈span〉Diamonds are the deepest accessible “fragments” of Earth, providing records of deep geological processes. Absolute ages for diamond formation are crucial to place these records in the correct time context. Diamond ages are typically determined by dating inclusions, assuming that they were formed simultaneously with their hosts. One of the most widely used mineral inclusions for dating diamond is garnet, which is amenable to Sm-Nd geochronology and is common in lithospheric diamonds. By investigating worldwide garnet-bearing diamonds, we provide crystallographic evidence that garnet inclusions that were previously considered to be syngenetic may instead be protogenetic, i.e., they were formed before the host diamond, raising doubts about the real significance of many reported diamond “ages.” Diffusion modeling at relevant pressures and temperatures, however, demonstrates that isotopic resetting would generally occur over geologically short time scales. Therefore, despite protogenicity, the majority of garnet-based ages should effectively correspond to the time of diamond formation. On the other hand, our results indicate that use of large garnet inclusions (e.g., 〉100 μm) and diamond hosts formed at temperatures lower than ~1000 °C is not recommended for diamond age determinations.〈/span〉

Beschreibung: 〈span〉Paleoaltimetry provides critical constraints on orogenic processes. Validation of paleoaltimeters enhances confidence in their application to geologic problems and requires investigations of proxy materials formed at known elevations over known time frames. We evaluated isotope-elevation relationships for late Pleistocene to Holocene (220–0 ka) hydrated volcanic glasses deposited over a 4 km elevation range spanning the Pacific coastal forearc and Andean magmatic arc of Ecuador (0°–1.5°S). Reconstructed δD values of paleometeoric water (δD〈sub〉pw〈/sub〉) in the glasses decrease systematically with elevation. Holocene δD〈sub〉pw〈/sub〉 values are similar to δD values of modern meteoric water (δD〈sub〉mw〈/sub〉), whereas late Pleistocene δD〈sub〉pw〈/sub〉 values are 10‰–30‰ lower than both Holocene δD〈sub〉pw〈/sub〉 values and δD〈sub〉mw〈/sub〉 values at high (〉2 km) elevations. An elevation reconstruction based on δD differences from a modern or late Pleistocene low-elevation datum (ΔδD〈sub〉mw〈/sub〉 or ΔδD〈sub〉pw〈/sub〉, respectively) using a one-dimensional thermodynamic Rayleigh distillation model parameterized with modern temperature accurately predicts Holocene sample elevations but overpredicts elevations of volcanic glass samples that were deposited during glacial marine isotope stage (MIS) 6 by 1–1.5 km. Late Pleistocene sample elevations are accurately predicted by applying a realistic correction for estimated lower air temperature (by ~5 °C) during glacial MIS 6. This study validates the applicability of volcanic glass δD values in paleoaltimetry studies, underscores the importance of accounting for climate-induced changes in isotope lapse rates when calculating paleoelevations, and suggests that ΔδD〈sub〉pw〈/sub〉 might be a sensitive proxy for climate change when applied on time scales over which elevation change was minimal.〈/span〉

Beschreibung: 〈span〉Long-lived, multi-event bedforms hundreds of meters in wavelength in outcrop are interpreted as deepwater antidunes, the first recognized of this nature. Deepwater sediment gravity flows commonly reach a Froude supercritical flow state, but interpretation of their deposits largely excludes antidunes, which are commonly assumed to be ephemeral. Well-exposed, extensive slope turbidites of the Fish Creek–Vallecito Basin of Southern California (USA) are organized into 3–10-m-thick bedsets of 20–30 distinguishable lenticular backset beds that build low-angle (〈10°), undulating geometries and accrete opposite to paleoflow. Bedsets lack high-angle geometries, deep scour surfaces, and structureless facies intrinsic to cyclic steps. Instead, bedsets are differentiated by rhythmic down-dip transitions from thin, subparallel fine-grained beds into thicker, inclined coarser-grained beds and back into thinner, flattening, and, in cases, downflow-dipping finer-grained beds. Within bedsets, compensationally stacked waveforms have ~3–7 m amplitudes and ~75–215 m wavelengths that increase upsection and are comparable to modern upstream-migrating sediment waves. Bioturbated fine-grained caps of each sand bed indicate that antidune bedforms evolved across multiple flow events. Recognition of antidunes in deep water can have important implications for paleoflow reconstruction.〈/span〉

