New Geology Articles Published Online Ahead of Print in April

Boulder, Colo., USA: Thirty-one new articles were published online ahead of print for Geology in April. Topics include shocked zircon from the Chicxulub impact crater; the Holocene Sonoran Desert; the architecture of the Congo Basin; the southern Death Valley fault; missing water from the Qiangtang Basin; sulfide inclusions in diamonds; how Himalayan collision stems from subduction; ghost dune hollows; and the history of the Larsen C Ice Shelf. These Geology articles are online at .

Nonlinear fault damage zone scaling revealed through analog modeling
Sylvain Mayolle; Roger Soliva; Stéphane Dominguez; Christopher Wibberley; Yannick Caniven
Abstract: Fault damage zones strongly influence fluid flow and seismogenic behavior of faults and are thought to scale linearly with fault displacement until reaching a threshold thickness. Using analog modeling with different frictional layer thicknesses, we investigate damage zone dynamic evolution during normal fault growth. We show that experimental damage zone growth with displacement is not linear but progressively tends toward a threshold thickness, being larger in the thicker models. This threshold thickness increases significantly at fault segment relay zones. As the thickness threshold is approached, the failure mode progressively transitions from dilational shear to isochoric shear. This process affects the whole layer thickness and develops as a consequence of fault segment linkage as inferred in nature when the fault matures. These findings suggest that fault damage zone widths are limited both by different scales of mechanical unit thickness and the evolution of failure modes, ultimately controlled in nature by lithology and deformation conditions.
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History of the Larsen C Ice Shelf reconstructed from sub–ice shelf and offshore sediments
J.A. Smith; C.-D. Hillenbrand; C. Subt; B.E. Rosenheim; T. Frederichs ...
Abstract: Because ice shelves respond to climatic forcing over a range of time scales, from years to millennia, an understanding of their long-term history is critically needed for predicting their future evolution. We present the first detailed reconstruction of the Larsen C Ice Shelf (LCIS), eastern Antarctic Peninsula (AP), based on data from sediment cores recovered from below and in front of the ice shelf. Sedimentologic and chronologic information reveals that the grounding line (GL) of an expanded AP ice sheet had started its retreat from the midshelf prior to 17.7 ± 0.53 calibrated (cal.) kyr B.P., with the calving line following ~6 k.y. later. The GL had reached the inner shelf as early as 9.83 ± 0.85 cal. kyr B.P. Since ca. 7.3 ka, the ice shelf has undergone two phases of retreat but without collapse, indicating that the climatic limit of LCIS stability was not breached during the Holocene. Future collapse of the LCIS would therefore confirm that the magnitudes of both ice loss along the eastern AP and underlying climatic forcing are unprecedented during the past 11.5 k.y.
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Confirmation of slow Ti diffusion in quartz by diffusion couple experiments and evidence from natural samples
Andreas Audétat; Nobuyoshi Miyajima; Dorothea Wiesner; Jean-Nicolas Audinot
Abstract: Titanium diffusion profiles in natural quartz crystals have become an increasingly popular tool to reconstruct the time scales of various magmatic, metamorphic, and hydrothermal processes. However, the original calibration of Ti diffusion rates in quartz has recently been challenged, and diffusivities were found to be about three orders of magnitude lower. We performed annealing experiments on crystal-crystal diffusion couples consisting of Ti-free synthetic quartz seeds over which Ti-rich quartz (100–3000 μg/g Ti) was grown hydrothermally. The annealing experiments were performed at 1000–1600 °C and 0.1 MPa to 2.0 GPa, and they lasted for 3–84 days. The resulting diffusion profiles were mapped by cathodoluminescence (CL), transmission electron microscope–energy-dispersive X-ray spectroscopy (TEM-EDXS), and, for the first time, by helium ion microscope–secondary ion mass spectrometry (HIM-SIMS). Obtained diffusion coefficients range from values similar to the lower range in previous research to values up to two orders of magnitude lower. In addition, inversely zoned quartz and sanidine phenocrysts in a natural rhyolite were studied. Comparison of the diffusion profiles suggests that at ~735 °C, the Ti diffusivity in quartz is ~1.5 and 3.0 orders of magnitude lower than that of Ba and Sr, respectively, in sanidine. The combined evidence confirms that Ti diffusion in quartz is very slow, potentially even slower than proposed earlier. Consequently, previous time scales derived from Ti diffusion profiles in quartz are likely orders of magnitude too short, and further experiments are necessary to fully clarify the issue.
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Did accretion of the Caribbean oceanic plateau drive rapid crustal thickening in the northern Andes?
