Publisher: Geological Society of America   (Total: 4 journals)   [Sort by number of followers]

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Bulletin of the Geological Society of America     Hybrid Journal   (Followers: 35, SJR: 2.329, CiteScore: 4)
Geology     Full-text available via subscription   (Followers: 56, SJR: 3.114, CiteScore: 4)
Geosphere     Open Access   (Followers: 2, SJR: 1.752, CiteScore: 3)
Lithosphere     Open Access   (Followers: 3, SJR: 1.892, CiteScore: 3)
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Geology
Journal Prestige (SJR): 3.114
Citation Impact (citeScore): 4
Number of Followers: 56  
 
  Full-text available via subscription Subscription journal
ISSN (Print) 0091-7613 - ISSN (Online) 1943-2682
Published by Geological Society of America Homepage  [4 journals]
  • A multidisciplinary approach to reconstructing the history of early animal
           life on Earth

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      Abstract: The 50th anniversary of Geology provides not only a welcome occasion to celebrate past decades of extraordinary advances in geoscience knowledge but also the impetus to cast our gaze forward and envision what new directions and discoveries will guide geoscience research through the next 50 years and beyond. My research broadly concerns reconstructing the factors that have shaped the emergence and evolution of complex life on our planet, as well as the fidelity of archives of past environmental and ecological change. This is an area that, in my view, is currently undergoing a renaissance—fueled by efforts leveraging multidisciplinary approaches; exploring increasingly highly resolved spatial and temporal scales; and harnessing the power of large, stratigraphically grounded data sets.
      PubDate: Thu, 01 Sep 2022 00:00:00 GMT
       
  • Crustal transpressional fault geometry influenced by viscous lower crustal
           flow

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      Abstract: AbstractThe San Andreas fault (California, USA) is near vertical at shallow (<10 km) depth. Geophysical surveys along the San Andreas fault reveal that, at depths of 10–20 km, it dips ~50–70° to the southwest near the Western Transverse Ranges and dips northeast in the San Gorgonio region. We investigate the possible origin of along-strike geometric variations of the fault using a three-dimensional thermomechanical model. For two blocks separated by transpressional faults, our model shows that viscous lower crustal material moves from the high-viscosity block into the low-viscosity block. Fault plane-normal flow in the viscous lower crust rotates the fault plane due to the simple shear flow at the brittle-ductile transition depth. This occurs irrespective of initial fault dip direction. Rheological variations used to model the lower crust of Southern California are verified by independent observations. Block extrusion due to lower crustal viscosity variation facilitates the formation of the Garlock Fault and sustains the geometric complexity of the fault.
      PubDate: Tue, 21 Jun 2022 00:00:00 GMT
       
  • Eruptive tempo of Emeishan large igneous province, southwestern China and
           northern Vietnam: Relations to biotic crises and paleoclimate changes
           around the Guadalupian-Lopingian boundary

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      Abstract: AbstractThe Emeishan large igneous province (LIP), southwestern China and northern Vietnam, is thought to have been a potential driver for the biotic crises and paleoclimate changes around the Guadalupian-Lopingian boundary (GLB; Permian), but the lack of high-precision radiometric dates to constrain the duration and eruption rates of the volcanism has limited the assessment of their relationship. We present new chemical abrasion–isotope dilution–isotope ratio mass spectrometry U-Pb zircon geochronology of three samples from the lowermost and uppermost parts of the volcanic succession in the central portion of the Emeishan LIP. The results demonstrate that Emeishan volcanism began by 260.55 ± 0.07 Ma and persisted until at least 257.22 ± 0.37 Ma. Combined with a previously published age of 259.1 ± 0.5 Ma for silicic ignimbrites, we estimate that ~85% of Emeishan LIP volume erupted within 1.45 ± 0.50 m.y. Our new results confirm that the Emeishan volcanism began slightly prior to the initiation of the associated mass extinction event and was contemporaneous with the associated warming interval. The new data support the hypothesis that the Emeishan LIP likely triggered the biotic crises and paleoclimate changes around the GLB.
      PubDate: Tue, 21 Jun 2022 00:00:00 GMT
       
  • Evidence for a lithospheric step and pervasive lithospheric thinning
           beneath southern New England, northeastern USA

