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New Geology articles published online ahead of print in June – EurekAlert

Geological Society of America
Boulder, Colo., USA: Article topics include the Great Unconformity of the Rocky Mountain region; new Ediacara-type fossils; the southern Cascade arc (California, USA); the European Alps and the Late Pleistocene glacial maximum; Permian-Triassic ammonoid mass extinction; permafrost thaw; the southern Rocky Mountains of Colorado (USA); “gargle dynamics”; invisible gold; and alluvial fan deposits in Valles Marineris, Mars. These Geology articles are online at https://geology.geoscienceworld.org/content/early/recent .
A new kind of invisible gold in pyrite hosted in deformation-related dislocations
Denis Fougerouse; Steven M. Reddy; Mark Aylmore; Lin Yang; Paul Guagliardo …
Abstract: Mining of “invisible gold” associated with sulfides in gold ores represents a significant proportion of gold production worldwide. Gold hosted in sulfide minerals has been proposed to be structurally bound in the crystal lattice as a sulfide-gold alloy and/or to occur as discrete metallic nanoparticles. Using a combination of microstructural quantification and nanoscale geochemical analyses on a pyrite crystal from an orogenic gold deposit, we show that dislocations hosted in a deformation low-angle boundary can be enriched in Ni, Cu, As, Pb, Sb, Bi, and Au. The cumulative trace-element enrichment in the dislocations is 3.2 at% higher compared to the bulk crystal. We propose that trace elements were segregated during the migration of the dislocation following the dislocation-impurity pair model. The gold hosted in nanoscale dislocations represents a new style of invisible gold.
View article : https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49028.1/604581/A-new-kind-of-invisible-gold-in-pyrite-hosted-in
A quantification of the effect of diagenesis on the paleoredox record in mid-Proterozoic sedimentary rocks
Alec M. Hutchings; Alexandra V. Turchyn
Abstract: Iron speciation in ancient sedimentary rocks is widely used to reconstruct oceanic redox conditions over geological time, specifically to assess the extent of oxic, euxinic (anoxic containing sulfide), and ferruginous (anoxic containing iron) conditions. We explore how post-depositional sedimentary processes can skew particular geochemical signals in the rock record. One such process is when aqueous sulfide—including that produced in the sediment column—reacts with sedimentary iron, converting non-sulfide, highly reactive iron minerals to iron-sulfide minerals; this can lead to increased preservation of iron as pyrite and an overestimation of seafloor euxinia. We show that sedimentary rocks with higher (>5 wt%) total iron content are more buffered to this effect and thus are a more reliable indicator of true water-column euxinia. When considering this effect in the geological past, we estimate that true euxinia in the mid-Proterozoic may have been as much as fourfold less than previously thought—more in line with other recent paleoredox proxies not based on iron minerals. Marine iron and sulfate concentrations were more equivalent in Proterozoic–Neoproterozoic oceans, suggesting this time period was particularly susceptible to this post-depositional alteration, explaining the extent of euxinia suggested for this geological interval.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48774.1/604580/A-quantification-of-the-effect-of-diagenesis-on
How fast do submarine fans grow? Insights from the Quaternary Golo fans, offshore Corsica
Michael L. Sweet; Gwladys T. Gaillot; Tammy M. Rittenour; Kathrine Love; Michael D. Blum
Abstract: High-resolution seismic, core, and chronological data from the Quaternary Golo deep-sea fans, offshore Corsica, France, give new insights into rates of submarine fan growth. Average vertical deposition rates for units that represent the Late Pleistocene glacial periods are 0.1–0.5 m/k.y. Glacial-age deposits are sand rich; in contrast, post-glacial deposits lack a significant sand fraction and are dominated by carbonate-rich mud. As a result, seismically constrained volumetric rates of deposition for glacial periods with low sea level and a subaerially exposed shelf are ~0.23 km 3/k.y., 2×–5× higher than rates during interglacials when sea level is high, the shelf is submerged, and sand is trapped in shallow-marine environments. At millennial time scales, variations in deposition rate reflect climate-driven sea-level changes, autogenic avulsion of river channels that extend across the shelf during low sea level, and autogenic avulsion of submarine channels that shift the locus of deposition laterally. Short-term deposition rates range from 8.6 m/k.y. at proximal portions of submarine fans to 0.4 m/k.y. along the distal fringe. Our data show that submarine fans can be dynamic environments with formation and evolution of levee-confined channels and lobe complexes in 10 3–104 yr, comparable to the time scales needed to form fluvial channel belts and delta lobes.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48911.1/604582/How-fast-do-submarine-fans-grow-Insights-from-the
Contrasting northern and southern European winter climate trends during the Last Interglacial
J. Sakari Salonen; Maria Fernanda Sánchez-Goñi; Hans Renssen; Anna Plikk