Sarah W.M. George; Brian K. Horton; Cristian Vallejo; Lily J. Jackson; E. Gabriela Gutierrez
Abstract: The onset of orogenic shortening in the northern Andes Mountains coincided with latest Cretaceous accretion of the Caribbean oceanic plateau. We present isotopic data (εHf in zircon and εNd) coupled with arc position to test whether accretion led to abrupt crustal thickening in the northern Andes of Ecuador and Colombia. A rapid isotopic excursion toward more evolved crustal compositions was synchronous with ca. 75–70 Ma collision in Ecuador and preceded a similar deviation in Colombia at ca. 70–55 Ma. The rapid but diachronous shift to more evolved isotopic signatures is attributed to progressive northward accretion of the oceanic plateau and associated thickening of continental crust. We emphasize the effects of accretion on the magmatic evolution of Cordilleran-type margins, initially provoking shortening and crustal thickening, and ultimately providing a substrate for subsequent arcs.
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How Himalayan collision stems from subduction
M. Soret; K.P. Larson; J. Cottle; A. Ali
Abstract: The mechanisms and processes active during the transition from continental subduction to continental collision at the plate interface are largely unknown. Rock records of this transition are scarce, either not exposed or obliterated during subsequent events. We examine the tectono-metamorphic history of Barrovian metamorphic rocks from the western Himalayan orogenic wedge. We demonstrate that these rocks were buried to amphibolite-facies conditions from ≤47 Ma to 39 ± 1 Ma, synchronously with the formation (46 Ma) and partial exhumation (45–40 Ma) of the ultrahigh-pressure eclogites. This association indicates that convergence during continental subduction was accommodated via development of a deep orogenic wedge built through successive underplating of continental material, including the partially exhumed eclogites, likely in response to an increase in interplate coupling. This process resulted in the heating of the subduction interface (from ~7 to ~20 °C/km) through advective and/or conductive heat transfer. Rapid cooling of the wedge from 38 Ma, coeval with the formation of a foreland basin, are interpreted to result from indentation of a promontory of thick Indian crust.
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Mud in sandy riverbed deposits as a proxy for ancient fine-sediment supply
N. Wysocki; E. Hajek
Abstract: The amount of silt and clay available to rivers reflects source-terrain composition and weathering and can be a primary control on the form and dynamics of channel networks. Fine sediment also affects the permeability of buried fluvial reservoirs. Despite this significance, there is currently a lack of methods for reconstructing how much fine sediment was transported by ancient rivers. Mud accumulations in sandy river deposits are often interpreted as indicators of variable flow conditions; however, these deposits may present an opportunity to constrain how much fine sediment was transported through ancient rivers. We report results from a series of experiments designed to evaluate how much clay and silt are preserved in sandy riverbed deposits under constant and variable discharge conditions. Our results demonstrate that (1) mud deposits, including drapes and lenses, form readily under constant, high-discharge conditions, (2) the amount of fine sediment recovered from bed-material deposits increases as fine-sediment supply increases, and (3) fine-sediment retention is higher during bed aggradation than during bypass conditions. These results indicate that the net retention of clay and silt in sandy riverbed deposits may be a simple but powerful proxy for comparing the overall amount of fine sediment supplied to ancient rivers.
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Subduction of the Izanagi-Pacific Ridge–transform intersection at the northeastern end of the Eurasian plate
Toru Yamasaki; Gen Shimoda; Kenichiro Tani; Jinichiro Maeda; Futoshi Nanayama
Abstract: Recent reconstructions of global plate motions suggest that the Izanagi-Pacific Ridge was subducted along the eastern margin of Eurasia at ca. 50 Ma. In the Hidaka magmatic zone (HMZ), which was located at the northeastern end of the Eurasian plate, three magmatic pulses occurred (46–45, 40–36, and 19–18 Ma). We report whole-rock geochemical and Sr-Nd-Pb isotopic data for 36 Ma high-Sr/Y (adakitic) rocks from the HMZ and show that these rocks formed by partial melting of oceanic crust and were emplaced as near-trench intrusions during ridge subduction. We reevaluate the nature of plutonic rocks in the HMZ and show that both the 46–45 and 40–36 Ma granitoids have essentially identical geochemical features. The distribution of plutons and magmatic cessation between 45 and 40 Ma are best explained by subduction of a ridge-transform intersection with a large offset of the ridge axis. The boundary between the Eocene granitoids corresponds to the position of a paleo–transform fault, and adakitic magmatism was caused by partial melting triggered by slab tearing at an overlapping spreading center. The paleoridge-transform configuration coincides with the locations of later large faults and a peridotite body.
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Climatically driven instability of marine methane hydrate along a canyon-incised continental margin
Richard J. Davies; Miguel Ángel Morales Maqueda; Ang Li; Mark Ireland
Abstract: Establishing how past climate change affected the stability of marine methane hydrate is important for our understanding of the impact of a future warmer world. As oceans shallow toward continental margins, the base of the hydrate stability zone also shallows, and this delineates the feather edge of marine methane hydrate. It is in these rarely documented settings that the base of the hydrate stability zone intersects the seabed and hydrate can crop out where it is close to being unstable and most susceptible to dissociation due to ocean warming. We show evidence for a seismically defined outcrop zone intersecting canyons on a canyon-incised margin offshore of Mauritania. We propose that climatic, and hence ocean, warming since the Last Glacial Maximum as well as lateral canyon migration, cutting, and filling caused multiple shifts of the hydrate stability field, and therefore hydrate instability and likely methane release into the ocean. This is particularly significant because the outcrop zone is longer on canyon-incised margins than on less bathymetrically complex submarine slopes. We propose considerably more hydrate will dissociate in these settings during future ocean warming, releasing methane into the world’s oceans.