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      Abstract: AbstractIn this study, we use data from the SEISConn seismic experiment to calculate Sp receiver functions in order to characterize the geometry of upper-mantle structure beneath southern New England (northeastern United States). We image robust negative-velocity-gradient discontinuities beneath southern New England that we interpret as corresponding to the lithosphere-asthenosphere boundary (LAB) and identify a well-defined step of 15 km in LAB depth at a longitude of 73°W, which we interpret to be the boundary between Laurentian and Appalachian lithosphere, although the offset may be larger if the putative LAB phase is reinterpreted to be a mid-lithospheric discontinuity. We infer that the lithosphere throughout the region is substantially thinner than elsewhere in the continental interior, consistent with regional tomographic studies and previously published Sp receiver function results. The presence of thinned lithosphere suggests that the low-velocity Northern Appalachian Anomaly (NAA) in the upper mantle may extend as far south as coastal Connecticut. The presence of regionally thinned lithosphere and a step in lithospheric thickness suggests that inherited structure may be preserved in present-day lithosphere, even in the presence of more recent dynamic processes associated with the NAA.
      PubDate: Tue, 21 Jun 2022 00:00:00 GMT
       
  • Silurian wildfire proxies and atmospheric oxygen

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      Abstract: AbstractThe earliest evidence of wildfire is documented from two localities: the early mid-Silurian Pen-y-lan Mudstone, Rumney, Wales (UK), and the late Silurian Winnica Formation, Winnica, Poland. Nematophytes dominate both charcoal assemblages. Reflectance data indicate low-temperature fires with localized intense conditions. Fire temperatures are greater in the older and less evolved assemblage. These charcoal assemblages and others, new and previously documented, from the Silurian and earliest Devonian are compared to box models of atmospheric oxygen concentration (pO2). Based on modern charring experiments, these data indicate pO2 is divergent from the broad trends predicted by the COPSE-revisited and GEOCARBSULFOR models. Sustained burns require a minimum pO2 threshold of 16%, or ~0.75 present atmospheric level. This threshold was first met and, our charcoal data indicate, was exceeded in the mid-Silurian and then, later in the Silurian, attained again repeatedly.
      PubDate: Mon, 13 Jun 2022 00:00:00 GMT
       
  • Epidote U-Pb geochronology and H isotope geochemistry trace pre-orogenic
           hydration of midcrustal granitoids

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      Abstract: AbstractHydrothermal veins and altered feldspar are evidence for fluid circulation in granitic rocks in the continental crust. The hydrothermal alteration of feldspar affects the deformation behavior of granitoids, especially if it occurs before orogeny. Geochronology can establish the timing of fluid circulation to determine if this fluid-driven alteration plays a role in crustal deformation. Although existing dating techniques cannot be applied to feldspar alteration directly, absolute ages of fluid circulation can be obtained from hydrothermal veins. We combined U-Pb geochronology and hydrogen isotope data (δD) from epidote [Ca2Al2(Al,Fe3+) Si3O12(OH)] to unravel the hydration of post-Variscan granitoids in the Alpine orogen. The recent protocol for epidote U-Pb dating proves for the first time that fluids of meteoric origin infiltrated the granitoids in Permian times by exploiting synrift faults, consistent with the δD values of the epidote-forming fluids. This hydration event caused at least some degree of feldspar hydration and weakening of the granitic rocks ~260 m.y. before their pervasive structural overprint by the Alpine orogeny. The preservation of Permian U-Pb ages despite Alpine orogenic processes confirms epidote as a powerful tool with which to unveil pre-orogenic hydration events in metagranitoids. Our analytical approach broadens insights into the water cycle in the middle continental crust in orogens.
      PubDate: Mon, 13 Jun 2022 00:00:00 GMT
       
  • Mid-loaf crisis: Internal breadcrust surfaces in rhyolitic pyroclasts
           reveal dehydration quenching