Abstract: The Last Interglacial (LIG; 130–115 ka) is an important test bed for climate science as an instance of significantly warmer than preindustrial global temperatures. However, LIG climate patterns remain poorly resolved, especially for winter, affected by a suite of strong feedbacks such as changes in sea-ice cover in the high latitudes. We present a synthesis of winter temperature and precipitation proxy data from the Atlantic seaboard of Europe, spanning from southern Iberia to the Arctic. Our data reveal distinct, opposite latitudinal climate trends, including warming winters seen in the European Arctic while cooling and drying occurred in southwest Europe over the LIG. Climate model simulations for 130 and 120 ka suggest these contrasting climate patterns were affected by a shift toward an atmospheric circulation regime with an enhanced meridional pressure gradient and strengthened midlatitude westerlies, leading to a strong reduction in precipitation across southern Europe.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49007.1/604583/Contrasting-northern-and-southern-European-winter
A record of syn-tectonic sedimentation revealed by perched alluvial fan deposits in Valles Marineris, Mars
J.M. Davis; P.M. Grindrod; S.G. Banham; N.H. Warner; S.J. Conway …
Abstract: On Mars, basins formed by tectonic processes are rare and mostly have unconstrained subsidence histories. One method for understanding this record of subsidence is through associated alluvial fans, which are sourced from uplifted areas and accumulate in downthrown basins. The source, morphology, and superposition of fan deposits can be used to reconstruct fault kinematics, the relative timing of accommodation space formation, and, in turn, the influence tectonic processes had on Martian fan formation. Here we use high-resolution orbital data sets to characterize sediment fan deposits associated with syn-tectonic sedimentation in two regions of the Valles Marineris canyons: Coprates Chasma and Juventae Chasma. These deposits comprise sediment fans on the current canyon floor and low-gradient surfaces perched several kilometers above the canyon floor. We interpret the low-gradient surfaces as remnant sediment fan deposits, which originally formed at the former canyon floor and have since been offset due to normal faulting. The preservation of vertically offset generations of sediment fan deposits supports a progressive, basinward migration of fault activity into the original hanging wall or repeat activity along a fault zone. Each episode of faulting was followed by a basinward shift in drainages, which led to fault-scarp degradation and formation of a new generation of fans. Multiple episodes of syn-tectonic sedimentation occurred during the evolution of the basins, with fluvial activity sporadically active. Our results demonstrate, for the first time on Mars, that depositional cyclicity was linked to tectonic deformation, possibly representative of regional processes throughout Valles Marineris.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48971.1/604584/A-record-of-syn-tectonic-sedimentation-revealed-by
Seismic fault weakening via CO2 pressurization enhanced by mechanical deformation of dolomite fault gouges
Hyun Na Kim; Byung-Dal So; Min Sik Kim; Kee Sung Han; Sol Bi Oh
Abstract: Carbon dioxide emissions from dolomite decarbonation play an essential role in the weakening of carbonate faults by lowering the effective normal stress, which is thermally activated at temperatures above 600–700 °C. However, the mechanochemical effect of low-crystalline ultrafine fault gouge on the decarbonation and slip behavior of dolomite-bearing faults remains unclear. In this study, we obtained a series of artificial dolomite fault gouges with systematically varying particle sizes and dolomite crystallinities using a high-energy ball mill. The laboratory-scale pulverization of dolomite yielded MgO at temperatures below 50 °C, indicating that mechanical decarbonation without significant heating occurred due to the collapse of the crystalline structure, as revealed by X-ray diffraction and solid-state nuclear magnetic resonance results. Furthermore, the onset temperature of thermal decarbonation decreased to ~400 °C. Numerical modeling reproduced this two-stage decarbonation, where the pore pressure increased due to low-temperature thermal decarbonation, leading to slip weakening on the fault plane even at 400–500 °C; i.e., 200–300 °C lower than previously reported temperatures. Thus, the presence of small amounts of low-crystalline dolomite in a fault plane may lead to a severely reduced shear strength due to thermal decomposition at ~400 °C with a small slip weakening distance.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48938.1/604585/Seismic-fault-weakening-via-CO2-pressurization
Alpine relief limited by glacial occupation time
Bernhard Salcher; Günther Prasicek; Sebastian Baumann; Florian Kober
Abstract: Glaciers exert a major control on the shape of mountain topography. They tend to reduce relief above and scour troughs below the equilibrium line altitude (ELA). While many studies report this dichotomy, relief-limiting effects are controversial due to difficulties in quantifying key factors such as the initial topography, the timing of glacial occupancy, or rock uplift counteracting glacial erosion. Consequently, effectivity and degree of glacial erosion remain ambiguous. In geologically and climatically well-investigated parts of the European Central Alps, our calculation of glacial occupation time (GOT) from Quaternary ELA variations allows the quantification of gradual topographic modifications generated by the cumulative impact of cirque erosion over the Quaternary. We show that under low uplift, relief is effectively limited by glacial and periglacial headwall retreat, leading to a decline in topographic relief as GOT increases. Conversely, higher uplift rates seem to induce more persistent valley glaciation, triggering a positive feedback loop in which steep slopes are protected against erosion and relief increases.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48639.1/604586/Alpine-relief-limited-by-glacial-occupation-time