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Enhanced Quaternary exhumation in the Namche Barwa syntaxis, eastern Himalaya
Rong Yang; Frédéric Herman; Ting Liu; Rabiul Haque Biswas; Maria Giuditta Fellin ...
Abstract: The Namche Barwa syntaxis in the eastern Himalaya is rapidly evolving in terms of its tectonics and topography. Here we constrain the exhumation history of the Yigong River to the immediate north of the syntaxis across different time scales using a multidisciplinary approach. Our new thermochronometric data reveal an acceleration of exhumation rates since 2 Ma in the downstream of the Yigong. Cosmogenic nuclides and thermoluminescence thermochronometry analyses confirm persistent rapid exhumation in the lower Yigong over the Quaternary with further increased exhumation in the last 100 ka. Together with the analysis of the morphology of the Yigong River profile, we interpret that northward expansion of the syntaxis together with capture of the Yigong by the Yarlung Tsangpo River during this expansion is responsible for the exhumation history of the Yigong River in the Quaternary.
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Protogenetic sulfide inclusions in diamonds date the diamond formation event using Re-Os isotopes
M.G. Pamato; D. Novella; D.E. Jacob; B. Oliveira; D.G. Pearson ...
Abstract: Sulfides are the most abundant inclusions in diamonds and a key tool for dating diamond formation via Re-Os isotopic analyses. The manner in which fluids invade the continental lithospheric mantle and the time scale at which they equilibrate with preexisting (protogenetic) sulfides are poorly understood yet essential factors to understanding diamond formation and the validity of isotopic ages. We investigated a suite of sulfide-bearing diamonds from two Canadian cratons to test the robustness of Re-Os in sulfide for dating diamond formation. Single-crystal X-ray diffraction (XRD) allowed determination of the original monosulfide solid-solution (Mss) composition stable in the mantle, indicating subsolidus conditions of encapsulation, and providing crystallographic evidence supporting a protogenetic origin of the inclusions. The results, coupled with a diffusion model, indicate Re-Os isotope equilibration is sufficiently fast in sulfide inclusions with typical grain size, at mantle temperatures, for the system to be reset by the diamond-forming event. This confirms that even if protogenetic, the Re-Os isochrons defined by these minerals likely reflect the ages of diamond formation, and this result highlights the power of this system to date the timing of fluid migration in mantle lithosphere.
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New statistical quantification of the impact of active deformation on the distribution of submarine channels
Marco Pizzi; Alexander C. Whittaker; Lidia Lonergan; Mike Mayall; W. Hamish Mitchell
Abstract: Submarine channel systems play a crucial role in governing the delivery of sediments and pollutants such as plastics from the shelf edge to deep water. Understanding their distribution in space and time is important for constraining the locus, magnitude, and characteristics of deep-water sedimentation and for predicting stratigraphic architectures and depositional facies. Using three-dimensional seismic reflection data covering the outer fold-and-thrust belt of the Niger Delta, we determined the pathways of Miocene to Pliocene channels that crossed, at 173 locations, 11 fold-thrust structures for which the temporal and spatial evolution of strain rates has been constrained over a period of 11 m.y. We use a statistical approach to quantify strain and shortening rate distributions recorded where channels have crossed structures compared to the fault array as a whole. Our results prove unambiguously that these distributions are different. The median strain rate where channels cross faults is <0.6%/m.y. (~40 m/m.y.), 2.5× lower than the median strain rate of active fault segments (1.5%/m.y.) with a marked reduction in the number of channel-fault crossings where fault strain rates are >1%/m.y. Our results quantify the sensitivity of submarine channels to active deformation at a population level for the first time and enable us to predict the temporal and spatial routing of submarine channels affected by structurally driven topography.
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Paleozoic evolution of crustal thickness and elevation in the northern Appalachian orogen, USA
Ian W. Hillenbrand; Michael L. Williams
Abstract: The Acadian and Neoacadian orogenies are widely recognized, yet poorly understood, tectono-thermal events in the New England Appalachian Mountains (USA). We quantified two phases of Paleozoic crustal thickening using geochemical proxies. Acadian (425–400 Ma) crustal thickening to 40 km progressed from southeast to northwest. Neoacadian (400–380 Ma) crustal thickening was widely distributed and varied by 30 km (40–70 km) from north to south. Doubly thickened crust and paleoelevations of 5 km or more support the presence of an orogenic plateau at ca. 380–330 Ma in southern New England. Neoacadian crustal thicknesses show a strong correlation with metamorphic isograds, where higher metamorphic grade corresponds to greater paleo-crustal thickness. We suggest that the present metamorphic field gradient was exposed through erosion and orogenic collapse influenced by thermal, isostatic, and gravitational properties related to Neoacadian crustal thickness. Geobarometry in southern New England underestimates crustal thickness and exhumation, suggesting the crust was thinned by tectonic as well as erosional processes.