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      Abstract: AbstractBreadcrust bombs are pyroclasts displaying fractured, dense surfaces enveloping expanded interiors, and are associated with Vulcanian explosions. We document pyroclasts from the 2008–2009 CE eruption of Chaitén (Chile) that are internally as well as externally breadcrusted. The pyroclasts are cut by intersecting micrometer-to millimeter-thick tuffisites with dense glassy walls, which grade into strongly inflated pumiceous material. We find H2O diffusion gradients proximal to the breadcrusted surfaces, such that H2O is depleted from far-field magma (0.68 ± 0.04 wt%) into dense, fractured vein walls (0.2–0.3 wt%), indicating a spatial association between H2O mass transfer within the pyroclast interior and both suppressed vesiculation and breadcrusting. We experimentally confirm that diffusive H2O depletion suppresses bubble growth at shallow conduit conditions. Therefore, we interpret the breadcrust formation to be induced by H2O diffusion and the associated rise in viscosity rather than by cooling in the classical breadcrust-formation models. We posit that a “dehydration quench” is important as degassing continues to very low H2O contents in shallowconduit magma that continues to vesiculate.
      PubDate: Mon, 13 Jun 2022 00:00:00 GMT
       
  • Natural levees increase in prevalence in the backwater zone: Coastal
           Trinity River, Texas, USA

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      Abstract: AbstractFlood dynamics in low-relief landscapes control the lateral exchange of water and sediment between a river and its floodplain. Locations where these exchanges occur for any given river discharge depend on local bank elevations, which in turn depend on the type of landform present immediately adjacent to the river channel. Our analysis separated landforms bordering a river into six categories: levee, scroll bar, counter point bar, channel-bend cutoff, erosional surface, and inactive surface. Each landform is associated with a different range of elevations. Levees are the highest, and counter point bars and cutoffs are the lowest. Using a combination of lidar-derived measurements of topography and water-surface profiles derived from U.S. Geological Survey gauge data, we show that landforms at the margins of the river change with downstream position on the coastal reach of the Trinity River in the southern United States. The fractions of counter point bars and cutoffs decrease downstream, while the fraction and continuity of levees increase to nearly 100%. This spatial change correlates with downstream reductions in channel-bend migration and deformation, and the measured range in river stage. As a result, the greatest range in bank elevations occurs upstream where variation in river stage is also highest. Meanwhile, the smallest range in bank elevation and river stage exists in the coastal backwater zone. Our analysis indicates that essentially all levees within the backwater zone are overtopped by flow associated with a single river discharge. Moving upriver, the discharge associated with levee-overtopping flow systematically increases. This study highlights the morphodynamic control on coastal river flooding.
      PubDate: Mon, 13 Jun 2022 00:00:00 GMT
       
  • Quantitative relationships between river and channel-belt planform
           patterns

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      Abstract: AbstractChannel planform patterns arise from internal dynamics of sediment transport and fluid flow in rivers and are affected by external controls such as valley confinement. Understanding whether these channel patterns are preserved in the rock record has critical implications for our ability to constrain past environmental conditions. Rivers are preserved as channel belts, which are one of the most ubiquitous and accessible parts of the sedimentary record, yet the relationship between river and channel-belt planform patterns remains unquantified. We analyzed planform patterns of rivers and channel belts from 30 systems globally. Channel patterns were classified using a graph theory-based metric, the Entropic Braided Index (eBI), which quantifies the number of river channels by considering the partitioning of water and sediment discharge. We find that, after normalizing by river size, channel-belt width and wavelength, amplitude, and curvature of the belt edges decrease with increasing river channel number (eBI). Active flow in single-channel rivers occupies as little as 1% of the channel belt, while in multichannel rivers it can occupy >50% of the channel belt. Moreover, we find that channel patterns lie along a continuum of channel numbers. Our findings have implications for studies on river and floodplain interaction, storage timescales of floodplain sediment, and paleoenvironmental reconstruction.
      PubDate: Mon, 13 Jun 2022 00:00:00 GMT
       
  • Ringwoodite and zirconia inclusions indicate downward travel of super-deep
           diamonds