Spatially variable provenance of the Chinese Loess Plateau
Haobo Zhang; Junsheng Nie; Xiangjun Liu; Alex Pullen; Guoqiang Li …
Abstract: Loess sequences of the Chinese Loess Plateau (CLP) compose one of the most complete Neogene–Quaternary terrestrial paleoclimatic archives. Understanding the CLP’s sediment sources is critical to tracing Asian aridification, atmospheric circulation patterns, and Asian monsoon evolution. Commonly, the sediments that compose the Quaternary strata of the CLP are considered largely homogeneous, and thus numerous studies have applied a uniform source model when attempting to use CLP-derived proxies as paleoclimate indicators. Here we present large-n detrital zircon U-Pb geochronology data from the Quaternary CLP. These data support spatial variability in sediment provenance across the CLP. At least three distinct provenance zones are recognized for Quaternary loess strata: central western, eastern, and northeastern. These zones received sediment primarily from their neighboring river systems. This finding conflicts with the classic views that attribute the Quaternary loess principally to the deserts north and west of the CLP. We conclude that fluvial processes, and thus precipitation, played an important and previously underemphasized role in Quaternary dust production in northern China. Furthermore, nonuniformity in CLP Quaternary sediment provenance raises questions about the validity of using paleoclimate information archived in the CLP to make sweeping regional interpretations.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48867.1/604587/Spatially-variable-provenance-of-the-Chinese-Loess
Igneous rock area and age in continental crust
Shanan E. Peters; Craig R. Walton; Jon M. Husson; Daven P. Quinn; Oliver Shorttle …
Abstract: Rock quantity and age are fundamental features of Earth’s crust that pertain to many problems in geoscience. Here we combine new estimates of igneous rock area in continental crust from the Macrostrat database (https://macrostrat.org/) with a compilation of detrital zircon ages in order to investigate rock cycling and crustal growth. We find that there is little or no decrease in igneous rock area with increasing rock age. Instead, igneous rock area in North America exhibits four distinct Precambrian peaks, remains low through the Neoproterozoic, and then increases only modestly toward the recent. Peaks in Precambrian detrital zircon age frequency distributions align broadly with peaks in igneous rock area, regardless of grain depositional age. However, detrital zircon ages do underrepresent a Neoarchean peak in igneous rock area; young grains and ca. 1.1 Ga grains are also overrepresented relative to igneous area. Together, these results suggest that detrital zircon age distributions contain signatures of continental denudation and sedimentary cycling that are decoupled from the cycling of igneous source rocks. Models of continental crustal evolution that incorporate significant early increase in volume and increased sedimentation in the Phanerozoic are well supported by these data.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49037.1/604588/Igneous-rock-area-and-age-in-continental-crust
Explosive caldera-forming eruptions and debris-filled vents: Gargle dynamics
Greg A. Valentine; Meredith A. Cole
Abstract: Large explosive volcanic eruptions are commonly associated with caldera subsidence and ignimbrites deposited by pyroclastic currents. Volumes and thicknesses of intracaldera and outflow ignimbrites at 76 explosive calderas around the world indicate that subsidence is commonly simultaneous with eruption, such that large proportions of the pyroclastic currents are trapped within the developing basins. As a result, much of an eruption must penetrate its own deposits, a process that also occurs in large, debris-filled vent structures even in the absence of caldera formation and that has been termed “gargling eruption.” Numerical modeling of the resulting dynamics shows that the interaction of preexisting deposits (fill) with an erupting (juvenile) mixture causes a dense sheath of fill material to be lifted along the margins of the erupting jet. This can cause an eruption that would otherwise produce a buoyant plume and fallout deposits to instead form pyroclastic currents as the dense sheath drives pulsing jet behavior. Increasing thickness of fill amplifies the time variation in jet height. Increasing the fill grain size relative to that of the juvenile particles can result in a much higher jet due to poorer mixing between the dense sheath and the dilute jet core. In all cases, material collapses along the entire height of the dense sheath rather than from the top of a simple fountain. These gargle dynamics provide strong backing for processes that have been inferred to result in intraplinian ignimbrites and simultaneous deposition from high- and low-energy pyroclastic currents.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48995.1/604589/Explosive-caldera-forming-eruptions-and-debris