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Diapiric relamination of the Orocopia Schist (southwestern U.S.) during low-angle subduction
James B. Chapman
Abstract: The Orocopia Schist and related schists are sediments subducted during the Laramide orogeny and are thought to have been underplated as a laterally extensive layer at the base of the crust in the southwestern United States Cordillera. This concept is hard to reconcile with the existence of continental mantle lithosphere in southeastern California and western Arizona. Analytical solutions and numerical modeling suggest that the Orocopia Schist may have ascended through the mantle lithosphere as sediment diapirs or subsolidus crustal plumes to become emplaced in the middle to lower crust. Modeled time-temperature cooling paths are consistent with the exhumation history of the Orocopia Schist and explain an initial period of rapid cooling shortly after peak metamorphism. The Orocopia Schist represents a potential example of relaminated sediment observable at the surface.
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Annually resolved sediments in the classic Clarkia lacustrine deposits (Idaho, USA) during the middle Miocene Climate Optimum
Daianne Höfig; Yi Ge Zhang; Liviu Giosan; Qin Leng; Jiaqi Liang ...
Abstract: The world-renowned Miocene Clarkia paleolake in northern Idaho (USA) is closely associated with Columbia River Basalt Group volcanism. The flood basalt dammed a local drainage system to form the paleolake, which preserved a plant fossil Lagerstätte in its deposits. However, the precise age and temporal duration of the lake remain unsettled. We present the first unequivocal U-Pb zircon ages from interbedded volcanic ashes at the P-33 type location, constraining the deposition to 15.78 ± 0.039 Ma. Using micro–X-ray fluorescence and petrographic and spectral analyses, we establish the annual characteristics of laminations throughout the stratigraphic profile using the distribution of elemental ratios, mineral assemblages, and grain-size structures, as well as organic and fossil contents. Consequently, the ~7.5-m-thick varved deposit at the type location P-33 represents ~840 yr of deposition, coincident with the end of the main phase of Columbia River Basalt Group eruptions during the Miocene Climate Optimum. The timing and temporal resolution of the deposit offer a unique opportunity to study climate change in unprecedented detail during global warming associated with carbon-cycle perturbations.
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The carbonate compensation depth in the South Atlantic Ocean since the Late Cretaceous
Adriana Dutkiewicz; R. Dietmar Müller
Abstract: Deep-sea carbonate deposition is a complex process that is encapsulated in the carbonate compensation depth (CCD)—a facies boundary separating calcareous sediments from non-carbonates. Knowing how the CCD has varied over time is important for understanding and predicting the distribution of seafloor sediments and assessing their role in the global carbon cycle. We focus on the South Atlantic Ocean where the most recent CCD curve is based on Deep Sea Drilling Project (DSDP) Leg 73 sites drilled in 1980 in the South Atlantic Ocean. We compute the South and central South Atlantic CCD from the Late Cretaceous to the present day using updated age models from 45 DSDP and Ocean Drilling Program sites and backtracking with lithology-specific decompaction, eustasy, and dynamic topography. Our models extend further back in time and show more fluctuations than previous reconstructions, with the CCD varying by hundreds of meters during a span of 2–3 m.y. The addition of eustasy and dynamic topography deepens the CCD by as much as 500 m between 74 Ma and 45 Ma, and by ~200 m during the Cenozoic. The central South Atlantic CCD diverges from the average South Atlantic CCD during the Eocene and Miocene, when it was ~1 km shallower. These regional deviations may be due to changes in primary productivity and/or carbonate dissolution leading to reduced carbonate accumulation rates. Our CCD curves highlight the importance of regional processes in carbonate deposition across the South Atlantic and provide improved constraints for the modeling of geochemical cycles.
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Transient Permian-Triassic euxinia in the southern Panthalassa deep ocean
S.E. Grasby; D.P.G. Bond; P.B. Wignall; R. Yin; L.J. Strachan ...
Abstract: Both the duration and severity of deep-water anoxic conditions across the Permian-Triassic mass extinction (PTME) are controversial. Panthalassa Ocean circulation models yield varying results, ranging from a well-ventilated deep ocean to rapidly developing northern-latitude, but not southern-latitude, anoxia in response to Siberian Traps–driven global warming. To address this uncertainty, we examined a southern-paleolatitude pelagic record. Trace metal and pyrite framboid data suggest bottom-water euxinic conditions developed in the southern Panthalassa Ocean at the PTME, coincident with enhanced volcanic activity indicated by Hg geochemistry. While a global ocean euxinic event at the PTME placed extraordinary stress on marine life, southern surface waters appear to have recovered more quickly as radiolarian populations returned several million years before they did in northern Panthalassa.