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      Abstract: AbstractNatural diamonds and their inclusions provide unique glimpses of mantle processes from as deep as ~800 km and dating back to 3.5 G.y. Once formed, diamonds are commonly interpreted to travel upward, either slowly within mantle upwellings or rapidly within explosive, carbonate-rich magmas erupting at the surface. Although global tectonics induce subduction of material from shallow depths into the deep mantle, mineralogical evidence for downward movements of diamonds has never been reported. We report the finding of an unusual composite inclusion consisting of ringwoodite (the second finding to date), tetragonal zirconia, and coesite within an alluvial super-deep diamond from the Central African Republic. We interpret zirconia + coesite and ringwoodite as prograde transformation products after zircon or reidite (ZrSiO4) and olivine or wadsleyite, respectively. This inclusion assemblage can be explained if the diamond traveled downward after entrapping olivine/wadsleyite + zircon/reidite, dragged down by a subducting slab, before being delivered to the surface. This indicates that the commonly assumed view that diamonds form at, and capture material from, a specific mantle level and then travel upward is probably too simplistic.
      PubDate: Fri, 27 May 2022 00:00:00 GMT
       
  • Explosive-effusive-explosive: The role of magma ascent rates and paths in
           modulating caldera eruptions

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      Abstract: AbstractOne of the biggest challenges in volcanology is assessing the role of magma properties (volatile budgets, storage depths, and ascent rates) in controlling eruption explosivity. We use a new approach based on apatite to estimate volatile contents and magma ascent rates from a sequence of sub-Plinian, effusive, and Vulcanian eruption deposits at Rabaul caldera (Papua New Guinea) emplaced in 2006 CE to probe the mechanisms responsible for the sudden transitions in eruption styles. Our findings show that all magmas were originally stored at similar conditions (2–4 km depth and 1.8–2.5 wt% H2O in the melt); only the magma that formed the lava flow stalled and degassed at a shallower level (0.2–1.5 km) for several months. A more energetic batch of magma rose from depth, bypassed the transient reservoir, and ascended within ≤8 h to Earth's surface (mean velocity ≥0.2 m/s), yielding the initial sub-Plinian phase of the eruption. The shallowly degassed magma was then able to reach the surface as a lava flow, likely through the path opened by the sub-Plinian magma. The magma of the last Vulcanian phase ascended without storage at a shallow depth, albeit more slowly (ascent rate 0.03–0.1 m/s) than the sub-Plinian magma. Our study illustrates how the complexity of plumbing systems may affect eruption styles, including at other volcanic systems, and have implications for interpreting volcano monitoring data.
      PubDate: Fri, 27 May 2022 00:00:00 GMT
       
  • Mayflies as resource pulses in Jurassic lacustrine ecosystems

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      Abstract: AbstractResource pulses, occasional events of ephemeral resource superabundance, represent a fundamental mechanism by which energy, nutrients, and biomass are transported across ecotones. They are widespread in extant ecosystems; however, little is known about their deep-time record. We report the earliest-known mayfly swarm from the Early Jurassic Xiwan biota of southern China. Our taphonomic and sedimentological analyses show that these mayflies were buried on the bottom of a calm lake after post-mating death. Our suite of analyses suggests that the complex mating-swarm behavior was already well established in mayflies by the Early Jurassic. More importantly, our find represents the earliest-known resource pulse of insects, a mechanism that can play a substantial role in nutrient transport from aquatic ecosystems to surrounding terrestrial ecosystems. Such an aquatic-terrestrial ecosystem linkage may be a key novelty in Mesozoic lacustrine ecosystems. Our results high-light the underappreciated ecological significance of insects in deep-time terrestrial ecology.
      PubDate: Fri, 27 May 2022 00:00:00 GMT
       
  • Gouge fabrics reset by thermal pressurization record stress on faults
           after earthquakes

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      Abstract: AbstractStress on seismogenic faults provides critical information about how much elastic energy is stored in the crust and released by earthquakes, which is crucial in understanding earthquake energetics and recurrence. However, determining post-earthquake stress states on faults remains challenging because current borehole methods are rarely applicable to damaged fault zone rocks. We applied neutron texture analysis to gouge samples of the 1999 Chi-Chi earthquake in Taiwan to infer the stress state after the earthquake. Results indicate that the clay fabric within the principal slip zone is orthogonal to the fault plane, whereas outside the principal slip zone the fabric is predominantly parallel to the bedding-parallel fault plane. We suggest that the clay fabric in the slip zone was first neutralized by the coseismic fluidization caused by thermal pressurization and later re-oriented to the new direction of post-earthquake principal stress. Such stress orientation is consistent with the orientations inferred from core-scale fault slip data and dislocation models constrained from global navigation satellite system displacements. If thermal pressurization is a ubiquitous process during earthquakes, gouge fabrics can be used to help probe the post-earthquake stress state of faults.
      PubDate: Fri, 27 May 2022 00:00:00 GMT
       
