ALBERT

Description: Constraining the pre-opening paleogeography of the Canadian and Alaskan margins of the Canada Basin is a first-order objective in resolving the plate tectonic evolution of the Amerasia Basin of the Arctic Ocean. The most widely accepted model for opening of the Canada Basin involves counterclockwise rotation of Arctic Alaska away from Arctic Canada about a pole of rotation in the Mackenzie Delta region, although numerous other kinematic models have been proposed. The rotation model is tested using detrital zircon U-Pb geochronology of 12 samples from Middle Mississippian to Early-Middle Jurassic strata (Ellesmerian and lower Beaufortian megasequences) obtained from wells and outcrop along Alaska’s North Slope. These northerly-derived strata were deposited in fluvial to nearshore marine environments along the south-facing (present-day) shelf margin of the Arctic Alaska Basin and contain 360–390 Ma, 415–470 Ma, 500–750 Ma, 0.9–2.1 Ga, and 2.4–3.2 Ga zircon populations. Detrital zircon age populations in Ellesmerian and lower Beaufortian strata are remarkably similar to detrital zircon populations from Devonian foreland clastic wedge strata in the Canadian Arctic Islands and northern Yukon Territory. A paleogeographic setting in which Arctic Alaska received sediments recycled from the Devonian foreland clastic wedge and underlying Franklinian Basin strata is most consistent with the model of Embry (1990) in which northern Alaska lay within the foreland fold and thrust belt of the Franklinian mobile belt prior to the opening of the Canada Basin. The sequences that are inferred to have been the long-lived source region for Ellesmerian and lower Beaufortian strata were uplifted by Paleozoic (predominantly Late Devonian) deformation that has been documented along the Canadian and Alaskan margins. Triassic and Jurassic strata deposited along the Arctic Canada, Arctic Alaska, and northern Yukon shelves have detrital zircon ages that are significantly older than the youngest detrital zircon ages (Mesozoic) in coeval strata that were deposited west of Hanna Trough and north of the Sverdrup Basin axis, supporting continuity of these bathymetric features prior to opening of the Canada Basin.

Description: Distributive submarine fans contain channel-lobe elements that compensationally stack to build a radially dispersive map pattern. The middle parts of some submarine fans contain juxtapositions of channel elements and lobe elements due to longitudinal and lateral shifts in their channel-lobe transition zones. This article uses an exceptionally well-exposed three-dimensional outcrop of the Ross Sandstone at Bridges of Ross (Ireland) to document the stratigraphic and plan-view manifestation of lateral juxtapositions of channel elements and lobe elements in submarine fans. Observations made herein compare favorably to those in seafloor studies of Navy Submarine Fan (offshore southern California, USA) by William Normark and others, indicating that these systems can be used as paired outcrop-seafloor analogs for distributive fans in which the channel-lobe transition zones are located in longitudinally variable positions. In addition, data from Bridges of Ross and Navy Submarine Fan are integrated to constrain a geometric model that predicts the fractional length of a fan that contains lateral juxtapositions of channel elements and lobe elements. Lateral juxtapositions of channel elements and lobe elements are important because they enhance vertical and lateral connectivity within subsurface reservoirs.

Description: The northeastern Tibetan Plateau constitutes a transitional region between the low-relief physiographic plateau to the south and the high-relief ranges of the Qilian Shan to the north. Cenozoic deformation across this margin of the plateau is associated with localized growth of fault-cored mountain ranges and associated basins. Herein, we combine detailed structural analysis of the geometry of range-bounding faults and deformation of foreland basin strata with geomorphic and exhumational records of erosion in hanging-wall ranges in order to investigate the magnitude, timing, and style of deformation along the two primary fault systems, the Qinghai Nan Shan and the Gonghe Nan Shan. Structural mapping shows that both ranges have developed above imbricate fans of listric thrust faults, which sole into décollements in the middle crust. Restoration of shortening along balanced cross sections suggests a minimum of 0.8–2.2 km and 5.1–6.9 km of shortening, respectively. Growth strata in the associated foreland basin record the onset of deformation on the two fault systems at ca. 6–10 Ma and ca. 7–10 Ma, respectively, and thus our analysis suggests late Cenozoic shortening rates of 0.2 +0.2/–0.1 km/m.y. and 0.7 +0.3/–0.2 km/m.y. along the north and south sides of Gonghe Basin. Along the Qinghai Nan Shan, these rates are similar to late Pleistocene slip rates of ~0.10 ± 0.04 mm/yr, derived from restoration and dating of a deformed alluvial-fan surface. Collectively, our results imply that deformation along both flanks of the doubly vergent Qilian Shan–Nan Shan initiated by ca. 10 Ma and that subsequent shortening has been relatively steady since that time.

Description: A recent focus of major international exploration in East Antarctica has been aimed at revealing its bedrock topography and imaging its tectonic architecture and evolution. Here we present the tectonic interpretation of regional-scale lineaments revealed by the Radarsat mosaic of Antarctica on the ice sheet surface in the Vostok–Dome C–Adventure Basin region. These lineaments appear in the radar backscatter textures as alignments of marked tonal variations with lengths of tens to hundreds of kilometers and were identified using an automated methodology. We explore the origin scenarios for the ice sheet surface lineaments by comparing their azimuthal trends and spatial distribution with the main morphotectonic features of the bedrock. Azimuthal analysis reveals that lineaments cluster around two preferential directions interpreted as structural or tectonic domains. These show strong correlations with azimuths of tectonic fabrics in the bedrock. The main lineament domain parallels the morphotectonic features of the study area, namely the Adventure Basin and the Concordia and Aurora Trenches. The second lineament set corresponds to the mean orientation of the Lake Vostok depression. The spatial analyses of the two lineament domains strengthen our findings and interpretations. Comparisons with wind and ice flow directions exclude their influence on the identified lineament pattern. Results reveal the tectonic origin of the lineament domains, and demonstrate the method’s usefulness as a tool for tectonic studies of regions characterized by thick covers. These regions include other areas of the East Antarctic craton such as the Gamburtsev Subglacial Mountains, as well as deserts or surfaces of other planets.

Description: Steep slopes mantled by pyroclastic deposits are favorable areas prone to generate hazardous volcaniclastic flows. In Italy, such a setting is well represented in the Campania Region, where pyroclastic deposits from the explosive activity of the Neapolitan volcanoes (Ischia, Campi Flegrei, and Somma-Vesuvius) cover the Apennine range bordering the Campanian Plain. In order to provide a useful contribution to the mitigation and prevention of these calamitous natural events, this work presents a multidisciplinary approach to improve the understanding of the volcaniclastic flow hazard zonation in an Apennine area of 340 km 2 surrounding the Somma-Vesuvius volcano. The disruption proneness index (DPI) was calculated in order to identify the drainage basins potentially prone to generate volcaniclastic flows. This index is obtained by combining satellite and morphometric data in a geographic information system (GIS) environment. It is calculated for 1100 drainage basins, considering the main parameters influencing the slope stability (slope angle, basin shape factor, curvature, relative relief, aspect, and land cover). The land cover mapping is obtained from Landsat data and airborne high-resolution images, while the morphometric parameters are derived from a digital elevation model (DEM) with a cell size of 10 m. The result is a zonation map that classifies the drainage basins according to different degrees of proneness to generate volcaniclastic flows (low, moderate, high, and very high). The drainage basins falling within high and very high classes are 66%, while 28% fall in the moderate class, and the remaining 6% fall in the low proneness class.

Description: The trachydacite complex of Mammoth Mountain and an array of contemporaneous mafic volcanoes in its periphery together form a discrete late Pleistocene magmatic system that is thermally and compositionally independent of the adjacent subalkaline Long Valley system (California, USA). The Mammoth system first erupted ca. 230 ka, last erupted ca. 8 ka, and remains restless and potentially active. Magmas of the Mammoth system extruded through Mesozoic plutonic rocks of the Sierra Nevada batholith and extensive remnants of its prebatholith wall rocks. All of the many mafic and silicic vents of the Mammoth system are west or southwest of the structural boundary of Long Valley caldera; none is inboard of the caldera’s buried ring-fault zone, and only one Mammoth-related vent is within the zone. Mammoth Mountain has sometimes been called part of the Inyo volcanic chain, an ascription we regard inappropriate and misleading. The scattered vent array of the Mammoth system, 10 x 20 km wide, is unrelated to the range-front fault zone, and its broad nonlinear footprint ignores both Long Valley caldera and the younger Mono-Inyo range-front vent alignment. Moreover, the Mammoth Mountain dome complex (63%–71% SiO 2 ; 8.0%–10.5% alkalies) ended its period of eruptive activity (100–50 ka) long before Holocene inception of Inyo volcanism. Here we describe 25 silicic eruptive units that built Mammoth Mountain and 37 peripheral units, which include 13 basalts, 15 mafic andesites, 6 andesites, and 3 dacites. Chemical data are appended for nearly 900 samples, as are paleomagnetic data for ~150 sites drilled. The 40 Ar/ 39 Ar dates (230–16 ka) are given for most units, and all exposed units are younger than ca. 190 ka. Nearly all are mildly alkaline, in contrast to the voluminous subalkaline rhyolites of the contiguous long-lived Long Valley magma system. Glaciated remnants of Neogene mafic and trachydacitic lavas (9.1–2.6 Ma) are scattered near Mammoth Mountain, but Quaternary equivalents older than ca. 230 ka are absent. The wide area of late Quaternary Mammoth magmatism remained amagmatic during the long interval (2.2–0.3 Ma) of nearby Long Valley rhyolitic eruptions.

Description: Sources of seismic hazard in the Puget Sound region of northwestern Washington include deep earthquakes associated with the Cascadia subduction zone, and shallow earthquakes associated with some of the numerous crustal (upper-plate) faults that crisscross the region. Our paleoseismic investigations on one of the more prominent crustal faults, the Darrington–Devils Mountain fault zone, included trenching of fault scarps developed on latest Pleistocene glacial sediments and analysis of cores from an adjacent wetland near Lake Creek, 14 km southeast of Mount Vernon, Washington. Trench excavations revealed evidence of a single earthquake, radiocarbon dated to ca. 2 ka, but extensive burrowing and root mixing of sediments within 50–100 cm of the ground surface may have destroyed evidence of other earthquakes. Cores in a small wetland adjacent to our trench site provided stratigraphic evidence (formation of a laterally extensive, prograding wedge of hillslope colluvium) of an earthquake ca. 2 ka, which we interpret to be the same earthquake documented in the trenches. A similar colluvial wedge lower in the wetland section provides possible evidence for a second earthquake dated to ca. 8 ka. Three-dimensional trenching techniques revealed evidence for 2.2 ± 1.1 m of right-lateral offset of a glacial outwash channel margin, and 45–70 cm of north-side-up vertical separation across the fault zone. These offsets indicate a net slip vector of 2.3 ± 1.1 m, plunging 14° west on a 286°-striking, 90°-dipping fault plane. The dominant right-lateral sense of slip is supported by the presence of numerous Riedel R shears preserved in two of our trenches, and probable right-lateral offset of a distinctive bedrock fault zone in a third trench. Holocene north-side-up, right-lateral oblique slip is opposite the south-side-up, left-lateral oblique sense of slip inferred from geologic mapping of Eocene and older rocks along the fault zone. The cause of this slip reversal is unknown but may be related to clockwise rotation of the Darrington–Devils Mountain fault zone into a position more favorable to right-lateral slip in the modern N-S compressional stress field.

Description: Central Oregon (northwestern USA), where northern Basin and Range extension diminishes in magnitude across the High Lava Plains, exhibits widespread extensional faulting and Quaternary volcanism, yet the relations between the processes are complex and chronology is poorly constrained. Here we use cosmogenic 3 He exposure dating of basalt lava flows to quantify the timing of normal faulting and emplacement of a lava field on the margins of pluvial Fort Rock Lake. The northwest-trending Christmas Valley fault system displaces High Lava Plains volcanic rocks, forming an ~3-km-wide graben that transects the eastern Fort Rock Basin. A portion of the western edge of the graben is marked by a normal fault displaying flexural shear folding with a prominent vertical hinge crack, called Crack in the Ground. Lava flows of the Four Craters Lava Field flowed into this crack. Exposure dating of the Four Craters Lava Field, on the eastern flank of the older Green Mountain shield volcano, indicates an emplacement age of 14 ± 1 ka. We dated Green Mountain basalt exposed in the walls of the crack (the fault wall), which also yielded exposure ages of 14 ± 1 ka. The similar ages suggest that substantial crack opening occurred at about the same time the Four Craters lava was emplaced. These data indicate a period of synchronous normal faulting and dike-fed cinder cone activity emanating from a fault-parallel fissure ~2 km northeast of the crack ca. 14 ka, with minimal displacement since.

Description: Cordilleran granitic batholiths (sensu lato) preserve information about time scales and processes of upper crustal magmatic arc construction during Mesozoic subduction and mountain building. The Bald Mountain batholith in northeastern Oregon (USA) is a classic example of a composite, incrementally constructed batholith that formed during terrane amalgamation outboard of the western U.S. Cordillera. Whole-rock geochemistry and zircon trace element, U-Pb, Lu-Hf, and O isotopic data reveal that batholith construction occurred over ~15 Ma, commencing with the syncollisional emplacement of small, low-Sr/Y (〈40) norite-granite plutons from 157 to 155 Ma. The next phase of magmatism was postcollisional and dominated by high-Sr/Y (〉40) tonalite-granodiorite magmatism that produced the main mass of the batholith, including the granodiorite of Anthony Lake (147 Ma) and the tonalite of Bald Mountain (145–141 Ma). Zircons from the norite-granite suite display a narrow range in initial Hf of 7.2–7.7 and elevated 18 O (Zrn) ranging from 8.2 to 10.0 (excluding one outlier). Zircons from the later granodiorite-tonalite suite show a similar range of initial Hf values (6.3–8.9) and 18 O (7.1–10.0), indicating a similar history of interaction with evolved crustal material. Modeling of whole-rock and zircon geochemistry indicates that both the low- and high-Sr/Y magmas composing the main phase of the batholith were generated by dehydration–partial melting of mafic arc crust (e.g., amphibolite), leaving behind a plagioclase-poor restite, which was garnet granulite in the case of the high-Sr/Y magmas. Final magma compositions in both suites were affected by assimilation of supracrustal material either at depth or during ascent. We suggest that high-Sr/Y magmas in the Bald Mountain batholith were generated by partial melting of thickened arc crust ~10 m.y. after arc-arc collision began at 159–154 Ma. Heat to drive lower crustal melting was conveyed by an increase in mantle power input as a result of renewed subduction-related magmatism. Mixing and homogenization in the lower crust involving mantle-derived basalts and crustally derived partial melts can account for the geochemical variation we observe in tonalites and granodiorites in the Bald Mountain batholith.