Prior oil and gas production can limit the occurrence of injection-induced seismicity: A case study in the Delaware Basin of western Texas and southeastern New Mexico, USA
Noam Z. Dvory; Mark D. Zoback
Abstract: We demonstrate that pore pressure and stress changes resulting from several decades of oil and gas production significantly affect the likelihood of injection-related induced seismicity. We illustrate this process in the Delaware Basin (western Texas and southeastern New Mexico, USA), in which hydraulic fracturing and waste-water injection have been inducing numerous earthquakes in the southernmost part of the basin where there has been no prior oil and gas production from the formations in which the earthquakes are now occurring. In the seismically quiescent part of the basin, we show that pore-pressure and poroelastic-stress changes associated with prior oil and gas production make induced seismicity less likely. The findings of this study have important implications for the feasibility of large-scale carbon storage in depleted oil and gas reservoirs.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49015.1/604590/Prior-oil-and-gas-production-can-limit-the
Excess ice loads in the Indian Ocean sector of East Antarctica during the last glacial period
Takeshige Ishiwa; Jun’ichi Okuno; Yusuke Suganuma
Abstract: An accurate reconstruction of the Antarctic Ice Sheet is essential in order to develop an understanding of ice-sheet responses to global climate changes. However, the erosive nature of ice-sheet expansion and the difficulty of accessing much of Antarctica make it challenging to obtain field-based evidence of ice-sheet and sea-level changes before the Last Glacial Maximum. Limited sedimentary records from Lützow-Holm and Prydz Bays in East Antarctica demonstrate that the sea level during Marine Isotope Stage 3 was close to the present level despite the global sea-level drop lower than –40 m. We demonstrate glacial isostatic adjustment modeling with refined Antarctic Ice Sheet loading histories. Our experiments reveal that the Indian Ocean sector of the Antarctic Ice Sheet would have been required to experience excess ice loads before the Last Glacial Maximum in order to explain the observed sea-level highstands during Marine Isotope Stage 3. As such, we suggest that the Antarctic Ice Sheet partly reached its maximum thickness before the global Last Glacial Maximum.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48830.1/600851/Excess-ice-loads-in-the-Indian-Ocean-sector-of
Across-arc variations in Mo isotopes and implications for subducted oceanic crust in the source of back-arc basin volcanic rocks
Xiaohui Li; Quanshu Yan; Zhigang Zeng; Jingjing Fan; Sanzhong Li …
Abstract: Molybdenum (Mo) isotope ratios provide a potential means of tracing material recycling involved in subduction zone processes. However, the geochemical behavior of Mo in subducted oceanic crust remains enigmatic. We analyzed Mo isotope ratios of arc and back-arc basin lavas from the Mariana subduction zone (western Pacific Ocean), combining newly obtained element and Sr-Nd-Pb-Li isotope data to investigate subduction zone geochemical processes involving Mo. The Mo isotope ratios (δ98/95Mo NIST3134; U.S. National Institute of Standards and Technology [NIST] Mo standard) of the volcanic rocks showed clear across-arc variations, decreasing with increasing depth to the Wadati-Benioff zone. The high δ98/95Mo values in the Mariana Islands (–0.18‰ to +0.38‰) correspond to high 87Sr/86Sr, low 143Nd/144Nd, and radiogenic Pb isotope ratios, suggesting that altered upper oceanic crust played an important role in the magma source. The low δ98/95Mo values in the Central Mariana Trough (–0.65‰ to –0.17‰) with mantle-like Sr-Nd-Pb but slightly low δ 7Li values provide direct evidence for the contribution of deep recycled oceanic crust to the magma source of the back-arc basin lavas. The isotopically light Mo magmas originated by partial melting of a residual subducted slab (eclogite) after high degrees of dehydration and then penetrated into the back-arc mantle. This interpretation provides a new perspective with which to investigate the deep recycling of subducted oceanic lithosphere and associated magma petrogenesis.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48754.1/600852/Across-arc-variations-in-Mo-isotopes-and
Tectonic controls on basement exhumation in the southern Rocky Mountains (United States): The power of combined zircon (U-Th)/He and K-feldspar 40Ar/39Ar thermochronology
Jason W. Ricketts; Jacoup Roiz; Karl E. Karlstrom; Matthew T. Heizler; William R. Guenthner …
Abstract: The Great Unconformity of the Rocky Mountain region (western North America), where Precambrian crystalline basement is nonconformably overlain by Phanerozoic strata, represents the removal of as much as 1.5 b.y. of rock record during 10-km-scale basement exhumation. We evaluate the timing of exhumation of basement rocks at five locations by combining geologic data with multiple thermochronometers. 40Ar/39Ar K-feldspar multi-diffusion domain (MDD) modeling indicates regional multi-stage basement cooling from 275 to 150 °C occurred at 1250–1100 Ma and/or 1000–700 Ma. Zircon (U-Th)/He (ZHe) dates from the Rocky Mountains range from 20 to 864 Ma, and independent forward modeling of ZHe data is also most consistent with multi-stage cooling. ZHe inverse models at five locations, combined with K-feldspar MDD and sample-specific geochronologic and/or thermochronologic constraints, document multiple pulses of basement cooling from 250 °C to surface temperatures with a major regional basement exhumation event 1300–900 Ma, limited cooling in some samples during the 770–570 Ma breakup of Rodinia and/or the 717–635 Ma snowball Earth, and ca. 300 Ma Ancestral Rocky Mountains cooling. These data argue for a tectonic control on basement exhumation leading up to formation of the Precambrian-Cambrian Great Unconformity and document the formation of composite erosional surfaces developed by faulting and differential uplift.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49141.1/600853/Tectonic-controls-on-basement-exhumation-in-the