  • Mercury isotopic composition of igneous rocks from an accretionary orogen:
           Implications for lithospheric recycling

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      Abstract: AbstractMercury (Hg) provides critical information on terrestrial planet formation and evolution due to its unique physicochemical properties and multiform isotopic compositions. Current knowledge of Hg is mainly limited to Earth's surface environments, and the understanding of Hg in the Earth's interior remains unclear. Accretionary orogens are major settings for continental crustal growth and crust-mantle interactions. We studied the Hg concentration and isotopic composition of igneous rocks in the eastern Central Asian orogenic belt, using Hg as a proxy to trace the recycling of surface materials in Earth's lithosphere. Our results show low Hg abundances in mafic through felsic igneous rocks (4.93 ± 4.35 ppb, standard deviation [SD], n = 267). Mafic rocks show slightly lower δ202Hg (−2.9‰ ± 0.5‰, SD, n = 24) than intermediate (−2.4‰ ± 0.8‰, SD, n = 58) and felsic (−1.5‰ ± 0.8‰, SD, n = 185) rocks, indicating a chemical stratification of Hg isotopic composition in the continental crust with isotopically lighter Hg in the lower part and heavier Hg in the upper part. Slightly positive Δ199Hg values are observed in mantle-derived mafic (0.07‰ ± 0.06‰, SD) and intermediate (0.06‰ ± 0.07‰, SD) rocks, which agree well with those reported for marine sediments, indicating the involvement of fluids or melts from the oceanic crust. Larger variations of Δ199Hg values (−0.26‰ to +0.21‰, average: 0.01‰ ± 0.08‰, SD, n = 185) are observed in felsic rocks, further indicating recycling of surface Hg from the marine reservoir via slab subduction (reflected by positive values) plus magmatic assimilation of terrestrial Hg (reflected by negative values). Our study demonstrates that Hg isotopes can be a promising tracer for the chemical dynamics of Earth's lithosphere.
      PubDate: Fri, 27 May 2022 00:00:00 GMT
       
  • Neodymium isotopes of central Mediterranean phosphatic hardgrounds reveal
           Miocene paleoceanography

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      Abstract: AbstractUnderstanding the causes of the formation of hardgrounds provides insights on the oceanographic evolution of a basin. Phosphate-rich hardground formation interrupted carbonate ramp deposition in the Mediterranean during the Miocene. We analyzed the εNd record of three central Mediterranean hardgrounds to identify the origin of the phosphate-rich waters that formed them within the frame of Mediterranean Miocene paleoceanographic evolution. The Nd isotopes suggest that eastern Mediterranean deep waters were controlled by runoff, in contrast to Atlantic and Indian Ocean waters. This Nd isotope record attests to the weakening of Mediterranean circulation during the Miocene due to closure of the Indian Gateway. Limited exchange with Atlantic shallow seawater led to long residence times for deep waters in the basin. This record indicates the role of upwelling in formation of phosphate hardgrounds and shows the influence of global climate change and local paleoceanographic conditions.
      PubDate: Fri, 27 May 2022 00:00:00 GMT
       
  • Deep-focus earthquakes: From high-temperature experiments to cold slabs

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      Abstract: AbstractDeep-focus earthquakes (DFEs) present an interesting scientific challenge in that they occur at depths where brittle failure should be impossible. The fact that their occurrence is confined to locations where subducting lithospheric slabs are crossing through the transition zone suggests that olivine phase transformations may be involved in the production of these earthquakes. Experimental studies have shown that olivine can persist metastably in subducting slabs and that olivine phase transformations can lead to faulting at high pressures. However, it has been argued that large DFEs are too large to be contained within a metastable olivine wedge preserved in the interior of subducting slabs. We demonstrate, using experiments on olivine-analog materials, that transformational faulting can continue to propagate via shear-enhanced melting into the stable high-pressure phase. We also show that transformational faulting is controlled by the ratio between strain rates and the olivine-ringwoodite transformation rates, and extrapolate this relationship to the natural conditions of DFEs. Counterintuitively, these results imply that cold and fast-subducting slabs produce transformational faulting at higher temperatures, which results in more numerous DFEs.
      PubDate: Fri, 27 May 2022 00:00:00 GMT
       