Description: Dissected caldera structures expose thick intracaldera tuff and, uncommonly, cogenetic shallow plutons, while remnants of correlative outflow tuffs deposited on the pre-eruption ground surface record elements of ancient landscapes. The Middle Fork caldera encompasses a 10 km x 20 km area of rhyolite welded tuff and granite porphyry in east-central Alaska, ~100 km west of the Yukon border. Intracaldera tuff is at least 850 m thick. The K-feldspar megacrystic granite porphyry is exposed over much of a 7 km x 12 km area having 650 m of relief within the western part of the caldera fill. Sensitive high-resolution ion microprobe with reverse geometry (SHRIMP-RG) analyses of zircon from intracaldera tuff, granite porphyry, and outflow tuff yield U-Pb ages of 70.0 ± 1.2, 69.7 ± 1.2, and 71.1 ± 0.5 Ma (95% confidence), respectively. An aeromagnetic survey indicates that the tuff is reversely magnetized, and, therefore, that the caldera-forming eruption occurred in the C31r geomagnetic polarity chron. The tuff and porphyry have arc geochemical signatures and a limited range in SiO 2 of 69 to 72 wt%. Although their phenocrysts differ in size and abundance, similar quartz + K-feldspar + plagioclase + biotite mineralogy, whole-rock geochemistry, and analytically indistinguishable ages indicate that the tuff and porphyry were comagmatic. Resorption of phenocrysts in tuff and porphyry suggests that these magmas formed by thermal rejuvenation of near-solidus or solidified crystal mush. A rare magmatic enclave (54% SiO 2 , arc geochemical signature) in the porphyry may be similar to parental magma and provides evidence of mafic magma and thermal input. The Middle Fork is a relatively well preserved caldera within a broad region of Paleozoic metamorphic rocks and Mesozoic plutons bounded by northeast-trending faults. In the relatively downdropped and less deeply exhumed crustal blocks, Cretaceous–Early Tertiary silicic volcanic rocks attest to long-term stability of the landscape. Within the Middle Fork caldera, the granite porphyry is interpreted to have been exposed by erosion of thick intracaldera tuff from an asymmetric resurgent dome. The Middle Fork of the North Fork of the Fortymile River incised an arcuate valley into and around the caldera fill on the west and north and may have cut down from within an original caldera moat. The 70 Ma land surface is preserved beneath proximal outflow tuff at the west margin of the caldera structure and beneath welded outflow tuff 16–23 km east-southeast of the caldera in a paleovalley. Within ~50 km of the Middle Fork caldera are 14 examples of Late Cretaceous (?)–Tertiary felsic volcanic and hypabyssal intrusive rocks that range in area from 〈1 km 2 to ~100 km 2 . Rhyolite dome clusters north and northwest of the caldera occupy tectonic basins associated with northeast-trending faults and are relatively little eroded. Lava of a latite complex, 12–19 km northeast of the caldera, apparently flowed into the paleovalley of the Middle Fork of the North Fork of the Fortymile River. To the northwest of the Middle Fork caldera, in the Mount Harper crustal block, mid-Cretaceous plutonic rocks are widely exposed, indicating greater total exhumation. To the southeast of the Middle Fork block, the Mount Veta block has been uplifted sufficiently to expose a ca. 68–66 Ma equigranular granitic pluton. Farther to the southeast, in the Kechumstuk block, the flat-lying outflow tuff remnant in Gold Creek and a regionally extensive high terrace indicate that the landscape there has been little modified since 70 Ma other than entrenchment of tributaries in response to post–2.7 Ma lowering of base level of the Yukon River associated with advance of the Cordilleran ice sheet.

Description: Detrital zircon provenance analysis is used to resolve the age of sandstone injectites together with source sandstones that form fault-bounded, tabular bodies within Mesoproterozoic crystalline rocks of the Colorado Front Range. Named Tava sandstone (informal), the unit is a product of liquefaction and remobilization of mature quartz sediment within source bodies having volumes ≥1 x 10 6 m 3 into dikes up to 6 m in width. To surmount the indeterminate age of emplacement, we obtained new U-Pb detrital zircon age data for two source sandstones, three dikes and one sill, for comparison to four Paleozoic arenites. Tava age distributions feature a dominant 1.33–0.97 Ga broad age group and narrow ca. 1.11, 1.44, and 1.70 Ga groups, with several smaller age groups 〉1.5 Ga. The Tava detrital zircon results are dissimilar to Paleozoic sandstones but closely resemble published detrital zircon reference data for Grenville orogen–derived siliciclastic units of the western United States. The similarity in age distributions is borne out by statistical comparisons among Tava sandstone, Paleozoic samples, and Neoproterozoic strata that reveal a high probability of correlation of Tava sandstone to ca. 800–680 Ma strata deposited during intracontinental extension. We conclude that Tava sandstone is Neoproterozoic in age and provides a new avenue to investigation of Rodinia’s terrestrial paleoenvironment.

Description: We present new multiscale structural, mineral chemical, and U-Pb isotope dilution–thermal ionization mass spectrometry (ID-TIMS) data in order to unravel part of the tectono-metamorphic evolution of the Shuswap complex in the southern Canadian Cordillera. We reconstructed the pressure-temperature-deformation-time ( P - T - d - t ) history of the Joss Mountain domain within the Shuswap complex. The west-dipping Greenbush shear zone separates the Joss Mountain domain from the structurally lower Thor-Odin culmination to the east, the southern culmination of the Monashee complex, and one of the structurally deepest parts of the Shuswap complex. At Joss Mountain, the protolith of an orthogneiss crystallized at ca. 360 Ma which is consistent with Late Devonian arc magmatism along the western paleomargin of North America. Joss Mountain metasedimentary rocks and orthogneiss were transposed at ~21–29 km depth over a period of at least 20 m.y., and possibly more than 38 m.y., during Late Cretaceous to Paleocene mature stages of Cordilleran continental collision. This mature collision took place while slow detachment of the subducted oceanic lithosphere occurred and thermal conditions were approaching those of a crust undergoing postorogenic thermal relaxation. Transposition at Joss Mountain ended earlier and exhumation started earlier than in the Monashee complex. Exhumation occurred under conditions of near-isothermal decompression and geothermal gradients consistent with lithospheric thinning. Earlier and slower exhumation of the Joss Mountain domain than of the adjacent northwestern Thor-Odin culmination may have resulted from normal movement along the Greenbush shear zone contributing to the exhumation of the Shuswap complex.

Description: Physical and numerical simulations of the development of mountain topography predict that asymmetric distributions of precipitation over a mountain range induce a migration of its drainage divide toward the driest flank in order to equilibrate erosion rates across the divide. Such migration is often inferred from existing asymmetries, but direct evidence for the migration is often lacking. New low-temperature apatite cooling ages from a transect across the northern North Cascades range (Washington, NW USA) and from two elevation profiles in the Skagit River valley record faster denudation on the western, wetter side of the range and lower denudation rates on the lee side of the range. This difference has already been documented further south along another transect across the range; however, in the south, the shift from young cooling ages to older ages occurs across the modern drainage divide. Here, further north, the shift occurs along a range-transverse valley within the Skagit Gorge. It has been proposed that the upper Skagit drainage was once a part of the leeward side of the range but started to drain toward the western side of the range across the Skagit Gorge in Quaternary time. Age-elevation profiles along the former drainage and in the Skagit Gorge restrict the onset of Skagit Gorge incision to the last 2 m.y., in agreement with 4 He/ 3 He data for the gorge floor. Breaching of the range drainage resulted in its displacement 40 km further east into the dry side of the range. In the 2000-m-deep, V-shaped Skagit Gorge, river stream power is still high, suggesting that incision of the gorge is still ongoing. Several other similar events have occurred along the range during the Pleistocene, supporting the proposed hypothesis that the repeated southward incursions of the Cordilleran ice sheet during this period triggered divide breaching and drainage reorganization by overflow of ice-dammed lakes at the front of the growing ice sheet. Since these events systematically rerouted streams toward the wet side of the range and resulted in leeward migration of the divide, we propose that in fact the Cordilleran ice sheet advance essentially catalyzed the adjustment of the mountain chain topography to the current orographic precipitation pattern.

Description: Calcite-filled extension veins and shear fractures are preserved in numerous travertine deposits along the western margin of the Albuquerque Basin of the Rio Grande rift. Calcite veins are banded and show geometries suggesting incremental cracking and calcite precipitation. U-series and 234 U model ages from calcite infillings indicate that vein formation was active in the Quaternary, from ca. 2 Ma to ca. 250 ka. Vein orientations are systematic within each deposit and record a dominant extension direction that was horizontal and varied from E-W to NW-SE, consistent with both the regional finite extensional strain in the rift and with the global positioning system (GPS)–constrained deformation field. Three sites contain three orthogonal vein sets that crosscut one another nonsystematically, suggesting alternating times of: (1) regional E-W horizontal extension (dominant), (2) alternating N-S and E-W vertical veins that suggest vertical s 1 and s 2 » s 3 , and (3) horizontal veins that are interpreted to reflect times of highest pore fluid pressures and subequal principal stresses. One site contains conjugate normal faults that also record the dominant E-W extensional tectonic stress. Quaternary extensional strain rates calculated from vein opening for three locations range from 3.2 ± 1.4 x 10 –16 s –1 to 3.2 x 10 -15 ± 2.7 x 10 –16 s –1 , which are up to ~40 times higher than the long-term (Oligocene–Holocene) finite strain rates calculated for different basins of the Rio Grande rift (8.5 x 10 –17 to 4.5 x 10 –16 s –1 ), and up to ~100 times higher than modern strain rates measured by GPS data (3.9 x 10 -17 ± 6.3 x 10 –18 to 4.4 x 10 -17 ± 6.3 x 10 –18 s –1 ). These high Quaternary rates are comparable to modern strain rates measured in the Basin and Range Province and East African Rift. Thus, this paper documents persistent E-W regional extension through the Quaternary in the Rio Grande rift that bridges geologic, paleoseismic, and GPS rates. Anomalously high strain rates in the Quaternary were facilitated by ascent of travertine-depositing CO 2 -rich waters along rift-bounding normal faults, leading to locally very high stain accumulations. These sites also provide examples of natural leakage of deeply sourced CO 2 interacting with regional tectonism, and they emphasize that rift maturation is a highly dynamic process, both spatially and temporally.

Description: Ten years of teleseismic earthquakes recorded by broadband seismic instruments from the Anza network–USArray stations around the San Jacinto fault were used to create P receiver function images of the lithospheric structure beneath this major strike-slip fault. Analyses of back-azimuthal variation and location of the conversion points near the fault suggest an ~8 km vertical offset of the Moho directly beneath the San Jacinto fault. This implies that the fault extends through the entire crust and into the mantle lithosphere, supporting the idea that the strain in the lower crust is localized within a narrow zone. The Moho offset and surface trace of the San Jacinto fault zone are coincident with a compositional boundary in the Peninsular Ranges batholith previously identified in potential field geophysical data and Sr isotope analyses. The position of the offset with respect to this relict geologic feature, which predates the pluton emplacement that formed the batholith, may be a controlling factor in strain location and plate-boundary fault initiation.

Description: The closure of ancient oceans created a dynamic setting suitable for craton formation via the thickening of continental material over a mantle downwelling. This process subjected the thickening lithosphere to extensive deformation, forming internal structure that can be preserved over the lifetime of the craton. Recent seismic imaging of cratonic lithosphere has led to observations of anomalous features colloquially known as midlithospheric discontinuities. These discontinuities are attributed to a range of sources, including the lithosphere-asthenosphere boundary, melt accumulation, and phase transitions. However, the internal structure imaged within these cratons might be reflective of their formation. In particular, the orientation and nature of the variable depths of the midlithospheric discontinuities suggest a more complicated origin such as that which could be introduced during the formation and thickening phase of cratonic lithosphere. Here, we present geodynamic models demonstrating the internal structures produced during the formation of cratonic lithosphere as well as new seismological observations of midlithospheric discontinuities in the West African craton, together with reassessment of midlithospheric discontinuities observed in the North American, South African, Fennoscandia, and Australian cratons. We suggest that the midlithospheric discontinuities observed in these cratons could be remnants of deformation structures produced during the formation of the cratons after ancient oceans closed.

Description: The timing of widespread continental emergence is generally considered to have had a dramatic effect on the hydrological cycle, atmospheric conditions, and climate. New secondary ion mass spectrometry (SIMS) oxygen and laser-ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) Lu-Hf isotopic results from dated zircon grains in the granitic Neoarchean Rum Jungle Complex provide a minimum time constraint on the emergence of continental crust above sea level for the North Australian craton. A 2535 ± 7 Ma monzogranite is characterized by magmatic zircon with slightly elevated 18 O (6.0–7.5 relative to Vienna standard mean ocean water [VSMOW]), consistent with some contribution to the magma from reworked supracrustal material. A supracrustal contribution to magma genesis is supported by the presence of metasedimentary rock enclaves, a large population of inherited zircon grains, and subchondritic zircon Hf ( Hf = –6.6 to –4.1). A separate, distinct crustal source to the same magma is indicated by inherited zircon grains that are dominated by low 18 O values (2.5–4.8, n = 9 of 15) across a range of ages (3536–2598 Ma; Hf = –18.2 to +0.4). The low 18 O grains may be the product of one of two processes: (1) grain-scale diffusion of oxygen in zircon by exchange with a low 18 O magma or (2) several episodes of magmatic reworking of a Mesoarchean or older low 18 O source. Both scenarios require shallow crustal magmatism in emergent crust, to allow interaction with rocks altered by hydrothermal meteoric water in order to generate the low 18 O zircon. In the first scenario, assimilation of these altered rocks during Neoarchean magmatism generated low 18 O magma with which residual detrital zircons were able to exchange oxygen, while preserving their U-Pb systematics. In the second scenario, wholesale melting of the altered rocks occurred in several distinct events through the Mesoarchean, generating low 18 O magma from which zircon crystallized. Ultimately, in either scenario, the low 18 O zircons were entrained as inherited grains in a Neoarchean granite. The data suggest operation of a modern hydrological cycle by the Neoarchean and add to evidence for the increased emergence of continents by this time.

Description: To illustrate the structural evolution of the Black Sea Basin in the context of Neotethyan subduction and subsequent continental collisions, we present the first lithosphere-scale, ~250-km-long, balanced and restored cross section across its southern continental margin, the Central Pontides. Cross-section construction and restoration are based on field, seismic-reflection, geophysical, and apatite fission-track data. The structure of the onshore Pontides belt is predominantly controlled by inverted normal faults, whereas the offshore areas are devoid of large structural inversion. The restored section indicates that Cretaceous crustal thinning occurred synchronously with (probably buoyancy-driven) exhumation of a forearc high-pressure blueschist wedge likely during Neotethyan slab retreat. Apatite fission-track data show that structural inversion of the forearc zone, which formed the Central Pontides fold-and-thrust belt, started at ca. 55 Ma. This Eocene structural inversion followed upon collision of the Kirsehir continental block and the arrest of Neotethyan oceanic subduction below the Central Pontides. Compared to the Central Pontides belt, which underwent significant shortening (~28 km, i.e., ~33%), the relatively colder and stronger Black Sea lithosphere prevented the northern offshore areas from undergoing inversion. We propose that the location of Cenozoic contractional deformation is related to the absence of lithospheric mantle below the southern Pontides (forearc) zone as a consequence of the Cretaceous high-pressure wedge exhumation.