Reorienting the West African craton in Paleoproterozoic–Mesoproterozoic supercontinent Nuna
Zheng Gong; David A.D. Evans; Nasrrddine Youbi; Abdelhak Ait Lahna; Ulf Söderlund …
Abstract: The location of the West African craton (WAC) has been poorly constrained in the Paleoproterozoic–Mesoproterozoic supercontinent Nuna (also known as Columbia). Previous Nuna reconstruction models suggested that the WAC was connected to Amazonia in a way similar to their relative position in Gondwana. By an integrated paleomagnetic and geochronological study of the Proterozoic mafic dikes in the Anti-Atlas Belt, Morocco, we provide two reliable paleomagnetic poles to test this connection. Incorporating our new poles with quality-filtered poles from the neighboring cratons of the WAC, we propose an inverted WAC-Amazonia connection, with the northern WAC attached to northeastern Amazonia, as well as a refined configuration of Nuna. Global large igneous province records also conform to our new reconstruction. The inverted WAC-Amazonia connection suggests a substantial change in their relative orientation from Nuna to Gondwana, providing an additional example of large-magnitude cumulative azimuthal rotations between adjacent continental blocks over supercontinental cycles.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48855.1/600854/Reorienting-the-West-African-craton-in
New Ediacara-type fossils and late Ediacaran stratigraphy from the northern Qaidam Basin (China): Paleogeographic implications
Ke Pang; Chengxi Wu; Yunpeng Sun; Qing Ouyang; Xunlai Yuan …
Abstract: Ediacara-type macrofossils characterize the late Ediacaran Period and are pivotal in understanding the early evolution of animals on the eve of the Cambrian explosion and useful in late Ediacaran biostratigraphy. They have been discovered on almost all major paleocontinents, except the North China and Tarim blocks, as well as on a series of northwestwest–oriented cratonic fragments between the two blocks, including the Olongbuluke terrane of the Qaidam block, where the terminal Ediacaran successions developed. We report a newly discovered terminal Ediacaran biotic assemblage, the Quanjishan assemblage, containing Ediacara-type fossils from the Zhoujieshan Formation of the Quanji Group in the Olongbuluke terrane, Qaidam Basin, northwestern China. The Quanjishan assemblage is dominated by the non-biomineralized tubular taxon Shaanxilithes, which has the potential to be a terminal Ediacaran index fossil, and by the iconic frondose rangeomorph Charnia, which represents the only unambiguous Ediacara-type fossil discovered in northwestern China. The co-occurrence of Charnia and Shaanxilithes from the Quanjishan assemblage likely constrains the depositional age of the Zhoujieshan Formation to be terminal Ediacaran (ca. 550–539 Ma) and the immediately underlying Hongtiegou diamictites to be late Ediacaran, probably representing post-Gaskiers glacial deposition. The occurrence of post-Gaskiers Ediacaran glaciation and similarities between the late Ediacaran–early Paleozoic lithostratigraphic and biostratigraphic sequences in the Olongbuluke terrane of the Qaidam block and the North China block suggest that these two blocks may have been located close to each other during this time period, and situated in the middle to high latitudes instead of the equatorial region.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48842.1/600855/New-Ediacara-type-fossils-and-late-Ediacaran
Slab-derived sulfate generates oxidized basaltic magmas in the southern Cascade arc (California, USA)
Michelle J. Muth; Paul J. Wallace
Abstract: Whether and how subduction increases the oxidation state of Earth’s mantle are two of the most important unresolved questions in solid Earth geochemistry. Using data from the southern Cascade arc (California, USA), we show quantitatively for the first time that increases in arc magma oxidation state are fundamentally linked to mass transfer of isotopically heavy sulfate from the subducted plate into the mantle wedge. We investigate multiple hypotheses related to plate dehydration and melting and the rise and reaction of slab melts with mantle peridotite in the wedge, focusing on electron balance between redox-sensitive iron and sulfur during these processes. These results show that unless slab-derived silicic melts contain much higher dissolved sulfur than is indicated by currently available experimental data, arc magma generation by mantle wedge melting must involve multiple stages of mantle metasomatism by slab-derived oxidized and sulfur-bearing hydrous components.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48759.1/600856/Slab-derived-sulfate-generates-oxidized-basaltic
Long-term Aptian marine osmium isotopic record of Ontong Java Nui activity
Hironao Matsumoto; Rodolfo Coccioni; Fabrizio Frontalini; Kotaro Shirai; Luigi Jovane …