Beschreibung: 〈span〉Deltas are dynamic systems that can provide important information on past climate conditions. Arctic deltas have the potential to preserve information about climate in one of the most temperature-sensitive regions of the Earth. We present experimental results assessing the effects of ice cover on delta morphodynamics to identify signatures of ice-cover presence during deposition. Ice cover drives spatial variation in sediment transport on the subaqueous delta clinoform through sub-ice channels, which leads to the development of (1) extended delta lobes built by elongated, subaqueous sediment wedges, and (2) bathymetry with increasing topographic roughness from the shoreline to a depth ≈ bottom-fast ice thickness. These unique seascape and stratigraphic features record past climate conditions, and can serve as indicators of climate change on vulnerable Arctic coasts.〈/span〉

Beschreibung: 〈span〉StraboSpot is a geologic data system that allows researchers to digitally collect, store, and share both field and laboratory data. StraboSpot is based on how geologists actually work to collect field data; although initially developed for the structural geology research community, the approach is easily extensible to other disciplines. The data system uses two main concepts to organize data: 〈span〉spots〈/span〉 and 〈span〉tags〈/span〉. A spot is any observation that characterizes a specific area, a concept applicable at any spatial scale from regional to microscopic. Spots are related in a purely spatial manner, and consequently, one spot can enclose multiple other spots that themselves contain other spots. In contrast, tags provide conceptual grouping of spots, allowing linkages between spots that are independent of their spatial position.The StraboSpot data system uses a graph database, rather than a relational database approach, to increase flexibility and to track geologically complex relationships. StraboSpot operates on two different platform types: (1) a field-based application that runs on iOS and Android mobile devices, which can function in either Internet-connected or disconnected environments; and (2) a web application that runs only in Internet-connected settings. We are presently engaged in incorporating microstructural data into StraboSpot, as well as expanding to include additional field-based (sedimentology, petrology) and lab-based (experimental rock deformation) data. The StraboSpot database will be linked to other existing and future databases in order to provide integration with other digital efforts in the geological sciences and allow researchers to do types of science that were not possible without easy access to digital data.〈/span〉

Beschreibung: 〈span〉Permian–Triassic strata in the Transantarctic Mountains and West Antarctica carry a significant detrital component derived from a contemporaneous magmatic belt along the Gondwana margin. Hf- and O-isotope characteristics were determined for near-contemporaneous (as shown by U-Pb zircon geochronology) detrital igneous zircons in Upper Permian and Triassic sandstones. Zircons from six granitoids in the contemporaneous magmatic belt were also analyzed for Hf and O isotopes in order to gain insight into the potential detrital zircon sources. Although the ages of these granitoids only loosely correspond with the depositional ages of the sandstones, the initial ε〈sub〉Hf〈/sub〉 and δ〈sup〉18〈/sup〉O isotope compositions for these igneous zircon grains, in general, overlap those recorded for the detrital igneous zircon grains. Results demonstrate a range of ε〈sub〉Hf〈/sub〉 and δ〈sup〉18〈/sup〉O values. Features of particular interest are the very low δ〈sup〉18〈/sup〉O values in two of the granitoids, and similar low values also recorded in the detrital igneous zircons in two sandstones. The distribution of Permian–Triassic granitoids must be much greater than is apparent from the existing outcrops in the extensively ice-covered region. The Permian and one of the Triassic granitoids have Hf-isotope characteristics similar to the Cretaceous granites and Devonian–Carboniferous plutons of West Antarctica, whereas the other Triassic granite differs from both. Importantly, the zircon isotopic data from the Permian–Triassic rocks suggest that an Hf-defined Upper Meso­protero­zoic lithosphere underlies much of the magmatic belt.〈/span〉

Beschreibung: 〈span〉Wind has been an enduring geologic agent throughout the history of Mars, but it is often unclear where and why sediment is mobile in the current epoch. We investigated whether eolian bed-form (dune and ripple) transport rates are depressed or enhanced in some areas by local or regional boundary conditions (e.g., topography, sand supply/availability). Bed-form heights, migration rates, and sand fluxes all span two to three orders of magnitude across Mars, but we found that areas with the highest sand fluxes are concentrated in three regions: Syrtis Major, Hellespontus Montes, and the north polar erg. All regions are located near prominent transition zones of topography (e.g., basins, polar caps) and thermophysical properties (e.g., albedo variations); these are not known to be critical terrestrial boundary conditions. The two regions adjacent to major impact basins (Hellas and Isidis Planitia) showed radially outward upslope winds driving sand movement, although seasonally reversing wind regimes were also observed. The northern polar dunes yielded the highest known fluxes on the planet, driven by summer katabatic winds modulated by the seasonal CO〈sub〉2〈/sub〉 cap retreat—processes not known to affect terrestrial dunes. In contrast, southern dune fields (〈45°S) were less mobile, likely as a result of seasonal frost and ground ice suppressing sand availability. Results suggest that, unlike on Earth, large-scale topographic and thermophysical variabilities play a leading role in driving sand fluxes on Mars.〈/span〉