Description: A new perspective on paleoenvironments of Ediacaran fossils of the upper Conception Group (Newfoundland) comes from geochemical and sedimentological study of volcanic tuffs and sedimentary rocks. Tuffs in the Conception Group have major- and trace-element compositions and U-Pb ages comparable with those of source volcanics on the nearby Burin and Bonavista Peninsulas and the islands of St. Pierre and Miquelon. Loss of silica and alkalies in some ashes indicates weathering on land, not marine diagenesis. Volcanic crystal and lapilli tuffs fail to show grading and have lapilli and highly vesicular scoria scattered in a fine-grained matrix, and so they were deposited on land, not in water. These as well as block-and-ash flows, volcanic spindle bombs, and degassing features are evidence of eruptions from nearby subaerial volcanic edifices. The fossiliferous Conception Group accumulated within a forearc basin, formed on continental crust, inboard of the Holyrood horst, and uplifted as part of an ancient subduction complex or accreted terrane. Like analogous forearc basins in Oregon-Washington, southern Chile, and Japan, the Conception Group includes not only marine bay turbidites, but also a variety of intertidal and terrestrial tsunamites, seismites, tempestites, and paleosols. Traditional marine turbidite models explain deposition of the Mall Bay, lower Drook, and lower Briscal Formations of the Conception Group, but the Gaskiers, upper Drook, upper Briscal, and Mistaken Point Formations were deposited in coastal plains and intertidal zones. Paleoenvironments of vendobiont fossils preserved in life position in Newfoundland were terrestrial to marginal marine, not deep sea.

Description: Detrital zircon laser ablation–inductively coupled plasma–mass spectrometry U-Pb age data from the Lower Ordovician Armorican Quartzite (deformed passive margin strata of Gondwanan affinity) of the Iberian Massif are presented herein. The S -shaped coupled Iberian oroclines defined within these zones palinspastically restore to a 2300 km linear Variscan orogen with a paleomagnetically constrained Late Carboniferous north-south trend. Detrital zircons are used to assess paleogeography and interpreted geometry of the Iberian portion of the Gondwana passive margin. A common signature is identified by (1) Neoproterozoic (ca. 500–850 Ma), (2) Stenian–Tonian (ca. 0.9–1.1 Ga), and lesser (3) Paleoproterozoic and (4) Archean populations (ca. 1.8–2.15 and 2.5–2.7 Ga, respectively). Minor site-to-site variation in relative proportion of widely ranging age groups suggests near-uniform distribution of a highly varied detrital input. Provenance analysis reveals strong correlations with Cambro-Ordovician clastic rocks from northeast African realms. Similarity with underlying sequences suggests a common paleogeography from the Ediacaran through early Paleozoic and persistence of a provenance distinction within the autochthonous Iberian Massif. Consistent northward paleoflow within widespread northeast African lower Paleozoic sedimentary cover suggests long-distance sedimentary transport across a North African peneplain from outlying basement terranes. We propose that the 2300-km-long Cantabrian–Central Iberian portion of the early Paleozoic Gondwana margin stretched east-west along the northern limits of the then low-lying Saharan Metacraton and Arabian-Nubian Shield. Accepting paleomagnetic constraints, a 90° counterclockwise rotation is required to reorient the Iberian portion to a pre-oroclinal (Late Carboniferous) north-south trend. The mechanisms for accommodating such a rotation are unclear.

Description: Terrestrial settings preceding the end-Permian crisis are reported to trend toward increasingly dry and arid conditions, resulting in landscape change and a shift in fluvial architectures and regimes. Much of the latest Permian (Changhsingian) stratigraphic record in the Karoo Basin, South Africa, consists of paleosols, which record the physical conditions across time and space. Preboundary sequences at Wapadsberg Pass, Eastern Cape Province, provide insight into the climate regime that influenced paleosol formation at that time. A high-resolution sedimentological and geochemical study of two, stacked aggradational paleosols, in conjunction with stable isotope geochemical characterization of paleosol carbonate-cemented concretions over a 90 m section at this locality, demonstrates that these landscapes were predominantly wetland terrains without a demonstrable trend in increasing drying up to the Permian-Triassic boundary, as defined by vertebrates in the area. Two paleosols examined 70 m below the Permian-Triassic boundary are identified as Protosols, and the former soil-air interface of each is marked by an autochthonous forest-floor litter in which canopy leaves of Glossopteris and groundcover plants of Trizygia are preserved. Molecular weathering ratios (e.g., base loss, clayeyness, chemical index of alteration minus potassium [CIA–K], etc.) determined from these horizons are indicative of immature soil development under water-saturated conditions. Assuming that paleosol-matrix concentrations of trace elements are indicative of Permian soil-solution chemistries, high concentrations of Ni, Cu, Ba, and Cr may have been growth-stress factors that may account for the small glossopterid leaf size in the megafloras, in contrast to current models that implicate stress in response to climate change. Stable isotope 18 O and 13 C values are presented for micritic and microspar (〈20 μm) calcite cements from carbonate nodules collected at 15 horizons through a 90 m stratigraphic interval up to, and including, the Permian-Triassic boundary. These isotopic ratios exhibit dissimilar trends. No clear trend exists in 18 O (Peedee belemnite [PDB] values range from –14.7 to –21.8). In contrast, a trend exists in 13 C values, where carbonate cements almost certainly precipitated under well-drained conditions in an interval that is 60 m below the Permian-Triassic boundary (–5.3), while 13 C values as low as –16.9, indicative of water-logged conditions, begin 90 m below and continue up to the Permian-Triassic boundary. Hence, no evidence is found for a preboundary trend toward increasing aridity at this locality. The first estimates of the latest Permian atmospheric pCO 2 from paleosols, based on coexisting calcite and organic matter 13 C values from paleosols that developed under well-drained conditions, provide a range of values from 900 to 1900, and 500 to 1300 ppmV, respectively, which are significantly lower than the latest Early Permian, when terrestrial biome replacement is documented to have occurred.

Description: Redox-sensitive detrital grains such as pyrite and uraninite in sedimentary successions provide one of the most conspicuous geological clues to a different composition of the Archean and early Paleoproterozoic atmosphere. Today, these minerals are rapidly chemically weathered within short transport distances. Prior to the rise of oxygen, low O 2 concentrations allowed their survival in siliciclastic deposits with grain erosion tied only to physical transport processes. After the rise of oxygen, redox-sensitive detrital grains effectively vanish from the sedimentary record. To get a better understanding of the timing of this transition, we examined sandstones recorded in a scientific drill core from the South African 2.415 Ga Koegas Subgroup, a mixed siliciclastic and iron formation–bearing unit deposited on the western deltaic margin of the Kaapvaal craton in early Paleoproterozoic time. We observed detrital pyrite and uraninite grains throughout all investigated sandstone beds in the section, indicating the rise of oxygen is younger than 2.415 Ga. To better understand how observations of detrital pyrite and uraninite in sedimentary rocks can quantitatively constrain Earth surface redox conditions, we constructed a model of grain erosion from chemical weathering and physical abrasion to place an upper limit on ancient environmental O 2 concentrations. Even conservative model calculations for deltaic depositional systems with sufficient transport distances (approximately hundreds of kilometers) show that redox-sensitive detrital grains are remarkably sensitive to environmental O 2 concentrations, and they constrain the Archean and early Paleoproterozoic atmosphere to have 〈3.2 x 10 –5 atm of molecular O 2 . These levels are lower than previously hypothesized for redox-sensitive detrital grains, but they are consistent with estimates made from other redox proxy data, including the anomalous fractionation of sulfur isotopes. The binary loss of detrital pyrite and uraninite from the sedimentary record coincident with the rise of oxygen indicates that atmospheric O 2 concentrations rose substantially at this time and were never again sufficiently low (〈0.01 atm) to enable survival and preservation of these grains in short transport systems.

Description: Dozens of mafic blueschist blocks are found in the Franciscan mélange, which is well exposed along 6+ km of nearly continuous sea cliffs and wave-cut benches near San Simeon, California. Thirty-four blocks were studied to discover all the varieties in this classic locality of mélange. The Na-amphibole and lawsonite ± epidote–rich schists were dynamically deformed, folded, and veined (including lawsonite and aragonite) before becoming incorporated into the shale-matrix mélange. Tectonically driven flowage of the matrix caused all lithologic components to pinch and swell, forming boudins that separated to become isolated ellipsoidal blocks. This megascopically ductile style of deformation was accommodated in mafic blocks by cataclasis concurrent with alteration to chlorite and pumpellyite. Shearing and alteration were most intense along block margins and within block faults. Petrographic analyses, aided by backscattered electron imaging, microprobe mineral analyses, bulk chemistry, and thermodynamic calculations, reveal these blueschists traveled along a counterclockwise pressure-temperature ( P - T ) path. Na-amphiboles have actinolitic cores that indicate an early greenschist-facies stage. Peak T and P conditions for the epidote-bearing blueschists were ~350 °C at pressures of 5–9 kbar. Many blocks have textural evidence of lawsonite replacing epidote and Na-amphibole with Fe 3+ -rich rims. This indicates epidote replacement occurred as the rocks cooled below ~250 °C at ~5 kbar. It is evident that the phase of dynamic epidote-blueschist-facies recrystallization that nearly obliterated evidence of an earlier greenschist-facies stage was followed by retrograde recrystallization under much less dynamic to nearly static, but still high- P , conditions. Cooling and much of the observed veining must have happened after these rocks were deeply underplated along the bottom of the ophiolitic leading edge of the North American plate. Pieces of the underplated blueschist terrane were probably detached from the hanging wall as slabs that boudinaged and reboudinaged while entrained in shale-matrix mélange upwelling from depths of at least 15 km.

Description: Abyssal peridotites and mid-oceanic ridge basalts (MORBs) represent complementary residue-liquid products of melting and melt migration in the oceanic mantle. Because MORBs are mixtures of melts from different mantle depths, their isotopic signature does not directly describe the isotopic composition of the mantle source, but instead describes the local average composition of different parts of the mantle. In contrast, abyssal peridotites, the residues of fractional melting and melt-rock reaction, should shed more light on the distribution of isotopic heterogeneities. We analyzed Pb isotopic compositions in sulfide grains from the Southwest Indian Ridge and the Gakkel Ridge (Arctic Ocean) using the high-resolution Cameca 1280 ion microprobe. Sulfide Pb isotope ratios show very large variations, with 16 grains from 1 sample covering ~25% of the entire range observed in the oceanic mantle. Pb isotopes in sulfides preserve a record of mantle compositions not seen in whole-rock MORBs from the same area. Sulfides from the Atlantis II Fracture Zone (Southwest Indian Ridge) confirm the presence of ancient refractory material scatter in the oceanic upper mantle. Gakkel Ridge sulfides define a high degree of isotopic variability, suggesting that oceanic mantle, not subcontinental lithospheric mantle, is the main source of such heterogeneity. Our results confirm that the source of MORBs, as represented by abyssal peridotites, is very heterogeneous and that other mantle end-member components are intimately mixed in. In-situ sulfide analysis is a powerful tool to detect the isotopic diversity of the MORB mantle source.

Description: The rivers of western Oregon have diverse forms and characteristics, with channel substrates ranging from continuous alluvial gravel to bare bedrock. Analysis of several measurable morphologic attributes of 24 valley reaches on 17 rivers provides a basis for comparing nonalluvial and alluvial channels. Key differences are that alluvial reaches have greater bar area, greater migration rates, and show systematic correlation among variables relating grain size to bed-material transport capacity. We relate these differences between channel types to bed-material transport rates as derived from a coupled regional analysis of empirical sediment yield measurements and physical experiments of clast attrition during transport. This sediment supply analysis shows that overall bed-material transport rates for western Oregon are chiefly controlled by (1) lithology and basin slope, which are the key factors for bed-material supply into the stream network, and (2) lithologic control of bed-material attrition from in-transport abrasion and disintegration. This bed-material comminution strongly affects bed-material transport in the study area, reducing transport rates by 50%–90% along the length of the larger rivers in the study area. A comparison of the bed-material transport estimates with the morphologic analyses shows that alluvial gravel-bed channels have systematic and bounding relations between bed-material transport rate and attributes such as bar area and local transport capacity. By contrast, few such relations are evident for nonalluvial rivers with bedrock or mixed-bed substrates, which are apparently more influenced by local controls on channel geometry and sediment supply. At the scale of western Oregon, the physiographic and lithologic controls on the balance between bed-material supply and transport capacity exert far-reaching influence on the distribution of alluvial and nonalluvial channels and their consequently distinctive morphologies and behaviors—differences germane for understanding river response to tectonics and environmental perturbations, as well as for implementing effective restoration and monitoring strategies.

Description: The Barberton Granite-Greenstone Belt (BGGB) of South Africa is an exceptionally well preserved Meso-Paleoarchean metamorphic supracrustal belt, one of only a few in the world. Studies of metamorphism in the BGGB have considerable potential to advance our understanding of tectonic processes in the Archean crust. Two current hypotheses persist to explain the origin of amphibolite-facies metamorphism in the southern BGGB. The first interprets these rocks to be the consequence of accretionary tectonics, while the second proposes a "dome-and-keel" vertical tectonic process driven by sinking of greenstone layers and the doming of the underlying granitoid crust. In this study, metamorphic pressure-temperature ( P-T ) analysis has been combined with garnet Lu-Hf and monazite U-Pb geochronology to directly date the amphibolite-facies metamorphism within the Stolzburg terrane of the BGGB. A garnet-biotite-chlorite–bearing sample yields a Lu-Hf garnet age of 3233 ± 17 Ma and a garnet-staurolite-kyanite–bearing sample produces a U-Pb monazite age of 3191 ± 9 Ma, whereas an andalusite-kyanite–bearing sample produces a U-Pb monazite age of 3436 ± 18 Ma. Phase diagrams and garnet compositional modeling produce a clockwise P-T evolution, with rocks reaching peak P-T conditions of 8.5 kbar and 640 °C for the ca. 3200 Ma event and minimum peak P-T conditions of ~4.5 kbar and 550 °C for the ca. 3435 Ma event. The duration of metamorphism for the ca. 3200 Ma event is estimated to be ~50–20 m.y. based on differences in age between U-Pb and Lu-Hf systems and durations needed to fit models of diffusionally modified garnet chemical zoning. Similarly shaped P-T paths over the Stolzburg terrane indicate that the metamorphism occurred in response to crustal thickening due to an accretionary tectonic process. Thus, the Stolzburg terrane constitutes an orogenic core, exhumed along the Komati fault.