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Water transfer to the deep mantle through hydrous, Al-rich silicates in subduction zones
Jörg Hermann; Shayne Lakey
Abstract: Constraining deep-water recycling along subduction zones is a first-order problem to understand how Earth has maintained a hydrosphere over billions of years that created conditions for a habitable planet. The pressure-temperature stability of hydrous phases in conjunction with slab geotherms determines how much H2O leaves the slab or is transported to the deep mantle. Chlorite-rich, metasomatic rocks that form at the slab-mantle interface at 50–100 km depth represent an unaccounted, H 2O-rich reservoir in subduction processes. Through a series of high-pressure experiments, we investigated the fate of such chlorite-rich rocks at the most critical conditions for subduction water recycling (5–6.2 GPa, 620–800 °C) using two different natural ultramafic compositions. Up to 5.7 GPa, 740 °C, chlorite breaks down to an anhydrous peridotite assemblage, and H2O is released. However, at higher pressures and lower temperatures, a hydrous Al-rich silicate (11.5 Å phase) is an important carrier to enable water transfer to the deep mantle for cold subduction zones. Based on the new phase diagrams, it is suggested that the deep-water cycle might not be in secular equilibrium.
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Eruption risks from covert silicic magma bodies
Shane M. Rooyakkers; John Stix; Kim Berlo; Maurizio Petrelli; Freysteinn Sigmundsson
Abstract: Unintentional encounters with silicic magma at ~2–2.5 km depth have recently occurred during drilling at three volcanoes: Kilauea (Hawaii), Menengai (Kenya), and Krafla (Iceland). Geophysical surveys had failed to warn about shallow magma before each encounter, and subsequent surveys at Krafla have been unable to resolve the size or architecture of its silicic magma body. This presents a conundrum for volcano monitoring: Do such shallow “covert” magma bodies pose an eruption risk? Here, we show that Krafla’s most recent explosive eruption, a mixed hydrothermal-magmatic event in 1724 C.E. that formed the Víti maar, involved rhyolite essentially indistinguishable in composition from magma encountered during drilling in 2009. Streaks of quenched basalt in some Víti pumices provide direct evidence for interaction between co-erupted rhyolitic and basaltic magmas, but crystals in these pumices show no evidence for late-stage heating or re-equilibration with more mafic melt, implying mixing time scales of at most several hours. Covert silicic magma thus presents an eruption risk at Krafla and may be mobilized with little warning. Difficulties in resolving magma bodies smaller than ~1 km3 with geophysical surveys mean that covert silicic magma may exist at many other volcanoes and should be considered in hazard and risk assessments.
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Ghost-dune hollows of the eastern Snake River Plain, Idaho: Their genesis, evolution, and relevance to Martian ghost-dune pits
David R. Gaylord; Tammy M. Rittenour; Paul K. Link; Brent D. Turrin; Mel A. Kuntz
Abstract: Ghost-dune hollows on the eastern Snake River Plain (ESRP), Idaho, USA, are topographically inverted, crescent-shaped depressions that record the partial encasement of sand dunes by ca. 61 ka basalt lava flows. Deflation of these “ghost” sand dunes produced approximately two dozen, 5–10-m-deep ghost-dune hollows now incompletely filled with pedogenically altered eolian and colluvial sediment. Optically stimulated luminescence (OSL) and 40Ar/39Ar ages constrain a ghost-dune hollow model that illuminates the late Pleistocene to Holocene environmental and climate history of the ESRP. Detrital zircon analyses indicate sand-dune supply routes changed following the burial of Pleistocene Henrys Fork (tributary of the Snake River) alluvium by ca. 70 ka basalt flows. Removal of Henrys Fork alluvium from the eolian supply system made Lake Terreton sediment the primary source for later ESRP sand dunes. Such sediment supply changes highlight the potential impacts of effusive volcanism on sand-dune histories and landscapes. Our results support stratigraphic and sedimentary modeling of comparable ghost-dune “pit” deposits older than ca. 2 Ga on Mars that may have served as refugia for early life on that planet. Analogous ancient ghost-dune hollow deposits on Earth may also have served as early life refugia.