  • Zircon-modeled melts shed light on the formation of Earth's crust from the
           Hadean to the Archean

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      Abstract: AbstractElucidating the compositions of melts from which Hadean zircons crystallized can provide insight into early crust construction. We calculated model melts using Ti-calibrated zircon/melt partition coefficients and trace element data for zircons from the Hadean, Archean, and possible analogue environments (e.g., rifts, hotspots, arcs) to constrain petrogenetic relationships. Model melts from oceanic settings (mid-ocean ridges, arcs, Iceland) showed higher heavy rare earth element (HREE) contents and shallower middle REE (MREE) to HREE/chondrite (ch) slopes than those from continental arcs and tonalite-trondhjemite-granodiorite suites (TTGs). However, Hadean and Archean model melts were consistently similar to each other and to those from continental arcs, hotspots, and TTGs (and dissimilar to oceanic settings), with depleted HREE contents and slope reversal in heaviest REEch. Despite close similarities that suggest comparable petrogenesis of Hadean and early Archean magmas from which Jack Hills detrital zircons crystallized, subtle variabilities in REEch and Zr/Hf suggest thickening crust and evolving igneous systems through time.
      PubDate: Fri, 27 May 2022 00:00:00 GMT
       
  • Triple oxygen isotope evidence for a hot Archean ocean

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      Abstract: AbstractTriple oxygen isotope (δ17O and δ18O) values of high- and low-temperature altered oceanic crust and products of basalt alteration experiments were measured to better constrain ocean isotope compositions in deep time. The data define an array of δ18O and Δ′17O (Δ′17O=δ′17O − λRL × δ′18O + γ) values from mantle values toward 1‰ and −0.01‰, respectively, with a λ of ~0.523. The altered oceanic crust data were used to construct a model for estimating δ18O-Δ′17O values of the ancient oceans if the continental weathering flux (FCW) and/or hydrothermal oceanic crust alteration flux (FHT) changed through time. A maximum lowering of 7‰ and 4‰, respectively, is achieved in the most extreme cases. The δ18O value of the ocean cannot be raised by more than 1.1‰. Eclogites from the Roberts Victor kimberlite (South Africa), with a protolith age of 3.1 Ga, have δ18O-Δ′17O values that precisely overlap with those of the modern altered oceanic crust, suggesting that the Archean oceans had similar isotope values as today. Published triple isotope data for Archean cherts show that all samples have been altered to some degree and suggest an Archean ocean surface temperature of ~70–100 °C. An ocean as light as −2‰ is still consistent with our eclogite data and reduce our temperature estimates by 10 °C.
      PubDate: Fri, 27 May 2022 00:00:00 GMT
       
  • The role of flat slab subduction, ridge subduction, and tectonic
           inheritance in Andean deformation

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      Abstract: AbstractConvergent plate boundaries show sharp variations in orogenic width and extent of intraplate deformation. Analysis of late Cenozoic contractile deformation along the Andean mountain front and adjacent foreland highlights the contrasting degrees of deformation advance toward the plate interior. The retroarc positions of the Andean topographic front (marked by frontal thrust-belt structures) and foreland deformation front (defined by isolated basement block uplifts) range from 300 to 900 km inboard of the trench axis. Over the ~8000 km arcuate length of the Andes (10°N to 55°S), four discrete maxima of inboard deformation advance are spatially co-located with the Peruvian (5°S–14°S) and Pampean (27°S–33°S) zones of flat slab subduction, the subducted Chile Ridge (45°S–48°S), and the anomalously thick Paleozoic stratigraphic wedge of Bolivia (17°S –23°S). The spatial correspondence of retroarc shortening with specific geodynamic configurations demonstrates the mechanical role of flat slab subduction, slab window development, and combined structural and stratigraphic geometries in shaping the orogenic architecture of Cordilleran margins, largely through lithospheric strengthening, weakening, and/or tectonic inheritance.
      PubDate: Fri, 27 May 2022 00:00:00 GMT
       