Description: Banded iron formations (BIFs) are central to interpretations about the composition of the Precambrian ocean, atmosphere, and biosphere. Hematite is an important component of many BIFs, and its presence has been used as evidence for the former presence of hydrous ferric oxyhydroxides that formed from the oxidation of dissolved ferrous iron in seawater. However, textural evidence for the origin of hematite is equivocal. New petrographic results show that hematite in unmineralized BIF from the ca. 2.5 Ga Dales Gorge Member of the Brockman Iron Formation, Hamersley Group, Western Australia, including morphologies previously interpreted to represent ferric oxyhydroxide precipitates, formed via fluid-mediated replacement of iron-silicates and iron-carbonates along sedimentary layering. The lateral transition from stilpnomelane- and siderite-rich laminae to hematite-dominated laminae is interpreted to reflect progressive stages of in situ alteration of reduced mineral assemblages by oxygen-bearing fluids rather than changes in the chemistry of the water column during deposition. Although morphologies previously ascribed to "primary" hematite are present, they are related to mineral replacement reactions, raising doubts about the petrographic criteria used to identify original hematite. Hematite replacement in unmineralized BIF postdated deposition and possibly metamorphism, and predated modern weathering. From a regional perspective, it appears to be a distal signature of the processes that were responsible for iron-ore mineralization, which involved the deep infiltration of oxygen-bearing meteoric fluids. The mineral replacement reactions recorded in the Dales Gorge Member are unlikely to be unique and probably occurred in BIFs elsewhere at some point in their history. The observation that at least some of the hematite in unmineralized BIF did not form directly from ferric oxyhydroxides implies that hematite is not a reliable proxy for the composition of the precursor sediment or the redox chemistry of the ocean. The oxidation of ferrous-rich phases after deposition suggests that the precursor sediments of BIF originally had a more reduced bulk composition. This raises the possibility that, in an ocean with negligible molecular oxygen and elevated Si and Fe, the growth of iron-rich clay minerals was favored over hematite.

Description: An anomalous body with low Vp (compressional wave velocity), low Vs (shear wave velocity), and high Vp/Vs anomalies is observed at 8–11 km depth beneath the upper east rift zone of Kilauea volcano in Hawaii by simultaneous inversion of seismic velocity structure and earthquake locations. We interpret this body to be a crustal magma reservoir beneath the volcanic pile, similar to those widely recognized beneath mid-ocean ridge volcanoes. Combined seismic velocity and petrophysical models suggest the presence of 10% melt in a cumulate magma mush. This reservoir could have supplied the magma that intruded into the deep section of the east rift zone and caused its rapid expansion following the 1975 M7.2 Kalapana earthquake.

Description: Current models for the assembly of Proterozoic Australia suggest that the North Australian craton (NAC), West Australian craton (WAC), and South Australian craton (SAC) had amalgamated by at least 1.6 Ga, with possible rafting and reattachment of the SAC by ca. 1.3 Ga. In this scenario, the younger (1.2–1.1 Ga) Grenvillian-aged Musgrave Province of central Australia, which separates all three cratons, has been considered postcollisional to intracratonic. However, new and recent U-Pb and Lu-Hf isotopic analyses of zircons from the Musgrave Province indicate continuous active-margin magmatic activity between 1.7 and 1.2 Ga. A distinctive inverted U-shaped pattern of the Hf array for this 500 m.y. period is evidence of part of a Proterozoic Wilson cycle, with subduction initiation at 1.7 Ga and eventual ocean closure by 1.2 Ga. We estimate that the cycle began at 2.2 Ga. Overlap of the Musgrave zircon age spectra and Hf isotopic array with the along-strike Albany-Fraser orogen (AFO) suggests derivation of the Musgrave Province from the WAC, not the NAC or SAC as previously thought. The Musgrave Province link to the WAC confirms that Australia did not assemble until at least early Grenvillian time (ca. 1.2 Ga). Moreover, because the SAC was part of the much larger Mawson continent, the 1.2 Ga collision was of transcontinental magnitude similar to that of the type-Grenville orogen in Laurentia. This favors an Australia-Mexico (AUSMEX) configuration at 1.2 Ga, rather than the southwestern United States and East Antarctica (SWEAT) or Proterozoic Australia–western United States (AUSWUS) models. The Musgrave-AFO marks a major, underestimated phase of Rodinian assembly.

Description: During periods of volcanic activity, hydrothermal fluid chemistry changes drastically, becoming unusually dilute due to enhanced degrees of phase separation. Despite decreases in nearly all other metals, these dilute fluids maintain surprisingly high dissolved Fe concentrations. This is demonstrated by a 17 yr time series from 9°50'N on the East Pacific Rise, where two eruption cycles are separated by a decade of steady-state chemical and physical conditions. We report experimental data confirming a sharp increase in Fe solubility in low-salinity and low-density vapors that constitutes a reversal in behavior exhibited in near-critical vapors characteristic of the steady-state condition. In accordance with field observations during the eruptions, a fundamental divergence between the otherwise similar behaviors of Fe and Mn also results. This helps explain how Fe fluxes are maintained during magmatic events, which may have important implications for the succession and temporal evolution of vent-related fauna. Calibrated geochemical proxies for subseafloor reaction conditions (pressure-temperature) now allow us to elucidate hydrothermal processes from steady state through eruptive and recovery stages at the 9°50'N system.

Description: The giant nitrate deposits of the hyperarid Atacama Desert (Chile) are one of the most extraordinary, yet enigmatic, mineral occurrences on Earth. These deposits are complex assemblages of highly soluble nitrates, chlorides, sulfates, perchlorates, iodates, and chromates, and their preservation is the result of prevalent hyperarid climate conditions in the Atacama Desert since the late Miocene, with average rainfall rates of 〈10 mm/yr in the past ~3 m.y. Although several hypotheses have been proposed since the mid-1800s, the formation of these extensive deposits still remains highly controversial despite the fact that recent studies have argued toward an atmospheric source for the nitrate, sulfate, and perchlorate components. In this report, we focus on the often overlooked and poorly studied iodine and chromium components of Atacama’s nitrates. We present the first cosmogenic iodine ( 129 I) and stable chromium ( 53/52 Cr) isotope data of nitrates showing that groundwater has played an unforeseen role in the formation of these massive deposits. The isotopic signature of I in the nitrates ( 129 I/I ~150–600 x 10 –15 ) share similarities with deep sedimentary (marine) pore waters and shales, deviating significantly from atmospheric iodine ( 129 I/I ~1500 x 10 –15 ), while the positive and highly fractionated 53/52 Cr SRM979 values (+0.7 to +3) are indicative of intense Cr redox cycling due to groundwater transport. Our evidence points toward a multi-source genetic model for the Atacama Desert nitrate deposits, where these extensive accumulations were the result of long-lived, near-surface mineral precipitation driven by groundwater (i.e., chromates, iodates) coupled with dry atmospheric deposition (i.e., nitrates, perchlorates) and sea spray inputs (i.e., sulfates, chlorides), triggered by increasing aridity and tectonic uplift.

Description: Sediment transport in mountain channels controls the evolution of mountainous terrain in response to climate and tectonics and presents major hazards to life and infrastructure worldwide. Despite its importance, we lack data on when sediment moves in steep channels and whether movement occurs by rivers or debris flows. We address this knowledge gap using laboratory experiments on initial sediment motion that cross the river to debris-flow sediment-transport transition. Results show that initial sediment motion by river processes requires heightened dimensionless bed shear stress (or critical Shields stress) with increasing channel-bed slope by as much as fivefold the conventional criterion established for lowland rivers. Beyond a threshold slope of ~22°, the channel bed fails, initiating a debris flow prior to any fluvial transport, and the critical Shields stress within the debris-flow regime decreases with increasing channel-bed slope. Combining theories for both fluvial and debris-flow incipient transport results in a new phase space for sediment stability, with implications for predicting fluvial sediment transport rates, mitigating debris-flow hazards, and modeling channel form and landscape evolution.

Description: Past glacials can be thought of as natural experiments in which variations in boundary conditions influenced the character of climate change. However, beyond the last glacial, an integrated view of orbital- and millennial-scale changes and their relation to the record of glaciation has been lacking. Here, we present a detailed record of variations in the land-ocean system from the Portuguese margin during the penultimate glacial and place it within the framework of ice-volume changes, with particular reference to European ice-sheet dynamics. The interaction of orbital- and millennial-scale variability divides the glacial into an early part with warmer and wetter overall conditions and prominent climate oscillations, a transitional mid-part, and a late part with more subdued changes as the system entered a maximum glacial state. The most extreme event occurred in the mid-part and was associated with melting of the extensive European ice sheet and maximum discharge from the Fleuve Manche river. This led to disruption of the meridional overturning circulation, but not a major activation of the bipolar seesaw. In addition to stadial duration, magnitude of freshwater forcing, and background climate, the evidence also points to the influence of the location of freshwater discharges on the extent of interhemispheric heat transport.

Description: Transient and episodic slow slip accommodates a great deal of tectonic strain and may be mechanically linked with locked regions of seismically hazardous faults. Best documented in subduction zones and associated with nonvolcanic tremor, most proposed mechanisms for slow slip revolve around the transition between stable and unstable frictional sliding. The dilemma is that slow slip is generated at a wide range of crustal depths, including at pressure-temperature conditions where frictional deformation mechanisms give way to predominantly viscous ones. We present a model for how fracture and viscous flow within mid-crustal shear zones can produce episodic creep transients. Our model for such transients stems from geological examples of shear zones that formed at temperatures and pressures of 〉500 °C and 〉0.6 GPa during early orogenesis, following Late Cretaceous subduction of a backarc ocean basin. Within these shear zones, relatively strong lenses of metabasalt localized fluid-filled fractures that were subsequently deformed by viscous flow in surrounding quartzofeldspathic gneiss. The spatial and temporal characteristics of the modeled creep events are similar to those of slow-slip events observed in modern subduction zones. We therefore suggest that some episodic tremor and slip can originate through combined fracture and viscous flow across shear zones comprising mixtures of strong and weak materials.

Description: During the first stage of the Late Miocene Messinian salinity crisis (5.97–5.60 Ma), deposition of sulfates (the Primary Lower Gypsum) occurred in shallow silled peripheral subbasins of the Mediterranean undergoing restricted water exchange with the Atlantic Ocean. Fluid inclusions in Messinian selenite crystals from the Piedmont Basin (northwest Italy) have surprisingly low salinities (average of 1.6 wt% NaCl equivalent), suggesting that parent waters were depleted in Na + and Cl – compared to modern seawater. Modern gypsum from a Mediterranean salt work, in contrast, contains fluid inclusions with elevated salinities that match the normal evaporation trend expected for seawater. The salinity data indicate that the Messinian sulfate deposits from the Piedmont Basin formed from hybrid parent waters: seawater mixed with Ca 2+ and SO 4 2– enriched freshwaters that dissolved coeval marginal marine gypsum. Such mixed parent waters and complex recycling processes should be taken into account when explaining the genesis of other Messinian gypsum deposits across the Mediterranean Basin.

Description: We show that a belt of clockwise vertical-axis block rotation associated with dextral-oblique rifting in the Basin and Range province in Mexico hosted the localization of plate-boundary strain that led to formation of the Gulf of California ocean basin. Paleomagnetism of Miocene ignimbrites distributed widely across the rift reveals the magnitude, distribution, and timing of rotation. Using new high-precision paleomagnetic vectors (α 95 1°) from tectonically stable exposures of these ignimbrites in Baja California, we determine clockwise rotations up to 76° for intrarift sites. Low reference-site error permits isolation of intrarift block rotation during proto-Gulf time, prior to rift localization ca. 6 Ma. We estimate that 48% (locally 0%–75%) of the net rotation occurred between 12.5 Ma and 6.4 Ma. Sites of large (〉20°) block rotation define an ~100-km-wide belt, associated with strike-slip faulting, herein named the Gulf of California shear zone, which was embedded within the wide rift Basin and Range province and kinematically linked to the San Andreas fault. After a protracted history of diffuse extension and transtension, rift localization was accomplished by focusing of Pacific–North America dextral shear into the Gulf of California, which increased strain rates and connected nascent pull-apart basins along the western margin of the province. Oblique rifting thus helped to localize and increase the rate of continental break up and strongly controlled the three-dimensional architecture of the resultant passive margins.

Description: It is widely proposed that the oceanic mantle is hydrated by outer rise normal faults, and carries large amounts of water to the deep mantle. However, the extent of oceanic mantle hydration is poorly constrained by existing observations, and is a major source of uncertainty in determining the total water delivered to the mantle. Full waveform modeling of dispersed P-wave arrivals from events deep within the Wadati-Benioff zone of northern Japan shows that hydrated fault zone structures are present at intermediate depths. Analysis of the P-wave coda associated with events 5–35 km below the top of the slab gives an overall indication of the bulk hydration of the subducting oceanic mantle, and can be explained by a 40-km-thick layer that is 17%–31% serpentinized. This suggests that the top of the oceanic mantle is 2.0–3.5 wt% hydrated, subducting 170–318 Tg/m.y. of water per meter of arc beneath northern Japan. This order-of-magnitude increase in the estimated H 2 O flux in this arc implies that over the age of the Earth, the equivalent of as many as 3.5 present-day oceans of water could be subducted along the Kuril and Izu-Bonin arcs alone. These results offer the first direct measure of the lower lithosphere hydration at intermediate depths, and suggest that regassing of the mantle is more vigorous than has previously been proposed.

Description: Rifting between large continental plates results in either continental breakup and the formation of conjugate passive margins, or rift abandonment and a set of aborted rift basins. The nonlinear interaction between key parameters such as plate boundary configuration, lithospheric architecture, and extension geometry determines the dynamics of rift evolution and ultimately selects between successful or failed rifts. In an attempt to evaluate and quantify the contribution of the rift geometry, we analyze the Early Cretaceous extension between Africa and South America that was preceded by ~20–30 m.y. of extensive intracontinental rifting prior to the final separation between the two plates. While the South Atlantic and Equatorial Atlantic conjugate passive margins continued into seafloor-spreading mode, forming the South Atlantic Ocean basin, Cretaceous African intraplate rifts eventually failed soon after South America broke away from Africa. We investigate the spatiotemporal dynamics of rifting in these domains through a joint plate kinematic and three-dimensional forward numerical modeling approach, addressing (1) the dynamic competition of Atlantic and African extensional systems, (2) two-stage kinematics of the South Atlantic Rift System, and (3) the acceleration of the South America plate prior to final breakup. Oblique rifts are mechanically favored because they require both less strain and less force in order to reach the plastic yield limit. This implies that rift obliquity can act as selector between successful ocean basin formation and failed rifts, explaining the success of the highly oblique Equatorial Atlantic rift and ultimately inhibiting the formation of a Saharan Atlantic Ocean. We suggest that thinning of the last continental connection between Africa and South America produced a severe strength-velocity feedback responsible for the observed increase in South America plate velocity.