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Controls on the origin and evolution of deep-ocean trench-axial channels
Adam D. McArthur; Daniel E. Tek
Abstract: The type and volume of sediment entering subduction zones affects the style of plate-boundary deformation and thus sedimentary and tectonic cycles. Because submarine channels significantly increase the transport efficiency of turbidity currents, their presence or absence in subduction trenches is a primary control on trench fill. To date, comprehensive architectural characterization of trench-axial channels has not been possible, undermining efforts to identify the factors controlling their initiation and evolution. Here, we describe the evolution of the Hikurangi Channel, which traverses the Hikurangi Trench, offshore New Zealand. Analysis of two- and three-dimensional seismic data reveals that the channel was present only during the last ~3.5 m.y. of the ~27 m.y. of the trench’s existence; its inception and propagation resulted from increased sediment supply to the trench following amplified hinterland exhumation. To test if the controls on the evolution of the Hikurangi Channel are universal, multivariate statistical analysis of the geomorphology of subduction trenches globally is used to investigate the formative conditions of axial channels in modern trenches. Terrigenous sediment supply and thickness of sediment cover in a trench are the dominant controls; subsidiary factors such as trench length and rugosity also contribute to the conditions necessary for trench-axial channel development. Axial channels regulate sediment distribution in trenches, and this varies temporally and spatially as a channel propagates along a trench. The presence of a trench-axial channel affects plate-boundary mechanics and has implications for the style of subduction-margin deformation.
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Joint inversion of surface wave and gravity data reveals subbasin architecture of the Congo Basin
A. Raveloson; A. Nyblade; R. Durrheim
Abstract: We investigated the architecture of the greater Congo Basin, one of the largest and least-well-studied sedimentary basins on any continent. Seismograms from a large number of M > 4.5 earthquakes within and surrounding the African plate were used to make event-to-station Rayleigh wave group velocity measurements between periods of 5 and 100 s. Group velocities for discrete periods across the basin, obtained by inverting the event-station measurements, were jointly modeled with gravity data to obtain a three-dimensional S-wave and density model of the basin. The model corroborates the existence of two previously suggested subbasins, one to the north and one to the south, each ~8 km deep and separated by an east-west structural high. Our results favor a salt tectonics origin for the structural high but cannot rule out uplifted basement rock. The northern subbasin is offset to the west from the southern subbasin, consistent with previous studies suggesting sinistral motion along basement faults during periods of transpressional tectonics in late Neoproterozoic–early Paleozoic times.
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Hydrothermal recycling of sedimentary ammonium into oceanic crust and the Archean ocean at 3.24 Ga
E.E. Stüeken; T.J. Boocock; A. Robinson; S. Mikhail; B.W. Johnson
Abstract: The Archean ocean supported a diverse microbial ecosystem, yet studies suggest that seawater was largely depleted in many essential nutrients, including fixed nitrogen. This depletion was in part a consequence of inefficient nutrient recycling under anoxic conditions. Here, we show how hydrothermal fluids acted as a recycling mechanism for ammonium (NH 4+) in the Archean ocean. We present elemental and stable isotope data for carbon, nitrogen, and sulfur from shales and hydrothermally altered volcanic rocks from the 3.24 Ga Panorama district in Western Australia. This suite documents the transfer of NH4 + from organic-rich sedimentary rocks into underlying sericitized dacite, similar to what is seen in hydrothermal systems today. On modern Earth, hydrothermal fluids that circulate through sediment packages are enriched in NH4+ to millimolar concentrations because they efficiently recycle organic-bound N. Our data show that a similar hydrothermal recycling process dates back to at least 3.24 Ga, and it may have resulted in localized centers of enhanced biological productivity around hydrothermal vents. Last, our data provide evidence that altered oceanic crust at 3.24 Ga was enriched in nitrogen, and, when subducted, it satisfies the elemental and isotopic source requirements for a low-N, but 15N-enriched, deep mantle nitrogen reservoir as sampled by mantle plumes.
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Missing water from the Qiangtang Basin on the Tibetan Plateau
Bin Yong; Chi-Yuen Wang; Jiansheng Chen; Jiaqi Chen; D.A. Barry ...
Abstract: The Qiangtang Basin is a large endorheic basin in the inner part of the Tibetan Plateau, and has been thought to be a dry region in contrast with the surrounding wet outer region that feeds all the major Asian rivers. Combining surface hydrological data with modeling and satellite data from 2002 to 2016 CE, our study reveals that an enormous amount of water, ~54 ± 4 km3, is unaccounted for annually in the Qiangtang Basin. The amount of missing water is comparable to the total annual discharge of the Yellow River. Data from the Gravity Recovery and Climate Experiment (GRACE) satellite mission show little increase of local terrestrial water storage. Thus, the missing water must have flowed out of the basin through underground passages. Interpreting this result in the context of recent seismic and geological studies of Tibet, we suggest that a significant amount of meteoric water in the Qiangtang Basin leaks out by way of groundwater flow through deep normal faults and tensional fractures along the nearly north-south rift valleys that are oriented subnormal to and cross the surficial hydrological divide on the southern margin of the basin. Cross-basin groundwater outflow of such a magnitude defies the traditional view of a basin-scale water cycle and leads to a very different picture from the previous hydrological view of the Qiangtang Basin. This finding calls for major rethinking of the regional water balance.