Abstract: The early to mid-Aptian was punctuated by episodic phases of organic-carbon burial in various oceanographic settings, which are possibly related to massive volcanism associated with the emplacement of the Ontong Java, Manihiki, and Hikurangi oceanic plateaus in the southwestern Pacific Ocean, inferred to have formed a single plateau called Ontong Java Nui. Sedimentary osmium (Os) isotopic compositions are one of the best proxies for determining the timing of voluminous submarine volcanic episodes. However, available Os isotopic records during the age are limited to a narrow interval in the earliest Aptian, which is insufficient for the reconstruction of long-term hydrothermal activity. We document the early to mid-Aptian Os isotopic record using pelagic Tethyan sediments deposited in the Poggio le Guaine (Umbria-Marche Basin, Italy) to precisely constrain the timing of massive volcanic episodes and to assess their impact on the marine environment. Our new Os isotopic data reveal three shifts to unradiogenic values, two of which correspond to black shale horizons in the lower to mid-Aptian, namely the Wezel (herein named) and Fallot Levels. These Os isotopic excursions are ascribed to massive inputs of unradiogenic Os to the ocean through hydrothermal activity. Combining the new Os isotopic record with published data from the lowermost Aptian organic-rich interval in the Gorgo a Cerbara section of the Umbria-Marche Basin, it can be inferred that Ontong Java Nui volcanic eruptions persisted for ~5 m.y. during the early to mid-Aptian.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48863.1/600857/Long-term-Aptian-marine-osmium-isotopic-record-of
Anisotropy-revealed change in hydration along the Alaska subduction zone
Colton Lynner
Abstract: Megathrust earthquake behavior in subduction zones is controlled by a variety of factors including the hydration state of the subducting slab. Increased hydration reduces the occurrence of great, damaging earthquakes by diminishing the strength of the material along the interface between tectonic plates. Understanding variations in hydration in subductions zones is necessary for properly assessing the overall hazard posed by each region. Fortunately, seismic anisotropy is strongly dependent upon hydration of the subducting crust and lithosphere. I present shear-wave splitting measurements that illuminate changes in anisotropy, and therefore hydration, of the subducting Pacific plate beneath the Alaska subduction zone (northern Pacific Ocean). Variations in shear-wave splitting directly correlate to changes in the behavior of great, megathrust earthquakes. My measurements show that the Shumagin seismic gap is characterized by a hydrated subducting slab, explaining the long-term lack of great earthquakes. Observations in the immediately adjacent Semidi segment, which experiences great events regularly, indicate a far less hydrated slab. These results are driven by the preferential alignment of paleo-spreading fabrics of the Pacific plate. Where fabrics are more closely aligned with the orientation of the trench, outer-rise faulting and plate hydration is enhanced. These results highlight the importance of changes in preexisting slab structures and subsequent hydration in the production of great, damaging earthquakes.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48860.1/600696/Anisotropy-revealed-change-in-hydration-along-the
Out-of-phase Late Pleistocene glacial maxima in the Western Alps reflect past changes in North Atlantic atmospheric circulation
Natacha Gribenski; Pierre G. Valla; Frank Preusser; Thibault Roattino; Christian Crouzet …
Abstract: Paleoglacier reconstructions in the northern and southern forelands of the European Alps indicate a synchronous Late Pleistocene glacial maximum during Marine Isotope Stage (MIS) 2, in phase with global ice volume records. However, strong controversy remains for the western foreland, where scarce and indirect dating as well as modeling studies suggest glacial maxima out of phase with the rest of the Alps. New luminescence dating brings the first direct Late Pleistocene glacial chronology for the western Alpine foreland and reveals two major glacier advances of similar maximum extent, at ca. 75–60 and ca. 40–30 ka, coinciding with MIS 4 and late MIS 3. We propose that asynchrony in glacial maxima between the western and the northern and southern Alpine forelands results from a progressive spatial reorganization of the atmospheric circulation over the North Atlantic in response to Northern Hemisphere ice-sheet fluctuations. While such a feedback mechanism has emerged from general circulation models, our Late Pleistocene paleoglacial reconstruction permits tracking of the spatiotemporal evolution of moisture advection patterns over Western Europe.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48688.1/600697/Out-of-phase-Late-Pleistocene-glacial-maxima-in
Highly localized upper mantle deformation during plate boundary initiation near the Alpine fault, New Zealand
Steven Kidder; David J. Prior; James M. Scott; Hamid Soleymani; Yilun Shao