Description: Continental rifts are commonly flanked by zones of high elevation, but the cause of uplift remains controversial. Proposed uplift mechanisms include active and induced asthenospheric upwelling, and isostatically driven lithospheric flexure. Discrimination between these hypotheses requires close constraint of the timing of rift flank uplift and crustal extension. Here, we focus on the well-preserved Neogene Gulf of California rift. The western rift margin is characterized by a prominent east-facing kilometer-scale escarpment, which bounds a west-tilted, topographically asymmetric rift flank. We exploit west-draining canyons incised into the rift flank to constrain the timing of uplift to between ca. 5.6 and 3.2 Ma using 40 Ar/ 39 Ar dating of lavas, which show cut-and-fill relationships to the canyons. Rift flank uplift closely followed the onset of slip on the principal fault of the Loreto rift segment at ca. 8–6 Ma, the age of which we obtain from apatite (U-Th)/He and fission-track thermochronologic analysis of rift escarpment exhumation. Uplift was therefore coeval with lithospheric rupture and the onset of oceanic spreading between ca. 6 and 3 Ma, but post-dates a proposed asthenospheric upwelling event by ~8–10 Ma. The timing of uplift is inconsistent with either active or induced upwelling as uplift mechanisms, and we conclude that rift flank uplift was driven by the flexural response to lithospheric unloading.

Description: The geomorphology, geometry, and sedimentary infill of buried gorges and V-shaped valleys observed at the base of major river valleys in the formerly glaciated southeastern Canadian Shield region have been revealed from excavation and drilling data acquired during the construction of hydro-electric dams and seismic data collected on lakes and offshore. Compilation of these previously published and unpublished data provided an exceptional opportunity to examine the morphology and spatial distribution of buried bedrock gorges and the mechanical processes responsible for their erosion. In some valleys, detailed observations of deep gorges have been allowed by their exhumation over large areas. Archive photographs show deep and large potholes, natural pillars, furrows, flutes, and scallops on the well-polished bedrock walls of the gorges. They also reveal that gorges and valleys have a sharp-ending V shape and very narrow base and are superimposed by a U-shaped valley, forming a buried valley-within-valley topography. The narrow and deep cross-profile of these gorges, their well-polished slopes, and the type of bedforms observed within them are typical features of fluvial bedrock channels. Drilling operations at many sites have also provided data on bedrock topography of these gorges within valleys and on the nature of their sedimentary infill. The different lines of evidence presented in this paper indicate that gorges and V-shaped valleys of the region were not eroded by the Laurentide Ice Sheet during Quaternary glaciations but are relics of a preserved preglacial fluvial system eroded during a lower base level. This paleofluvial system is interpreted to be linked to fluvially cut channels observed on the seafloor of the Estuary and Gulf of St. Lawrence.

Description: We develop a new high-resolution stratigraphic age model to unravel the contributions of tectonic and climatic processes on early to late Pleistocene synorogenic growth strata. We capitalize on excellent, continuous exposures along the flank of the Po foreland in northern Italy to elucidate hydrologic, geomorphic, and sedimentologic processes that are regularly attributed to, but rarely proven to be caused by, glacial-interglacial climatic changes and unsteady rock uplift. We perform our analysis on the Enza section, a succession of marine and terrestrial strata exposed along the Enza River, between Parma and Reggio Emilia, northern Italy. Bedding in the Enza section displays synorogenic growth strata geometry, with bedding dips that range from 2° to 55°, that becomes progressively shallower upsection. We develop an age model that incorporates biostratigraphy, magnetostratigraphy, rock-magnetic cyclostratigraphy, cosmogenic radionuclide burial dating, and optically stimulated luminescence dating and shows that the Enza section spans the interval between 0.04 and 1.65 Ma. Furthermore, the age model pins the time of deposition for several lithostratigraphic units of regional significance and shows that sediment accumulation was unsteady, ranging from 14–31 cm/k.y. in the marine part of the section to 5–362 cm/k.y. in the overlying littoral and terrestrial part of the section. Unsteady deposition is most pronounced in the terrestrial deposits where thick fluvial gravel packages accumulated in short (~10–15 k.y.) time periods that coincide with Quaternary glacial intervals. There is direct evidence for a dominant tectonic control in the older, marine part of the section. Here, sediment accumulation rates on the limb of the fold growing along this portion of the Northern Apennine mountain front show that between 1.07 and 1.65 Ma, repetitive progradation of neritic sand units directly followed pulses of rapid, punctuated uplift. In contrast, the cyclic terrestrial facies variations in the Enza section reveal that once the section became emergent at ca. 1 Ma and uplift slowed, climate was the dominant control on sediment production and deposition.

Description: Lake sediments contain valuable information about past volcanic and seismic events that have affected the lake catchment, and they provide unique records of the recurrence interval and magnitude of such events. This study uses a multilake and multiproxy analytical approach to obtain reliable and high-resolution records of past natural catastrophes from ~600-yr-old annually laminated (varved) lake sediment sequences extracted from two lakes, Villarrica and Calafquén, in the volcanically and seismically active Chilean Lake District. Using a combination of micro–X-ray fluorescence (µXRF) scanning, microfacies analysis, grain-size analysis, color analysis, and magnetic-susceptibility measurements, we detect and characterize four different types of event deposits (lacustrine turbidites, tephra-fall layers, runoff cryptotephras, and lahar deposits) and produce a revised eruption record for Villarrica Volcano, which is unprecedented in its continuity and temporal resolution. Glass geochemistry and mineralogy also reveal deposits of eruptions from the more remote Carrán–Los Venados volcanic complex, Quetrupillán Volcano, and the Huanquihue Group in the studied lake sediments. Time-series analysis shows 112 eruptions with a volcanic explosivity index (VEI) ≥2 from Villarrica Volcano in the last ~600 yr, of which at least 22 also produced lahars. This significantly expands our knowledge of the eruptive frequency of the volcano in this time window, compared to the previously known eruptive history from historical records. The last VEI ≥2 eruption of Villarrica Volcano occurred in 1991. Based on the last ~500 yr, for which we have a complete record from both lakes, we estimate the probability of the occurrence of future eruptions from Villarrica Volcano and statistically demonstrate that the probability of a 22 yr repose period (anno 2013) without VEI ≥2 eruptions is ≤1.7%. This new perspective on the recurrence interval of eruptions and historical lahar activity will help improve volcanic hazard assessments for this rapidly expanding tourist region, and it highlights how lake records can be used to significantly improve historical eruption records in areas that were previously uninhabited.

Description: Two fault-bounded sequences of metamorphic rocks are exposed in the Blue Mountains of eastern Jamaica. Westphalia Schist is dominated by amphibolite facies hornblende schist and mica schist. Mt. Hibernia Schist is dominated by blueschist-greenschist facies metabasalts. New whole-rock geochemistry and 40 Ar/ 39 Ar ages clarify the tectonic setting of the protoliths, timing of post-metamorphic cooling, and evolution of the northern margin of the Caribbean plate. Westphalia Schist is geochemically variable, with mafic igneous protoliths or volcaniclastic sedimentary protoliths. Regardless of the protolith, the trace-element geochemistry is consistent with an island-arc tectonic environment. These rocks most likely represent metamorphosed equivalents of the regionally extensive Early Cretaceous Greater Antilles arc that is preserved discontinuously along the present-day northern margin of the Caribbean plate. Mt. Hibernia Schist shows little geochemical variability, with an igneous protolith of subalkaline basaltic composition. Flat rare-earth-element patterns and flat extended trace-element patterns are consistent with an enriched mid-ocean ridge basalt or oceanic plateau environment. However, in terms of immobile elements, Mt. Hibernia Schist is geochemically indistinguishable from nearby ca. 90 Ma basalt of the Bath-Dunrobin Formation, which is a product of Caribbean plate–forming ocean plateau magmatism; i.e., Caribbean large igneous province. Hence, an ocean plateau environment is inferred for the Mt. Hibernia protolith. The Westphalia and Mt. Hibernia Schists are currently juxtaposed along the Blue Mountain fault, yet were subjected to very different subduction-related metamorphic histories. Stratigraphic relationships require that the metamorphic rocks were uplifted, and exposed at the surface by the Early Paleocene. 40 Ar/ 39 Ar ages indicate that the two units were affected differently by burial metamorphism related to Paleocene–Early Eocene transtensional tectonics. Final juxtaposition of Westphalia Schist and Mt. Hibernia Schist was accomplished through a combination of vertical and horizontal displacements during Neogene transpression along the Plantain Garden fault.

Description: Using cosmogenic isotopes and solute load analysis, we quantify chemical weathering (solutional erosion) and denudation rates over variable time scales in a tectonically stable, moderate-relief, carbonate terrain (Soreq drainage, Judea Hills, Israel), located in a semihumid Mediterranean climate. Long-term (〉10 4 yr) denudation rates were calculated from 36 Cl concentrations in 51 bedrock and sediment samples. Bedrock samples range in elevation (340–850 m), hillslope gradient (0°–30°), and mean annual precipitation (MAP; 500–630 mm) and vary in soil cover thickness (0–75 cm), Mg/Ca ratio (0.0–1.0 mol), clay mineral contents (0–6 wt%), and mechanical strength (41–58 Schmidt hammer rebound units). Soil p CO 2 values at a single location during the course of 1 yr, range between 0.4 and 9.0 mmol mol –1 . Average long-term denudation rate of exposed bedrock samples is 21 ± 7 mm k.y. –1 . Field observations and 36 Cl measurements indicate that soil pockets undergo cycles in the rate of deepening, and that over 10 5 yr time scale, average denudation rates beneath soil pockets are similar to those of exposed bedrock. Sediment samples yield even higher denudation rates, which are probably anthropogenically induced, but could also indicate that the sediment source is soil pockets. Long-term denudation rates are decoupled from hillslope gradient, elevation, and rock strength. Denudation rates show a positive correlation with present-day MAP values, exhibit a complex relation with rock Mg content, and show a weak correlation with clay content. Annual chemical weathering rates were calculated from modern-day solute load measured in waters of perched springs and the regional carbonate aquifer. Our results indicate that on annual, decadal, and 10 4 yr time scales, chemical weathering and denudation are controlled by carbonate dissolution, while mechanical processes are far less significant. Overlap between the distributions of HCO 3 – concentrations measured in runoff, springs, and the regional aquifer water suggests that chemical weathering focuses at the bedrock surface and therefore is comparable with solutional denudation. This result is in contrast to the features of ancient fluvial and colluvial activity (steep nonconcave hillslopes and stream profiles and knickzones in the streams) preserved in the present landscape. Such features were formed in response to mid-Pleistocene uplift and could have been preserved due to a decrease in stream power following the formation of subsurface drainage and the lowering in abrasive clast supply that followed the stabilization of hillslopes in the drainage. Long-term denudation rates calculated from exposed bedrock samples are higher by factor of 1.4 relative to annual, contemporary chemical weathering rates. Increased precipitation by a similar factor, averaged over the last glacial and present interglacial, can explain this difference.

Description: 2013 GSA Presidential Address Delivered at the GSA Annual Meeting in Denver, Colorado, USA, on Sunday, 27 October 2013 . ABSTRACT In the 125th year of The Geological Society of America, it is appropriate to look at the past accomplishments and the future of the Society. With this comes the understanding that it is through curiosity-driven geoscience, whose promotion and communication are hallmarks of GSA, that strides are made in understanding resources, hazards, the global environment, and the factors that influence our survival on planet Earth. Over the years, these advances have come from deductive reasoning based on new and accumulated field and laboratory observations and theoretical modeling, which are continuously taken to new levels by incorporating the latest scientific technology. With this in mind, we examine advances in understanding the origin of mountain belts in the context of the formation and evolution of Earth as our governing concepts have evolved from geosynclinal theory to plate interactions and continental collisions to concepts of continental lithospheric growth and destruction by processes like lower crustal and lithospheric delamination and forearc subduction erosion.

Description: Crustal structure in southeastern Egypt has been investigated to elucidate the nature of crustal thinning across the northern Red Sea. P-wave receiver function modeling for 7 stations in southeastern Egypt yields typical Proterozoic crustal thicknesses of 35–38 km around Lake Aswan, and thinner crust (25–26 km) within 50 km of the Red Sea coast. The Vp/Vs ratios are on average 1.78 and indicate an intermediate-composition crust. These results, when combined with other estimates of crustal thickness in the region, reveal a symmetric pattern of crustal thickness beneath the conjugate margins of the northern Red Sea. Such a pattern is consistent with a pure shear model of extension, and suggests that the greater amounts of uplift and volcanism on the eastern side of the Red Sea compared to the western side may be the result of deeper flow in the mantle associated with the African superplume and not a direct consequence of the rifting process.

Description: Detailed knowledge of lithospheric structure is essential for understanding the long-term evolution and dynamics of continents. We present an image of lithospheric structure across the central and western North China Craton (NCC), derived using S and P receiver functions from a dense seismic array. A negative velocity discontinuity is identified at ~80–100 km depth within the thick lithosphere (~160–200 km), similar to that observed in many other cratonic regions and roughly at the same depth as the base of the lithosphere in the eastern NCC. The intralithospheric discontinuity may indicate an ancient, mechanically weak layer within the overall strong cratonic lithosphere, and probably also existed beneath the eastern NCC before the Mesozoic. The presence of such a weak layer could have facilitated simultaneous lithospheric modification at the base and the middle of the lithosphere in the eastern NCC, especially under the strong influence of the Mesozoic Pacific subduction, leading to the severe lithospheric thinning and destruction recorded in this region. The weak layer probably did not strongly affect the stability and evolution of the central and western NCC and other cratonic regions where effects from plate boundary processes were weak. Our seismic images, integrated with geological data, provide new insights into structural heterogeneities in the subcontinental lithospheric mantle and their roles in the dynamic evolution of continents.

Description: We characterize the eastern Gulf of Cadiz, proximal to the Strait of Gibraltar, using a multidisciplinary approach that combines oceanographic, morphosedimentary, and stratigraphic studies. Two terraces (upper and lower) were identified along the middle slope. They are composed of several associated morphologic elements, including two large erosive channels, which allow us to determine a new and more detailed understanding of the Mediterranean Outflow Water (MOW) pathway and its deceleration upon exiting the Strait of Gibraltar. There is evidence for along-slope circulation and additional secondary circulation oblique to the main flow. The present upper core of the MOW flows along the upper terrace and the lower core flows along the lower terrace. However, the lower terrace shows larger and better defined erosive features on the seafloor than does the upper terrace; we attribute this to a denser, deeper, and faster MOW circulation that prevailed during past cold climates. Development of the present features started ca. 3.8–3.9 Ma, but the present morphology was not established until the late Pliocene–early Quaternary (3.2 to older than 2.0 Ma), when the MOW was enhanced, coeval with global cooling, a sea-level fall, and an increase in thermohaline circulation. We propose a direct link between the MOW and the Atlantic Meridional Overturning Circulation and therefore between the MOW and both the Northern Hemisphere and global climate. Our results have enabled a better understanding of a major overflow related to an oceanic gateway, and are of broad interest to geologists, climatologists, oceanographers, and petroleum geologists.