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Alternating wet and dry depositional environments recorded in the stratigraphy of Mount Sharp at Gale crater, Mars
W. Rapin; G. Dromart; D. Rubin; L. Le Deit; N. Mangold ...
Abstract: The Curiosity rover is exploring Hesperian-aged stratigraphy in Gale crater, Mars, where a transition from clay-bearing units to a layered sulfate-bearing unit has been interpreted to represent a major environmental transition of unknown character. We present the first description of key facies in the sulfate-bearing unit, recently observed in the distance by the rover, and propose a model for changes in depositional environments. Our results indicate a transition from lacustrine mudstones into thick aeolian deposits, topped by a major deflation surface, above which strata show architectures likely diagnostic of a subaqueous environment. This model offers a reference example of a depositional sequence for layered sulfate-bearing strata, which have been identified from orbit in other locations globally. It differs from the idea of a monotonic Hesperian climate change into long-term aridity on Mars and instead implies a period characterized by multiple transitions between sustained drier and wetter climates.
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Evidence for 40–41 km of dextral slip on the southern Death Valley fault: Implications for the Eastern California shear zone and extensional tectonics
Terry L. Pavlis; Ghislain Trullenque
Abstract: Recognition of a pair of pre-Neogene markers together with analysis of published data indicate ~40 km of dextral slip across the southern Death Valley fault zone, California, USA. Stratigraphic overlaps on fault rocks indicate much of the dextral slip predates the late Miocene, placing a significant fraction of the dextral slip in the same time window as regional extension and challenging interpretations that the modern strike-slip system became active post–6–3 Ma. However, these results are consistent with regional evidence that dextral transtension began by ca. 12 Ma.
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A metasedimentary source of gold in Archean orogenic gold deposits
Iain K. Pitcairn; Nikolaos Leventis; Georges Beaudoin; Stephane Faure; Carl Guilmette ...
Abstract: The sources of metals enriched in Archean orogenic gold deposits have long been debated. Metasedimentary rocks, which are generally accepted as the main metal source in Phanerozoic deposits, are less abundant in Archean greenstone belts and commonly discounted as a viable metal source for Archean deposits. We report ultralow-detection-limit gold and trace-element concentrations from a suite of metamorphosed sedimentary rocks from the Abitibi belt and Pontiac subprovince, Superior Province, Canada. Systematic decreases in the Au content with increasing metamorphic grade indicate that Au was mobilized during prograde metamorphism. Mass balance calculations show that over 10 t of Au, 30,000 t of As, and 600 t of Sb were mobilized from 1 km3 of Pontiac subprovince sedimentary rock metamorphosed to the sillimanite metamorphic zone. The total gold resource in orogenic gold deposits in the southern Abitibi belt (7500 t Au) is only 3% of the Au mobilized from the estimated total volume of high-metamorphic-grade Pontiac sedimentary rock in the region (25,000 km3), indicating that sedimentary rocks are a major contributor of metals to the orogenic gold deposits in the southern Abitibi belt.
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Similar curvature-to-width ratios for channels and channel belts: Implications for paleo-hydraulics of fluvial ridges on Mars
Alistair T. Hayden; Michael P. Lamb; Alexander J. Carney
Abstract: The surface of Mars contains abundant sinuous ridges that appear similar to river channels in planform, but they stand as topographic highs. Ridges have similar curvature-to-width ratios as terrestrial meandering rivers, which has been used to support the hypothesis that ridges are inverted channels that directly reflect channel geometry. Anomalously wide ridges, in turn, have been interpreted as evidence for larger rivers on Mars compared to Earth. However, an alternate hypothesis is that ridges are exhumed channel-belt deposits—a larger zone of relatively coarse-grained deposits formed from channel lateral migration and aggradation. Here, we measured landform wavelength, radius of curvature, and width to compare terrestrial channels, terrestrial channel belts, and martian ridges. We found that all three landforms follow similar scaling relations, in which ratios of radius of curvature to width range from 1.7 to 7.3, and wavelength-to-width ratios range from 5.8 to 13. We interpret this similarity to be a geometric consequence of a sinuous curved line of finite width. Combined with observations of ridge-stacking patterns, our results suggest that wide ridges on Mars could indicate fluvial channel belts that formed over significant time rather than anomalously large rivers.
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Outer forearc uplift and exhumation during high-flux magmatism: Evidence from detrital zircon geochemistry of the Nacimiento forearc basin, California, USA
Scott M. Johnston; Andrew R.C. Kylander-Clark
Abstract: We present new coupled detrital zircon trace-element and U–Pb age data from Valanginian–Santonian strata of the Nacimiento forearc basin (California, USA) to enhance provenance discrimination and investigate the evolution of the late Mesozoic California margin. Our data document at least five different Jurassic–earliest Cretaceous zircon populations with variable U/Yb ratios, and zircon that displays systematically increasing U/Yb from 130 to 80 Ma. Based on the presence of a distinctive population of geochemically primitive, 168–157 Ma low-U/Yb zircon that is found in Albian–Lower Cenomanian strata but not in older Valanginian strata, we infer a period of uplift and Albian–early Cenomanian erosion of forearc basement (the Coast Range ophiolite) that was coincident with increasing Cordilleran arc magmatic flux.