Abstract: Peridotite xenoliths entrained in magmas near the Alpine fault (New Zealand) provide the first direct evidence of deformation associated with the propagation of the Australian-Pacific plate boundary through the region at ca. 25–20 Ma. Two of 11 sampled xenolith localities contain fine-grained (40–150 mm) rocks, indicating that deformation in the upper mantle was focused in highly sheared zones. To constrain the nature and conditions of deformation, we combine a flow law with a model linking recrystallized fraction to strain. Temperatures calculated from this new approach (625–970 °C) indicate that the observed deformation occurred at depths of 25–50 km. Calculated shear strains were between 1 and 100, which, given known plate offset rates (10–20 mm/yr) and an estimated interval during which deformation likely occurred (<1.8 m.y.), translate to a total shear zone width in the range 0.2–32 km. This narrow width and the position of mylonite-bearing localities amid mylonite-free sites suggest that early plate boundary deformation was distributed across at least ~60 km but localized in multiple fault strands. Such upper mantle deformation is best described by relatively rigid, plate-like domains separated by rapidly formed, narrow mylonite zones.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48532.1/600699/Highly-localized-upper-mantle-deformation-during
Morphological selectivity of the Permian-Triassic ammonoid mass extinction
Xu Dai; Dieter Korn; Haijun Song
Abstract: Ammonoids suffered a diversity bottleneck during the Permian-Triassic mass extinction (PTME) and experienced a rapid diversification in the Early Triassic. However, the kinds of ammonoids that were more likely to survive the PTME and that fueled subsequent diversification are still poorly known. We compiled a comprehensive morphological data set and used the nonmetric multidimensional scaling method to reveal the impact of the PTME on the morphological selectivity of ammonoids. Our results show that postextinction taxa occupied a quite different morphospace when compared with the pre-extinction assemblages. The survivors were mainly smooth and weakly ornamented forms, while the late Permian species were dominated by coarsely ornamented forms. Contrary to previously recognized nonselective patterns, these results suggest a morphological selectivity of the Permian-Triassic crisis. Newcomers in the Griesbachian were mainly compressed and smooth forms. This morphological shift from the coarsely ornamented ammonoids dominating the Changhsingian to the smooth ammonoids dominating the Griesbachian possibly suggests an ecological turnover of ammonoids during the PTME.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48788.1/600700/Morphological-selectivity-of-the-Permian-Triassic
Permafrost thaw induced abrupt changes in hydrology and carbon cycling in Lake Wudalianchi, northeastern China
Yuan Yao; Yongsong Huang; Jiaju Zhao; Li Wang; Youhua Ran …
Abstract: Lakes in the permafrost zone have been proposed to serve as key outlets for methane and carbon dioxide emissions. However, there has been no geological record of the hydrological and biogeochemical responses of lakes throughout the thawing of surrounding permafrost. We use multiple biomarker and isotopic proxies to reconstruct hydrological and biogeochemical changes in Lake Wudalianchi in northeastern China during regional thawing of the permafrost. We show permafrost thawing, as indicated by lignin degradation, initiated rapid lake water freshening as a result of the opening of groundwater conduits, and negative organic δ13C excursion due to increased inorganic and organic carbon fluxes. These hydrological changes were followed, with an ~5–7 yr delay, by abrupt and persistent increases in microbial lake methanotrophy and methanogenesis, indicating enhanced anaerobic organic decomposition and methane emissions from lakes as permafrost thaws. Our data provide a detailed assessment of the processes involved during permafrost thaw, and highlight the importance of lakes in ventilating greenhouse gases to the atmosphere.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48891.1/600701/Permafrost-thaw-induced-abrupt-changes-in
Erosion of the Himalaya-Karakoram recorded by Indus Fan deposits since the Oligocene
Han Feng; Huayu Lu; Barbara Carrapa; Hanzhi Zhang; Jun Chen …
Abstract: The Cenozoic erosion history of the Himalaya-Karakoram, which is a function of tectonically driven uplift and monsoon climatic evolution in South Asia, remains elusive, especially prior to the Miocene. Here, we present a multiproxy geochemical and thermochronological analysis of the oldest samples available from the Arabian Sea, which we used to investigate the erosion history of the Himalayan and Karakoram orogenic system. The Indus Fan records rapid and sustained erosion of the Himalayan-Karakoram mountains from before 24 Ma (ca. 30) to ca. 16 Ma concurrent with changing provenance from the Indian (Himalayan) and Eurasian plates. Our data, combined with previous studies of younger Indus Fan deposits, indicate that the mid-to-late Cenozoic erosion history of the Himalayan-Karakoram mountains is overall consistent with a vigorous monsoonal climate from the late Oligocene to middle Miocene and with changes in global climate in the late Miocene, whereas erosion and deposition are relatively insensitive to changes in sources and rock erodibility. Although tectonic processes were active throughout, we suggest that the erosional signatures of the Himalayan-Karakoram mountains from the Indus Fan largely preserve a record of climate changes since the Oligocene.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48445.1/600703/Erosion-of-the-Himalaya-Karakoram-recorded-by
Raising the West: Mid-Cenozoic Colorado-plano related to subvolcanic batholith assembly in the Southern Rocky Mountains (USA)?