Description: Basement rocks exposed in the Acatlán Complex of the Mixteca terrane in southern Mexico record two tectonothermal events: (1) a Devonian–Mississippian (ca. 365–318 Ma) event, recording extrusion and exhumation of high-pressure rocks; and (2) an Early to Middle Permian (ca. 289–263 Ma) event, involving N-S dextral shearing, transtensional deformation, and local S-vergent thrusting in a magmatic arc environment. We document an additional, regionally significant, tectonothermal event during the Middle to Late Triassic recorded by 40 Ar/ 39 Ar step-heating laser-probe ages ranging from ca. 239 and 219 Ma ( cooling from ca. 525 °C to 300 °C) for amphibole, muscovite, and biotite from: (1) the Carboniferous Amarillo unit, consisting of medium-grade, metasedimentary rocks intruded by mafic dikes; and (2) the Pennsylvanian–Middle Permian, low-grade, clastic-calcareous, arc-related Tecomate Formation. U-Pb laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) data yield an age of 339 ± 6 Ma for the youngest population of detrital zircon grains in the Amarillo unit. Lithogeochemical and Sm-Nd isotopic data for the Amarillo unit dikes are very similar to those of other Carboniferous meta-igneous rocks in the eastern and southwestern part of the Acatlán Complex, displaying affinities transitional between mid-ocean-ridge basalt (MORB) and continental tholeiites, and initial Nd ( t = 339 Ma) values from -6.6 to +6.4, indicating both depleted and enriched mantle sources, as well as variable contamination by continental crust or by subduction-derived fluids. The 40 Ar/ 39 Ar cooling ages coincide with an apparent hiatus in magmatic activity in southern Mexico, which is inferred to record a change from steep to flat subduction.

Description: The timing of onset of modern plate tectonics on Earth is one of the fundamental unsolved problems in geology: How similar were the tectonic processes on early Earth, when the mantle was hotter and the crust more ductile, to those operating today? A key line of evidence for Archean (pre–2.7 Ga) plate tectonics rests on the presence of andesites, intermediate lavas that are the signature rock type of modern subduction zones. The 2.7 Ga Eastern Goldfields superterrane of the Yilgarn craton (herein east Yilgarn craton) in Western Australia is a richly mineral-endowed crustal element that has been a prime focus of debate between proponents of an uniformitarian, plate-tectonic–driven interpretation, and advocates of an alternative model wherein the entire assemblage of igneous rocks is derived ultimately from mantle plume activity. Andesites are a key component of the volcanic stratigraphy and potentially provide critical clues to the evolution of this piece of Archean lithosphere. Whereas east Yilgarn craton andesites have incompatible trace-element characteristics similar to those of modern island-arc andesites, they are distinguished by unusually high Ni, Cr, and MgO contents. Numerical modeling of fractionation of plume-related tholeiitic basalts, coupled with contamination by contemporaneous partial melts of preexisting continental crust, provides a good fit to this feature, along with all of the essential major- and trace-element characteristics of the east Yilgarn craton andesites. Thus, a rock type previously taken as a key line of evidence for plate-tectonic processes in the east Yilgarn craton can be explained just as well by a plume-driven mechanism, which is more consistent with the overwhelmingly plume-derived character of basalts and komatiites across the entire craton. This explains a paradox noted in many pre–2.7 Ga volcanic rock sequences around the world, namely, that apparently subduction-related rocks are interleaved with voluminous basaltic magmatism derived from 1000-km-scale plume-head arrival events. The problem is moot if Archean andesites are products of plume, not subduction-zone, magmatism.

Description: In this study, we utilize multiple thermochronometric methods, including apatite and zircon fission track, (U-Th)/He, and zircon U-Pb, to evaluate the cooling history and provenance of sedimentary strata of the late Carboniferous to Late Permian eastern Paganzo basin and adjacent basement rocks (Argentina). The strata in the study area represent a long-lived, composite basin system that is interpreted to have experienced multiple periods of deformation, and to have received sediment from a number of different source terranes. These strata are well exposed in the Sierra de Chepes of west-central Argentina. New thermochronometric data and field observations, together with published data from the surrounding mountains, allow us to reconstruct: (1) the cooling history of the underlying basement rocks and the highlands surrounding the basin, (2) the thermal history of the source areas that provided sediment to the basin, and (3) the timing of structural inversion of the basin. Our data suggest that parts of the Sierra de Chepes were rapidly exhumed in Late Devonian–Carboniferous times; these exhuming areas supplied sediment to the adjacent basin. In contrast, the overlying red-bed strata originated from a slowly exhuming region located farther east or north of the basin within the Pampean orogenic belt or the Famatinian belt, respectively. Burial by latest Carboniferous and younger strata and an elevated geothermal gradient resulted in heating of the underlying Upper Carboniferous strata and underlying granitoid basement to temperatures between 80 °C and 140 °C. During Triassic time, the eastern Paganzo basin was structurally inverted; this event was marked by rapid cooling and may be related to regional extension and the development of rift basins to the west. The basement and the Upper Paleozoic strata of the eastern Paganzo basin in the study area have remained below 50 °C since latest Jurassic–Early Cretaceous times and are characterized by very slow cooling. Results of this study provide a thermochronometric view along an ~330 m.y. path defining the geologic evolution of the eastern Paganzo basin and the upper crust of west-central Argentina.

Description: Passage of North America over the Yellowstone hotspot has had a profound influence on the topography of the northern Rocky Mountains region. One of the most prominent hotspot-related topographic features is the Yellowstone crescent of high terrain, which consists of two elevated shoulders bounding the eastern Snake River Plain and converging at a topographic swell centered on the Yellowstone region. We have applied single-grain (U-Th)/He dating to apatites (AHe) collected from the Pioneer-Boulder Mountains on the northern arm of the Yellowstone crescent of high terrain to constrain the timing, rates, and spatial distribution of exhumation. These data provide constraints on the timing and processes responsible for uplift related to passage of the hotspot. The Pioneer-Boulder Mountains represent a topographic and structural culmination defined by elevation and by the geometry of preserved strata of the Eocene Challis volcanic province. AHe ages indicate that ≥2–3 km of exhumation has occurred in the core of the Pioneer-Boulder Mountains culmination, where no Challis volcanics are preserved, since ca. 11 Ma. Challis volcanics are extensively preserved and Eocene topographic highs are locally preserved to the north and south of the Pioneer-Boulder Mountains, indicating minimal erosion in those areas. Age-elevation relationships suggest an exhumation rate of ~0.3 mm/yr between ca. 11 and 8 Ma for the culmination core; this relatively rapid interval of exhumation followed a period of 〉30 m.y. during which little to no regional-scale exhumation occurred. Spatial patterns of both exhumation and topography indicate that faulting was not the primary control on uplift and exhumation of the culmination. Instead, NNW-trending normal faults are superimposed on the culmination, with the AHe ages from the footwall of the Copper Creek fault indicating that faulting began at or after ca. 10–9 Ma. Regional exhumation at 11–8 Ma was synchronous with silicic eruptions from the ca. 10.3 Ma Picabo volcanic field located immediately to the south and with S tilting of the southern flank of the Pioneer-Boulder Mountains culmination, which was likely the result of loading of the eastern Snake River Plain by midcrustal mafic intrusions. This synchroneity suggests a causal relationship between hotspot processes and exhumation through potential contributions of flexure and mantle dynamics to uplift, as well as changes in drainage networks and base level.

Description: We synthesized published data on the erosion of the Alpine foreland basin and apatite fission-track ages from the Alps to infer the erosional sediment budget history for the past 5 m.y. The data reveal that erosion of the Alpine foreland basin is highest in front of the western Alps (between 2 and 0.6 km) and decreases eastward over a distance of 700 km to the Austrian foreland basin (~200 m). For the western Alps, erosion rates are 〉0.6 km/m.y., while erosion rates for the eastern foreland basin and the adjacent eastern Alps are 〈0.1 km/m.y., except for a small-scale signal in the Tauern Window. The results yield a large ellipsoidal, orogen-crossing pattern of erosion, centered along the western Alps. We suggest that accelerated erosion of the western Alps and their foreland basin occurred in response to regional-scale surface uplift, related to lithospheric unloading of the Eurasian slab along the Eurasian-Adriatic plate boundary. While we cannot rule out recent views that global climate change led to substantial erosion of the European Alps since 5 Ma, we postulate that regional-scale tectonic processes have driven erosion during this time, modulated by an increased erosional flux in response to Quaternary glaciations.

Description: ABSTRACT The Brunswick magnetic anomaly (BMA) in southern Georgia is coincident with seismic reflectivity marking the deep crustal suture between Laurentia and a crustal block of Gondwanan affinity. The source of the BMA remains enigmatic because of its apparent relationship with both the Permo-Carboniferous Alleghanian orogeny (ca. 315–270 Ma) and the emplacement of the Central Atlantic Magmatic Province (ca. 200 Ma). In this paper, the BMA is modeled using relatively weak (〈0.5 A/m) reversed-polarity remanent magnetization in lower crustal rocks (16–24 km depth) outboard of the Laurentian margin. The acquisition of this magnetic signature is consistent with transpression and strike-slip motion along the margin during the initial stage of Alleghanian convergence, which overlaps with the Kiaman Reversed Superchron (ca. 320–263 Ma). Simple magnetic models show that the onshore segment of the BMA can be explained as an effect of continental collision rather than voluminous magmatism along the suture zone. If Central Atlantic Magmatic Province intrusions were not focused along the suture zone, then evidence for tectonic wedging at the crust-mantle boundary associated with Alleghanian convergence may be preserved along the onshore segment of the BMA, rather than over-printed by Mesozoic magmatism.

Description: The Bouse Formation in the lower Colorado River trough holds an important record of the onset of the modern drainage patterns in the southwestern United States. It comprises calcareous and clastic infill deposited during flooding of several basins, including the Bristol and Blythe subbasins of Lake Bouse. An intercalated ash bed, which is key to constraining its depositional age, is exposed in two locations, Buzzards Peak and Amboy. Comparative zircon tephrochronology by secondary ion microprobe analysis of U-Pb zircon crystallization ages, U-Th trace element abundances, and oxygen isotopic composition confirm a correlation between the Bouse Formation tephra and the 4.834 ± 0.011 Ma Lawlor Tuff ( 40 Ar/ 39 Ar eruption age). Zircon in a coeval tephra associated with the Heise volcanic complex in the Snake River Plain has distinctly lower (by ~4.8, 18 O VSMOW [Vienna standard mean ocean water]) oxygen isotopic compositions than zircon from Bouse tephra, and can be ruled out as a source. The ca. 4.834 Ma depositional age for the Bouse Formation tephra in fine-grained sedimentary beds of the flooded Bristol and Blythe subbasins requires widespread Colorado River inundation in the lower Bouse basins at that time.

Description: The Tualatin basin, west of Portland (Oregon, USA), coincides with a 110 mGal gravity low along the Puget-Willamette lowland. New gravity measurements (n = 3000) reveal a three-dimensional (3-D) subsurface geometry suggesting early development as a fault-bounded pull-apart basin. A strong northwest-trending gravity gradient coincides with the Gales Creek fault, which forms the southwestern boundary of the Tualatin basin. Faults along the northeastern margin in the Portland Hills and the northeast-trending Sherwood fault along the southeastern basin margin are also associated with gravity gradients, but of smaller magnitude. The gravity low reflects the large density contrast between basin fill and the mafic crust of the Siletz terrane composing basement. Inversions of gravity data indicate that the Tualatin basin is ~6 km deep, therefore 6 times deeper than the 1 km maximum depth of the Miocene Columba River Basalt Group (CRBG) in the basin, implying that the basin contains several kilometers of low-density pre-CRBG sediments and so formed primarily before the 15 Ma emplacement of the CRBG. The shape of the basin and the location of parallel, linear basin-bounding faults along the southwest and northeast margins suggest that the Tualatin basin originated as a pull-apart rhombochasm. Pre-CRBG extension in the Tualatin basin is consistent with an episode of late Eocene extension documented elsewhere in the Coast Ranges. The present fold and thrust geometry of the Tualatin basin, the result of Neogene compression, is superimposed on the ancestral pull-apart basin. The present 3-D basin geometry may imply stronger ground shaking along basin edges, particularly along the concealed northeast edge of the Tualatin basin beneath the greater Portland area.

Description: New mapping combined with fault-slip and thermochronological data show that Middle Miocene to recent extension and exhumation of the Slate Range, eastern California, is produced by the active Searles Valley fault system and the Slate Range detachment, an older Middle Miocene low-angle normal fault. Offset Middle Miocene rocks record a combined ~9 km of west-directed extension over the past ~14 m.y. for the fault zones. (U-Th)/He apatite cooling ages of samples from the central and southern Slate Range indicate that footwall cooling began ca. 14 Ma; we interpret this as the age of initiation of motion on the Slate Range detachment. This timing is consistent with inferences made using stratigraphic and structural criteria. Data from the northern Slate Range show that rapid fault slip began along the Searles Valley fault ca. 4 Ma; data from the central and southern Slate Range can be interpreted as indicating cooling at 5–6 Ma. This timing correlates to the results of nearby studies, suggesting a strain transition in the surrounding area between ca. 6 and 3 Ma. The data collected are most consistent with a westward migration in the locus of transtensional deformation, and show that the initiation of that deformation commonly lags the timing predicted by plate reconstructions by a few million years.

Description: In this study we investigate the relationship between the dips of seismic reflectors, which are used to define sequence boundaries, and the orientation (dip and dip direction) of bedding surfaces at core scales. Sequence boundaries from seismic data and lithostratigraphic boundaries from cores and logs are compared with the bedding orientations measured on borehole images of Miocene siliciclastic sediments at Integrated Ocean Drilling Program Expedition 313 Site M28. It is not surprising that bedding orientations show huge variations at scales that are too small to be detected on seismic profiles. However, changes of orientation defined as rotation between two successive intervals match the depths of approximately half of the seismic sequence boundaries. While they do not match boundaries between lithostratigraphic units, changes of orientation frequently correlate with maxima and minima in the gamma ray signal, suggesting that they are related to changes in depositional processes rather than to changes in lithology. This study suggests for the first time that bedding attitudes can be used as a stratigraphic tool at various scales from bed to bed across depth intervals of tens of meters.

Description: The magnitude of late Cenozoic rock uplift of the Sierra Nevada (California, USA) remains unresolved despite more than a century of investigation, with estimates ranging from essentially zero to ~3 km of uplift at the range crest. Two sets of two-dimensional end-member mechanical models bracket how normal faulting along the eastern escarpment of the Sierra Nevada contributed to uplift of the range over a time span of millions of years. The short-term models are based on dislocations in an elastic half-space. The long-term models involve thin elastic beams resting on an inviscid fluid. Both sets of models predict that if the regional topography were entirely a response to faulting along the eastern escarpment, then the bedrock floors immediately east of the range should consistently lie thousands of meters below sea level, instead of thousands of meters above sea level as they generally do. Both sets of analyses indicate that although faulting would lift the range crest, it would drop the rock east of the range-front faults at least as much, and perhaps much more; model results suggest that ~66%–85% of the current escarpment relief stems from subsidence of the grabens east of the Sierra Nevada, with only ~15%–34% resulting from crestal uplift. Our results strongly indicate that range-front faulting in the last 3–10 m.y. uplifted rock at the Sierra Nevada crest by hundreds of meters to as much as 1 km, and that this uplift was superposed on high topography that predated the origin of the eastern escarpment. These conclusions are compatible with diverse geologic observations and measurements.