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Shutting down dust emission during the middle Holocene drought in the Sonoran Desert, Arizona, USA
Guy Tau; Onn Crouvi; Yehouda Enzel; Nadya Teutsch; Paul Ginoux ...
Abstract: Long-term relationships between climate and dust emission remain unclear, with two prevailing but opposite hypotheses for effects of climate shifts: (1) increased dust emission due to increasing aridity imposing a vegetation change, or (2) decreased dust emission due to increasing aridity which imposes less stormy climate and reduced sediment supply. Here we test these hypotheses by analyzing an ~11-m-long core archiving Holocene dust trapped in Montezuma Well, a natural sinkhole in Arizona (southwestern United States), alongside current dust sources and transport pathways. Major elements indicate that Montezuma Well sediments originate from two end members: local carbonate bedrock and external siliceous dust. Core sediments are similar to the adjacent siliceous soils accumulated atop the bedrock, pointing to their eolian origin. Particle-size distributions reveal fine dust transported during winter from the northwestern Sonoran Desert and the Mojave Desert and coarse dust transported during summer from the southwestern Sonoran Desert, similar to current climate systems and dust pathways. A survey of potential dust sources indicates that current summer and winter dust sources in the Sonoran Desert are under a supply-limited state. Dust fluxes were higher during wetter phases of the Holocene when winter sources dominated. During the middle Holocene drought, dust fluxes were minimal and dominated by summer sources until dust input ceased as drought conditions did not produce enough floods to refill sources with sediments. We propose that in the Sonoran Desert, dust emission is strongly connected with climate, increasing during humid intervals and enhanced by fluvial sediment replenishment at dust sources.
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Extensive decentralized hydrogen export from the Atlantis Massif
Susan Q. Lang; Marvin D. Lilley; Tamara Baumberger; Gretchen L. Früh-Green; Sharon L. Walker ...
Abstract: Hydrogen is an important energy source for subsurface microbial communities, but its availability beyond the flow focused through hydrothermal chimneys is largely unknown. We report the widespread export of H2 across the Atlantis Massif oceanic core complex (30°N, Mid-Atlantic Ridge; up to 44 nM), which is distinct from the circulation system feeding the Lost City Hydrothermal Field (LCHF) on the massif’s southern wall. Methane (CH4) abundances are generally low to undetectable (<3 nM) in fluids that are not derived from the LCHF. Reducing fluids exit the seafloor over a wide geographical area and depth range, including the summit of the massif and along steep areas of mass wasting east of the field. The depth of the fluids in the water column and their H2/CH4 ratios indicate that some are sourced separately from the LCHF. We argue that extensive H2 export is the natural consequence of fluid flow pathways strongly influenced by tectonic features and the volume and density changes that occur when ultramafic rocks react to form serpentinites, producing H 2 as a by-product. Furthermore, the circulation of H2 -rich fluids through uplifted mantle rocks at moderate temperatures provides geographically expansive and stable environmental conditions for the early evolution of biochemical pathways. These results provide insight into the spatial extent of H2- and CH4-bearing fluids associated with serpentinization, independent of the focused flow emanating from the LCHF.
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Shock-deformed zircon from the Chicxulub impact crater and implications for cratering process
Jiawei Zhao; Long Xiao; Zhiyong Xiao; Joanna V. Morgan; Gordon R. Osinski ...
Abstract: Large impact structures with peak rings are common landforms across the solar system, and their formation has implications for both the interior structure and thermal evolution of planetary bodies. Numerical modeling and structural studies have been used to simulate and ground truth peak-ring formative mechanisms, but the shock metamorphic record of minerals within these structures remains to be ascertained. We investigated impact-related microstructures and high-pressure phases in zircon from melt-bearing breccias, impact melt rock, and granitoid basement from the Chicxulub peak ring (Yucatán Peninsula, Mexico), sampled by the International Ocean Discovery Program (IODP)/International Continental Drilling Project (IODP-ICDP) Expedition 364 Hole M0077A. Zircon grains exhibit shock features such as reidite, zircon twins, and granular zircon including “former reidite in granular neoblastic” (FRIGN) zircon. These features record an initial high-pressure shock wave (>30 GPa), subsequent relaxation during the passage of the rarefaction wave, and a final heating and annealing stage. Our observed grain-scale deformation history agrees well with the stress fields predicted by the dynamic collapse model, as the central uplift collapsed downward-then-outward to form the peak ring. The occurrence of reidite in a large impact basin on Earth represents the first such discovery, preserved due to its separation from impact melt and rapid cooling by the resurging ocean. The coexistence of reidite and FRIGN zircon within the impact melt–bearing breccias indicates that cooling by seawater was heterogeneous. Our results provide valuable information on when different shock microstructures form and how they are modified according to their position in the impact structure, and this study further improves on the use of shock barometry as a diagnostic tool in understanding the cratering process.
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For Immediate Release
29 April 2021
GSA Release No. 21-25

Kea Giles