  • Protogenetic clinopyroxene inclusions in diamond and Nd diffusion
           modeling—Implications for diamond dating

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      Abstract: AbstractDiamonds are witnesses of processes that have operated in Earth's mantle over more than 3 b.y. Essential to our understanding of these processes is the determination of diamond crystallization ages. These cannot be directly determined on diamond, but they can be calculated using radiogenic isotopic systematics of suitable minerals included in a diamond. This method relies on the assumption that the mineral inclusions were in isotopic equilibrium with the diamond-forming medium. We evaluated the validity of Sm-Nd ages yielded by clinopyroxene inclusions by combining crystallographic orientation analyses and Nd diffusion modeling at the relevant conditions for Earth's cratonic mantle. We investigated the crystallographic orientation relationships (CORs) for 54 clinopyroxene inclusions within 18 diamonds from South Africa and Siberia. Clinopyroxene inclusions in some diamonds showed specific CORs with their hosts, indicating possible syngenesis. Other samples had clusters of clinopyroxene inclusions sharing the same orientation but no specific orientation relative to their hosts, indicating that the inclusions are older than the diamond (i.e., they are protogenetic). Diffusion modeling in the temperature range typical for lithospheric diamonds (900–1400 °C) showed that resetting of the Sm-Nd isotopic system in clinopyroxene grains larger than 0.05 mm requires geologically long interaction with the diamond-forming fluid/melt (>3.5 m.y. at average temperature of ~1150 °C). Depending on inclusion size and temperature regime, protogenetic clinopyroxene inclusions may not fully reequilibrate during diamond-formation events. We suggest that small clinopyroxene inclusions (<0.2 mm) that equilibrated at temperatures higher than 1050–1080 °C may be the most suitable for age determinations.
      PubDate: Fri, 27 May 2022 00:00:00 GMT
       
  • Accumulation of windblown sand in impact craters on Mars

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      Abstract: AbstractLoose sand, blown away from source regions by winds, is transported across Mars's surface into sand sheets and dunes and accumulates within topographic sinks. In the absence of plate tectonics, impact craters constitute a dominant sink for windblown sediments on Mars today. We analyzed the volume of all mapped eolian sands in martian craters >1 km in diameter to reveal spatiotemporal patterns of sediment accumulation on the planet's surface. We combined our results with global climate simulations, maps of depth to the ice table and dust cover, as well as lithologic and age information of the underlying geologic units, to better understand the controls on intracrater sand accumulation rates. We find that crater age, latitude, and lithology influence the accumulation rate of windblown sand and, notably, that it is enhanced in mechanically weaker substrates, high-latitude craters (suggesting that modern cryogenic processes may enhance sand production), and in Late Noachian and Early Hesperian craters (possibly hinting at increased erosion rates at that time).
      PubDate: Thu, 12 May 2022 00:00:00 GMT
       
  • Seismic evidence for lithospheric boudinage and its implications for
           continental rifting

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      Abstract: AbstractThe continental rifting that precedes the breakup of a continent and the formation of a new ocean basin is one of the key processes of plate tectonics. Although often viewed as a two-dimensional process, rifted margins exhibit significant variations along strike. We document along-strike variations developed during the ca. 200–160 Ma continental rifting that formed the margins of the Gulf of Mexico ocean basin. Rayleigh-wave ambient noise tomography reveals a zone of high and low seismic velocity resembling large scale geologic boudins in the mantle lithosphere of the northwestern Gulf of Mexico margin. These features become progressively less prominent eastward following the transition from a magma-poor to a magma-rich passive margin. We infer that mantle refertilization and thickness of the pre-rift lithosphere control deformation style and the along-strike variations in continental rifting. Our results also suggest that deformation during rifting produces long-lived features that persist long after breakup and, therefore, can be used to study rifted margins globally.
      PubDate: Thu, 12 May 2022 00:00:00 GMT
       
 
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