Description: Alkaline basalts with geochemical features similar to those of intraplate ocean islands have been emplaced along the main trace of the Tepic-Zacoalco rift (TZR), a unique tectonic structure of the western Trans-Mexican Volcanic Belt in which extension is superimposed to a convergent margin. New geochemical and petrologic data on mafic volcanic rocks along the rift indicate the existence of a highly heterogeneous pre-subduction mantle wedge that has been slightly overprinted by slab-derived chemical agents. Most mafic volcanic rocks display geochemical and isotopic compositions that are indistinguishable from those of the Pacific islands Socorro and Isabel, and confirm the existence of an ancient, recycled, high-μ component (HIMU; μ = 238 U/ 204 Pb) in their mantle source. Olivines separated from samples carrying the HIMU signature have NiO and CaO contents similar to olivines from mid-ocean ridge basalt (MORB), indicating that the source of enrichment must be entirely hosted in peridotite. In contrast, more evolved rocks within the TZR have stronger subduction signatures and water contents, and display a distinctive isotopic array that points to slab-derived contributions. Olivines from these rocks are slightly less forsteritic but also extend to higher NiO and lower CaO contents than those from more mafic magmas, suggesting provenance from a secondary pyroxenite source. The overall geochemical evidence thus indicates that the pre-subduction background mantle wedge in western Mexico must be identical, and just as diverse, as that below the Pacific basin. Extension-driven mantle upwelling in a continental setting can only melt a dry peridotitic mantle to its lowest extents, and therefore preferentially sample its most enriched and easily fusible components. Yet the addition of even a small amount of slab-derived silica promotes a secondary petrologic transformation to pyroxene-rich lithologies that upon melting create magmas with compositions that are more akin to a volcanic arc setting.

Description: From the mid-nineteenth through twentieth centuries, geologists attained a good, if imperfect, view of the development of the Colorado River and the Grand Canyon. Beginning in the late 1850s and continuing through the 1880s, fundamental concepts such as fluvialism, antecedence, and superposition were invoked to explain the development of the Colorado River. Early proposals envisioned the Colorado River as "old" relative to the surrounding landscape. Challenges to antecedence were slow to emerge, and it remained the most viable theory into the early twentieth century. At that time two distinct periods (and styles) of erosion were proposed: a plateau cycle with lateral stripping of strata and a canyon cycle of deep, vertical dissection. Beginning in the 1930s, newer ideas proposed that the Colorado River was "young," having been integrated by sequential basin spillover, the timing of which was constrained by interior basin deposits lying across the mouth of the Grand Canyon at the Grand Wash Cliffs (the Muddy Creek constraint). The field entered a period of uncertainty related to the conflicting evidence for an old (Paleogene) river upstream from the Grand Canyon versus a young (Neogene) river at Grand Wash Cliffs. Results from a symposium convened in 1964 offered a solution with a poly-phase history for the Colorado River. The poly-phase theory suggested that the river formed in a complex manner by the integration of two separate drainages, although some aspects became untenable. Efforts to resolve outstanding dilemmas from 1964, such as the ages of the Colorado River and the Grand Canyon, have ultimately led to a modern resurgence in research.

Description: The 1.85 Ga Belomorian Belt, Karelia, Russia, hosts ultralow 18 O and D (as low as –27.3 and –235 standard mean ocean water [SMOW], respectively), high-Al gneisses and amphibolites that we attribute to the Paleoproterozoic "Slushball Earth" glaciation. They now occur in at least 11 localities spanning 450 km. To constrain distribution of 18 O-depleted rocks, we performed detailed field mapping in Khitostrov, where 18 O values are the lowest. Using 430 new and previously published laser fluorination isotope analyses, we show that the elongated, concentrically zoned area of 18 O depletion is greater than 6 x 2 km in areal extent, ~10 times larger than previously thought. Relationships between 17 O versus 18 O strictly adhere to the equilibrium terrestrial mass-dependent fractionation with a slope of 0.527. We also report the results of ion microprobe U-Pb geochronology of zircons coupled with co-registered oxygen isotope spot analyses for mafic intrusions and host gneisses in six localities. The 2.9–2.7 Ga gneiss zircon cores are normal in 18 O (5–7). They show truncated oscillatory cathodoluminescence (CL) patterns and rounded shape indicative of original igneous crystallization with subsequent detrital overprinting. A younger 2.6–2.55 Ga metamorphic zircon domain with normal 18 O, low Th/U, dark cathodoluminescence, and also with rounded crystal morphology is commonly preserved. Cores are surrounded by ubiquitous rims highly depleted in 18 O (re-)crystallized with Svecofennian (1.85–1.89 Ga) ages. Rims are interpreted as metamorphic due to bright and uniform CL and Th/U 〈0.05. Mafic intrusions preserve few igneous zircon crystals between ca. 2.23 and 2.4 Ga in age, but neoblastic zircon in these intrusions originated mostly during 1.85 Ga Svecofennian metamorphism. The 18 O-age relationship for metamorphic rims in zircon and corundum grains suggests that 18 O values of fluids were subtly increasing with time during metamorphism. Large metamorphic corundum grains have ~3 intracrystalline 18 O isotope zonation from –24 to –21, which likely developed during interaction with metamorphic fluids. The Zr-in-rutile geothermometer temperatures are in the range of 760 to 720 °C, in accordance with mineral assemblages and amphibolite metamorphic grade. High and irregular rare-earth element (REE) abundance in cores and rims of many zircons correlates with high phosphorus content and is explained by nanometer-scale xenotime and monazite inclusions, likely in metamict zones during 1.85 Ga Svecofennian metamorphism. A survey of oxygen isotopes in ultrahigh-pressure diamond and coesite-bearing metamorphic terrains around the world reveals normal to high- 18 O values, suggesting that the low 18 O in metamorphic rocks of Dabie Shan, Kokchetav, and in Karelia, are genetically unrelated to metamorphism. We discuss alternative ways to achieve extreme 18 O depletion by kinetic, Rayleigh, and thermal diffusion processes, and by metamorphism. We prefer an interpretation where the low- 18 O and high-Al signature of the rocks predates metamorphism, and is caused by shallow hydrothermal alteration and partial dissolution of the protolith surrounding shallow mafic intrusions by glacial meltwaters during pan-global Paleoproterozoic "Slushball Earth" glaciations between ca. 2.4 and ca. 2.23 Ga.

Description: Detailed field study of the Likhu Khola region in east-central Nepal has provided the basis for new lithological, metamorphic, and structural interpretations of the region. Metamorphic mineral assemblages define an inverted metamorphic field gradient from middle greenschist facies rocks (micaceous phyllitic schist) in the structurally lowest levels observed to upper amphibolite facies migmatitic rocks (sillimanite migmatite) in the structurally highest levels observed in this study. Quartz textures and c -axis orientations indicate that deformational temperatures also generally increase upward in the structural section, ranging from ~490 °C to 〉650 °C. These temperatures are compatible with observed metamorphic mineral assemblages at lower structural levels, indicating that deformation and metamorphism may have been contemporaneous. All rocks in the mapped area are pervasively deformed and typically record a top-to-the-south sense of shear; no discrete large-scale thrust or normal-sense structures were observed. The deformational temperatures recorded, however, are similar to those observed in the immediate hanging wall of the Main Central thrust, the base of the exhumed metamorphic core. The new observations from the Likhu Khola region are compatible with nearby studies that highlight structural, metamorphic, and temporal discontinuities within the exhumed Himalayan mid-crust.

Description: Volcanic rocks near Yampa, Colorado (USA), represent one of several small late Miocene to Quaternary alkaline volcanic fields along the northeast margin of the Colorado Plateau. Basanite, trachybasalt, and basalt collected from six sites within the Yampa volcanic field were investigated to assess correlations with late Cenozoic extension and Rio Grande rifting. In this paper we report major and trace element rock and mineral compositions and Ar, Sr, Nd, and Pb isotope data for these volcanic rocks. High-precision 40 Ar/ 39 Ar geochronology indicates westward migration of volcanism within the Yampa volcanic field between 6 and 4.5 Ma, and the Sr, Nd, and Pb isotope values are consistent with a primary source in the Proterozoic subcontinental lithospheric mantle. Relict olivine phenocrysts have Mg- and Ni-rich cores, whereas unmelted clinopyroxene cores are Na and Si enriched with finely banded Ca-, Mg-, Al-, and Ti-enriched rims, thus tracing their crystallization history from a lithospheric mantle source region to one in contact with melt prior to eruption. A regional synthesis of Neogene and younger volcanism within the Rio Grande rift corridor, from northern New Mexico to southern Wyoming, supports a systematic overall southwest migration of alkaline volcanism. We interpret this Neogene to Quaternary migration of volcanism toward the northeast margin of the Colorado Plateau to record passage of melt through subvertical zones within the lithosphere weakened by late Cenozoic extension. If the locus of Quaternary alkaline magmatism defines the current location of the Rio Grande rift, it includes the Leucite Hills, Wyoming. We suggest that alkaline volcanism in the incipient northern Rio Grande rift, north of Leadville, Colorado, represents melting of the subcontinental lithospheric mantle in response to transient infiltration of asthenospheric mantle into deep, subvertical zones of dilational crustal weakness developed during late Cenozoic extension that have been migrating toward, and subparallel to, the northeast margin of the Colorado Plateau since the middle Miocene. Quaternary volcanism within this northern Rio Grande rift corridor is evidence that the rift is continuing to evolve.

Description: Large-volume travertine deposits in the southeastern Colorado Plateau of New Mexico and Arizona, USA, occur along the Jemez lineament and Rio Grande rift. These groundwater discharge deposits reflect vent locations for mantle-derived CO 2 , which was conveyed by deeply sourced hydrothermal fluid input into springs. U-series dating of stratigraphic sections shows that major aggradation and large-volume (2.5 km 3 ) deposition took place across the region episodically at 700–500 ka, 350–200 ka, and 100–40 ka. These pulses of travertine formation coincide with the occurrence of regional basaltic volcanism, which implies an association of travertine deposits with underlying low-velocity mantle that could supply the excess CO 2 . The calculation of landscape denudation rates based on basalt paleosurfaces shows that travertine platforms developed on local topographic highs that required artesian head and fault conduits. Episodic travertine accumulation that led to the formation of the observed travertine platforms represents conditions when fault conduits, high hydraulic head, and high CO 2 flux within confined aquifer systems were all favorable for facilitating large-volume travertine formation, which was therefore controlled by tectonic activity and paleohydrology. By analogy to the active Springerville–St. Johns CO 2 gas field, the large volumes and similar platform geometries of travertine occurrences in this study are interpreted to represent extinct CO 2 gas reservoirs that were vents for degassing of mantle volatiles into the near-surface system.

Description: The central approximation of plate tectonics is that the plates are rigid, which gives the theory its rigor and predictive power. Space geodetic measurements are consistent with the rigidity of stable plate interiors, but some failures of plate-circuit closure, in particular of oceanic plates, indicate that plates may be measurably non-rigid. We explore the hypothesis that horizontal thermal contraction causes deformation of oceanic plates. Here we show significant expected displacement fields due to thermal contraction for the Pacific plate based on a previously proposed relationship between seafloor age and strain rate and on two end-member assumptions on how strain compatibility is enforced. The predicted maximum 2.2 mm/yr southeastward motion of the northeastern part of the plate relative to the Pacific-Antarctic Rise may contribute to a large part of the non-closure of the Pacific–North America plate motion circuit. Our predicted displacement rates cannot (yet) be confirmed by current space geodetic data and will require seafloor geodesy with 1 mm/yr accuracy. The spatial distribution of predicted moment rate agrees reasonably well with that of intraplate earthquake epicenters, similar to what is observed for plate boundary zones. Our results suggest that plate-scale horizontal thermal contraction is significant, and that it may be partly released seismically.

Description: The mid-lithospheric discontinuity (MLD) is a seemingly sharp decrease in seismic velocity at depths internal to the lithosphere and appears to be a pervasive feature beneath continental interiors. Its presence within cratons, which have remained relatively stable since formation, suggests that the MLD may result from processes associated with continent formation. We use P- to S-wave receiver functions to interrogate seismic anisotropy across the MLD within the ca. 1.35–1.55 Ga Granite-Rhyolite Province of the central United States. Our analysis reveals strong evidence for sharp changes in the orientation of anisotropy across multiple MLDs, with an approximately north to northwest fast orientation of anisotropy in the upper lithosphere. The consistency of this signature over a large region suggests that the observed anisotropy is a relic of North American craton formation. In addition, the presence of several distinct anisotropic layers within the cratonic lithosphere supports models for craton formation via stacked subducted slabs or a series of underthrusting events.

Description: Volcanic ash layers preserved within the geologic record represent precise time markers that correlate disparate depositional environments and enable the investigation of synchronous and/or asynchronous behaviors in Earth system and archaeological sciences. However, it is generally assumed that only exceptionally powerful events, such as supereruptions (≥450 km 3 of ejecta as dense-rock equivalent; recurrence interval of ~10 5 yr), distribute ash broadly enough to have an impact on human society, or allow us to address geologic, climatic, and cultural questions on an intercontinental scale. Here we use geochemical, age, and morphological evidence to show that the Alaskan White River Ash (eastern lobe; A.D. 833–850) correlates to the "AD860B" ash (A.D. 846–848) found in Greenland and northern Europe. These occurrences represent the distribution of an ash over 7000 km, linking marine, terrestrial, and ice-core records. Our results indicate that tephra from more moderate-size eruptions, with recurrence intervals of ~100 yr, can have substantially greater distributions than previously thought, with direct implications for volcanic dispersal studies, correlation of widely distributed proxy records, and volcanic hazard assessment.

Description: Remotely operated and autonomous underwater vehicle technologies were used to image and sample exceptional deep sea outcrops where an ~100-m-thick section of turbidite beds is exposed on the headwalls of two giant submarine scours on Eel submarine fan, offshore northern California (USA). These outcrops provide a rare opportunity to connect young deep-sea turbidites with their feeder system. 14 C measurements reveal that from 12.8 ka to 7.9 ka, one turbidite was being emplaced on average every 7 yr. This emplacement rate is two to three orders of magnitude higher than observed for turbidites elsewhere along the Pacific margin of North America. The turbidites contain abundant wood and shallow-dwelling foraminifera, demonstrating an efficient connection between the Eel River source and the Eel Fan sink. Turbidite recurrence intervals diminish fivefold to ~36 yr from 7.9 ka onward, reflecting sea-level rise and re-routing of Eel River sediments.