Beschreibung: 〈span〉Several decades of studies on shelf-margin evolution have led to recognition that both accommodation-dominated and supply-dominated sediment-delivery systems are capable of transporting sediments from the shelf down into deep-water basins. The former case relies on falling sea level and lowstands to move deltas to the shelf edge, whereas the latter depends on well-supplied deltas reaching the shelf edge regardless of sea-level rise. However, it remains unclear how to distinguish between the two sediment-dispersal alternatives, and which of these is more efficient in delivering sediments to deep water. We explore sediment-volume partitioning into deep-water areas by analyzing 〉1600 runs of a geometric delta model with varying eustatic, shelf-morphologic, and sediment-supply conditions. Previous studies suggest that greenhouse eustatic (low amplitude and frequency) conditions generate lower shelf accommodation, and permit the shoreline to arrive at the shelf edge quickly. Further investigation reveals that (1) this argument works only for the supply-dominated system, and (2) the proportion of total sediment that reaches deep water is not correlated to the frequency of sea-level change, but depends strongly on the shelf width and the amplitude of sea-level change. We suggest a ratio between (1) the product of shelf width and the amplitude of sea-level change and (2) total sediment supply to quantitatively characterize the sediment dispersal system. A ratio of 0.4 forms a good boundary between accommodation- and supply-dominated systems in the modeling results, and in three well-studied ancient systems (the Maastrichtian Washakie Basin, Wyoming, USA; the Pliocene paleo–Orinoco margin, Trinidad and Tobago; and the Miocene New Jersey margin, northeastern USA). This work also suggests that the sediment mass balance becomes more important for continental margin building regardless of sea-level scenarios over the longer term.〈/span〉

Beschreibung: 〈span〉We present a novel approach to examining the detrital zircon record, using similarity analysis of regionally defined populations to track crust production, preservation, and the efficacy of crustal recycling and homogenization. We compared temporally binned detrital zircon age data from geographically defined regions using Kolmogorov-Smirnov similarity measures, multidimensional scaling, and cluster analysis. This approach tracks disparity in the global detrital zircon record from 4 Ga to the present. Disparity values increase dramatically in the Neoarchean and are interpreted to reflect the emergence and preservation of isolated crustal fragments. Disparity values decrease through the early Paleoproterozoic, associated with the onset of plate tectonics and craton assembly. Oscillating disparity values through the remaining Proterozoic and Phanerozoic correlate ( 〈span〉p〈/span〉 = 〈0.01) with the supercontinent cycle, where regional detrital zircon populations are more similar during assembly and more distinct during dispersal. This link between the detrital zircon record and the supercontinent cycle necessitates well-coupled crustal recycling mechanisms that operate via both magmatic and sedimentary processes.〈/span〉

Beschreibung: 〈span〉Neoproterozoic−Cambrian rocks of the Windermere Supergroup and overlying units record the breakup of Rodinia and formation of the northwestern Laurentian ancestral continental margin. Understanding the nature and timing of this transition has been hampered by difficulty correlating poorly dated sedimentary successions from contrasting depositional settings across Mesozoic structures. Here we present new litho- and chemo-stratigraphic data from a Cryogenian−lower Cambrian succession in east-central Yukon (Canada), establish correlations between proximal and distal parts of the upper Windermere Supergroup and related strata in the northern Canadian Cordillera, and consider implications for the formation of the northwestern Laurentian margin. The newly defined Nadaleen Formation hosts the first appearance of Ediacaran macrofossils, while the overlying Gametrail Formation features a large negative carbon isotope anomaly with δ〈sup〉13〈/sup〉C〈sub〉carb〈/sub〉 values as low as −13‰ that correlates with the globally developed Shuram-Wonoka anomaly. We also define the Rackla Group, which includes the youngest (Ediacaran) portions of the Windermere Supergroup in the northern Cordillera. The top of the Windermere Supergroup is marked by an unconformity above the Risky Formation that passes into a correlative conformity in the Nadaleen River area. This surface has been interpreted to mark the top of the rift-related succession, but we draw attention to evidence for tectonic instability through the early-middle Cambrian and argue that the transition from rifting to post-rift thermal subsidence is marked by a widespread unconformity that underlies upper Cambrian carbonate rocks. This is younger than the interpreted age of the rift to post-rift transition elsewhere along the ancestral western Laurentian continental margin.〈/span〉