Description: The Permian-Triassic boundary extinction event was the largest biological crisis of the Phanerozoic. One of the principle triggers for the mass extinction is thought to be greenhouse warming resulting from the release of CH 4 from basalt-coal interaction during the extensive Siberian Traps (Russia) eruptions. Observations of organic matter interpreted to be coal combustion products (fly ash) in latest Permian marine sediments have been used to support this hypothesis. However, this interpretation is dependent upon vesicular chars being fly ash (coal combustion derived) and not formed by alternative mechanisms. Here we present reflectance microscopy images of vesicular chars from Russian Permian coals, and chars from modern tundra, peatland, and boreal forest fires, to demonstrate that despite a difference in precursor fuels, wildfires are capable of generating vesicular chars that are morphologically comparable to end-Permian fly ash. These observations, coupled with extensive global evidence of wildfires during this time interval, call into question the contribution of coal combustion to the end-Permian extinction event.

Description: We present high-resolution field measurements of five sites along the United States Atlantic Coast, and cellular automata simulations, to investigate the erosion of marsh boundaries by wave action. According to our analysis, when salt marshes are exposed to high wave energy conditions their boundaries erode uniformly. The resulting erosion events follow a Gaussian distribution, yielding a relatively smooth shoreline. On the contrary, when wind waves are weak and the local marsh resistance is strong, jagged marsh boundaries form. In this case, erosion episodes have a long-tailed frequency magnitude distribution with numerous low-magnitude events, but also high-magnitude episodes. The logarithmic frequency magnitude distribution suggests the emergence of a critical state for marsh boundaries, which would make the prediction of failure events impossible. Internal physical processes allowing salt marshes to reach this critical state are geotechnical and biological, and related to the nonhomogeneity of salt marshes whose material discontinuities act as stress raisers.

Description: Subduction of mid-ocean ridges is a common feature in recent convergent margins, but is rarely documented in Proterozoic to Paleozoic orogenic belts. Here we describe evidence for ridge-trench interaction in the deeply eroded late Neoproterozoic Damara orogenic belt, central Namibia. The earliest interaction is indicated by primary intrusive contacts between amphibolite facies mid-ocean ridge metabasalts and trench metasediments. U-Pb zircon ages of 550–540 Ma from syntectonic granites in the forearc indicate the timing of partial melting and mafic underplating of the prism in response to ridge subduction. The thermal peak in the Damara belt, associated widespread granitic and alkalic plutonism, and hydrothermal activity coincide with the waning stages of tectonism at 530–520 Ma and are interpreted to indicate slab window widening and slab delamination. We suggest that the proposed two-stage thermal evolution of the Damara belt, comprising latest Neoproterozoic ridge subduction and early Cambrian slab delamination, represents a fingerprint of ridge subduction in ancient orogens.

Description: The Farewell terrane of western Alaska is one of the more remote and understudied crustal fragments in the North American Cordillera. Although it is generally accepted that the oldest, Precambrian parts of the Farewell terrane originated along the Arctic margin (i.e., Siberia), the paleogeographic history of the Farewell terrane during much of the middle and late Paleozoic remains unknown. Here, we present new sedimentologic and provenance data from upper Paleozoic clastic strata of the Mystic subterrane, which represents the youngest part of the Farewell terrane. Sedimentary facies consist of high- and low-density sediment-gravity-flow deposits and are interpreted to represent a submarine fan depositional system. Sandstone modal composition trends show a relative abundance of lithic volcanic fragments (~65%) and subordinate occurrences of lithic sedimentary fragments (~15%) and chert (~13%). Laser-ablation–inductively coupled plasma–mass spectrometry analyses of detrital zircons reveal a bulk U-Pb age distribution of Precambrian–Paleozoic grains. U-Pb detrital zircon age spectra from Mississippian strata have a primary peak age between 400 and 325 Ma and secondary peak ages between 480 and 415 Ma and 2000 and 1800 Ma. Devonian–Mississippian zircons exhibit enriched Hf isotopic values (–3 to –35), whereas Ordovician–Silurian zircons have both enriched (–5 to –25) and depleted (+5 to +14) Hf values. Age spectra from Permian strata show primary peaks between 320 and 275 Ma and 460 and 415 Ma, with isolated occurrences of Precambrian-age zircons. Pennsylvanian–Permian zircons exhibit depleted Hf values (+2 to +14). Youngest peak ages support a Mississippian–Early Permian maximum depositional age for this part of the Mystic subterrane. Overall, provenance trends reflect primary detrital contributions from arc and recycled orogen source areas, which included both enriched and primitive magmatic sources. New U-Pb and Hf isotope analyses from the Mystic assemblage match most closely with magmatic source areas of the Alexander and Wrangellia terranes. Findings are consistent with a model where the Farewell terrane was proximal to both the Alexander and Wrangellia terranes by Mississippian–Permian time.

Description: The exhumed Himalayan midcrustal core exposed in the Likhu Khola region of east-central Nepal includes upper-greenschist- to upper-amphibolite-grade metamorphic rocks that record pervasive, top-to-the-south sense deformation. Metamorphic temperature estimates are within error across the mapped area ranging from 772 ± 37 °C in the structurally lower, southern part of the study area to 853 ± 58 °C in the structurally higher, northern area. Estimated metamorphic pressures are relatively constant at lower structural levels, but they decrease from 11.8 ± 1.4 kbar to a minimum of 6.5 ± 1.3 kbar up structural section. The decrease in pressures coincides with an abrupt change in pressure estimates up structural section that is interpreted to mark a tectonometamorphic discontinuity that separates two domains with distinct structural, thermal, and metamorphic histories. In situ laser-ablation split-stream monazite U-Th/Pb and rare earth element (REE) petrochronology outlines dates ranging from ca. 27.8 Ma to 15.1 Ma in the hanging wall of the interpreted discontinuity; monazite REE data indicate the spread in ages is the result of a protracted metamorphic history and late-stage anatexis. Metamorphic and petrochronologic data from the Likhu Khola are consistent with a kinematic model in which material structurally above the discontinuity was metamorphosed in the deep hinterland of the orogen and was subsequently incorporated into the foreland of the orogen. The transition from hinterland- to foreland-style processes was marked by a shift to discrete deformation and the development of the discontinuity. Movement across the discontinuity is interpreted to have driven metamorphism and deformation of the rocks structurally below at or after 15 Ma. Discontinuities similar to that identified in this study are being identified and described across the orogen, indicating they are important, orogenwide features.

Description: Normal faults occur in the Niobrara Chalk and Pierre Shale in numerous localities in South Dakota, Nebraska, and Kansas. Lakeshore outcrops provide a window into strata that otherwise outcrop poorly. Various criteria were used to distinguish mass-wasting features from faults that formed at the subsurface. Locally, these faults have variable orientations and throws that vary from centimeters to tens of meters. In the Niobrara Chalk, the fault zones typically display well-developed striae, occasional slickensides, limited to absent damage zones, and dilational jogs with coarse, microvein calcite fill. In places, the faults are associated with gentle monoclines and folds, but elsewhere they occur in horizontal strata. A weathering bias suggests these faults are more common than observed. At any one locality, these faults have been or could be attributed to local tectonism, differential compaction, and/or glacial rebound. If due to tectonism, taken in aggregate, they indicate fairly widespread (albeit low-strain) activity, which is consistent with the idea of a critically stressed continental interior with migrating sites of faulting. An alternate and preferred hypothesis is that these are polygonal faults due to diagenetically driven deformation in fine-grained mud rocks. Subsurface polygonal fault systems have been described from the Niobrara Chalk and Pierre Shale in the Denver-Julesburg Basin area in eastern Colorado. Smectitic clays, associated with polygonal faulting elsewhere, are abundant in these units. The fault kinematics (extension in multiple directions), widespread character, and other traits are consistent with an origin by diagenetically driven deformation.

Description: U-Pb ages ( n = 403) of detrital zircons from the Dakota Formation in western Iowa and eastern Nebraska provide evidence for westward-flowing fluvial systems that stretched from the Appalachian highlands to the western U.S. Cordilleran foreland basin during Albian–Cenomanian time. Approximately 78% of detrital zircon grains match the ages of Grenvillian (1.3–1.0 Ga), Pan-African (750–500 Ma), and Paleozoic (500–310 Ma) bedrock sources located within the present-day Appalachian Mountains. The presence of minor detrital zircon grains of Paleoproterozoic (2.5–1.5 Ga) or Archean age (〉2.5 Ga) indicates that northern source regions in Minnesota, Wisconsin, and Canada did not contribute a significant volume of sediment, as had been previously interpreted. Based on similarities between detrital zircon signatures in the midcontinent strata and time-equivalent Cordilleran foreland basin strata, Appalachian sources may have contributed a previously unrecognized volume of sediment to the Albian–Cenomanian foreland basin system. Sediment flux from the Appalachian region to the Cordilleran foreland basin during middle Cretaceous time may have been related to increased uplift and exhumation due to passage over a mantle plume track.

Description: The elevation of continental interiors over time is demonstrably variable. A major part of change in elevation within the continental interior is likely driven by density changes within the upper mantle and by global mantle convection. For example, upper-mantle flow has been invoked as the cause of Neogene uplift of the interior Rocky Mountains and Colorado Plateau, warping and tilting sediment transport slopes that link to the widespread deposition of gravel units within the Great Plains. These geomorphic and sedimentologic features, however, can also be generated by an increase in runoff, since erosion will promote change in elevation due to isostatic compensation and the loading of the lithosphere by the deposition of sediment. To explore the consequences of change in topography and climate, we use a general length-dependent diffusive sediment transport law to model both erosion and deposition that includes the concentrative effects of river systems. The simplicity of the approach means that we can collapse sediment transport to one dimension and couple erosion and deposition with plate flexure. We find that for a landscape that is gently tilted (slope of order of 10 –3 ), a change in runoff has a minor effect on transport gradient, as sediment transport and associated flexural response maintain topography at a similar elevation. However, there can be a significant change in depositional style when the degree of tilt is altered by, for example, a local change in upper-mantle density. An increase in buoyancy within the upper mantle, which increases slopes, leads to a transient reduction in grain sizes deposited at a fixed location. This behavior is due to a temporary retreat of the zone of erosion into the catchment and a transient increase in accommodation space relative to sediment supply. A reduction in tilt has the opposite effect, the older deposits are eroded, and the erosion-deposition transition rapidly moves down system. There is convincing evidence that the formation of thin and laterally extensive conglomeratic units of the Great Plains was due to a reduced rate of subsidence. Based on the results of our model, we suggest that the deposition of widespread conglomeratic units within continental interiors is generally a consequence of a reduction in slope, as the dynamic support for regions of high topography is reduced.

Description: We present a comprehensive petrologic study of lower-crust mafic and felsic xenoliths hosted by Quaternary alkaline basalts of the Valle de Santiago monogenetic volcanic field. This is the only locality along the entire Trans-Mexican volcanic belt where the abundance and size of xenoliths allow the understanding in great detail of processes associated with interactions of young subduction-related magmatism and the deep continental crust. Mafic xenoliths (two pyroxene ± spinel granulites and metanorthosites), olivine-rich gabbroic xenoliths, and transitional xenoliths compose the bulk of the population, although a few belong to the charnockitic suite (enderbite and faersundite). Thermobarometric calculations (two-pyroxene, ilmenite-magnetite, Ti-in amphibole, amphibole-plagioclase, and phase equilibria in the system NCMAS (Na-Ca-Mg-Si)) result in pressures around 9 kbar and temperatures of 1000–1100 °C for the granulite-facies metamorphism, which would give a very hot lower crust, ~33 km thick, beneath Valle de Santiago and a mean geothermal gradient of ~30 °C/km. Igneous zircons (Th/U = 0.03–0.87) extracted from one of the felsic granulites yielded a major peak of latest Cretaceous age (67.1 Ma), interpreted as the crystallization age of the granitic protolith, without inheritance from Precambrian or Paleozoic crust. Minor peaks at 45.1 and 25.5 Ma are interpreted as partial Pb losses from some of the Cretaceous zircons. Trace-element geochemistry, as well as Sr, Nd, and Pb isotopic studies performed on two granulites, is consistent with the juvenile and coeval nature of both the mafic metagabbroic xenoliths and the alkaline basaltic magmas that lifted the xenoliths from the lower crust. Two intermediate stages in the thermal evolution of the sampled xenoliths include the emplacement at different depths of volatile and K-Fe-Ti–rich oxidized melts represented by igneous assemblages with kaersutite, biotite, titanomagnetite, spinel, plagioclase, Fe-rich epidote, clinopyroxene, fayalitic olivine, and glasses that pervasively invaded most granulite xenoliths before being taken to the surface. A preferred plumbing system model is presented depicting a protracted Miocene to Quaternary basaltic intraplate magmatic system that sampled former basaltic batches stationed in the lower crust, together with the Late Cretaceous deep-seated granitoids beneath Valle de Santiago in the backarc of the central Trans-Mexican volcanic belt. Both components were later subjected to granulite-facies conditions in the lower crust, most probably related to the continued heating of the crust by basaltic magmas underplated in the central Trans-Mexican volcanic belt backarc region.

Description: Three main rivers—the Ganges, Brahmaputra, and Meghna—coalesce in the Bengal basin to form the world’s largest delta system, which serves as filter and gateway between the Himalayan collision and vast Bengal fan repository. New insights into the Holocene construction of the Ganges-Brahmaputra-Meghna delta, with a focus on river sedimentation, channel migration, and avulsion history, are presented here using the Sr geochemistry of bulk sediments as a provenance tracer. The sediment load of each river transmits a distinct Sr signature owing to differences in source rocks from the Himalaya, Tibet, and local regions, allowing for effective tracking of river channels and stratigraphic development within the delta. In the early Holocene, vigorous delta aggradation occurred under rapid sea-level rise and high river discharge and supported the construction of sand-dominated stratigraphy by laterally mobile, braided-stream channels. However, the vertically (i.e., temporally) uniform, but geographically distinct, Sr signatures from these deposits indicate that the Ganges, Brahmaputra, and Meghna fluvial systems remained isolated from one another and apparently constrained within their lowstand valleys. By the mid-Holocene, though, delta stratigraphy records spatially and temporally nonuniform Sr signatures that hallmark the onset of avulsions and unconstrained channel migration, like those that characterize the modern Ganges and Brahmaputra fluvial systems. Such mobility developed in the mid-Holocene despite declining discharge and sea-level rise, suggesting that earlier channel behavior had been strongly influenced by antecedent topography of the lowstand valleys. It is only after the delta had aggraded above the valley margins that the fluvial systems were able to avulse freely, resulting in numerous channel reorganizations from mid-Holocene to present. These channel-system behaviors and their role in delta evolution remain coarsely defined based only on this initial application of Sr-based provenance tools, but the approach is promising and suggests that a more complete understanding can be achieved with continued study.

Description: The extremely high viscosities of high-silica liquids mean that intermediate and high-silica magmas exist deep within the laminar range of fluid behavior. Many common interpretations of layering in plutonic rocks rely on analogies with sedimentation in water, but for such high-viscosity liquids (including interstitial liquid in partially crystallized intermediate magmas), these analogies cannot be correct. Particle Reynolds numbers for high-silica liquids are exceedingly small, on the order of 10 –6 or less. In such a regime, inertia is negligible, and everyday experience is inapplicable. Layering and other pseudo-sedimentary structures in granodiorites and granites likely arise from processes such as chemical diffusion, self-organization, and crystal ripening.