Peter W. Lipman
Abstract: The Southern Rocky Mountains of Colorado, United States, have the highest regional elevation in North America, but present-day crustal thickness (~42–47 km) is no greater than for the adjacent, topographically lower High Plains and Colorado Plateau. The chemistry of continental-arc rocks of the mid-Cenozoic Southern Rocky Mountain volcanic field, calibrated to compositions and Moho depths at young arcs, suggests that paleocrustal thickness may have been 20%–35% greater than at present and elevations accordingly higher. Thick mid-Cenozoic Rocky Mountain crust and high paleo-elevations, comparable to those inferred for the Nevadaplano farther west in the United States from analogous volcanic chemistry, could be consistent with otherwise-perplexing evidence for widespread rapid erosion during volcanism. Variable mid-Cenozoic crustal thickening and uplift could have resulted from composite batholith growth during volcanism, superimposed on prior crustal thickening during early Cenozoic (Laramide) compression. Alternatively, the arc–crustal thickness calibration may be inappropriate for high-potassium continental arcs, in which case other published interpretations using similar methods may also be unreliable.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48963.1/600704/Raising-the-West-Mid-Cenozoic-Colorado-plano
Labyrinth patterns in Magadi (Kenya) cherts: Evidence for early formation from siliceous gels
Kennie Leet; Tim K. Lowenstein; Robin W. Renaut; R. Bernhart Owen; Andrew Cohen
Abstract: Sedimentary cherts, with well-preserved microfossils, are known from the Archean to the present, yet their origins remain poorly understood. Lake Magadi, Kenya, has been used as a modern analog system for understanding the origins of nonbiogenic chert. We present evidence for synsedimentary formation of Magadi cherts directly from siliceous gels. Petrographic thin-section analysis and field-emission scanning electron microscopy of cherts from cores drilled in Lake Magadi during the Hominin Sites and Paleolakes Drilling Project in 2014 led to the discovery of two-dimensional branching “labyrinth patterns” in chert, which are a type of fractal “squeeze” pattern formed at air-liquid interfaces. Labyrinth patterns preserved in chert from Lake Magadi cores indicate invasion of air along planes in dewatering gels. These patterns support the precipitation of silica gels in the saline-alkaline Lake Magadi system and syndepositional drying of gels in contact with air as part of chert formation. Recognizing cherts as syndepositional has been critical for our use of them for U-Th dating. Identification of labyrinth patterns in ancient cherts can provide a better understanding of paleoenvironmental and geochemical conditions in the past.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48771.1/600705/Labyrinth-patterns-in-Magadi-Kenya-cherts-Evidence
Serpentinized peridotite versus thick mafic crust at the Romanche oceanic transform fault
Emma P.M. Gregory; Satish C. Singh; Milena Marjanović; Zhikai Wang
Abstract: The crust beneath transform faults at slow-spreading ridges has been considered to be thin, comprising a thin mafic layer overlying serpentinized peridotite. Using wide-angle seismic data, we report the presence of a Moho at ~6 km depth and a low-velocity anomaly extending down to 9 km beneath the 20-km-wide Romanche transform valley floor in the equatorial Atlantic Ocean. The low crustal velocities above the Moho could be due to either highly serpentinized mantle peridotite or fractured mafic rocks. The existence of clear Moho reflections and the occurrence of a large crustal-depth rupture during the 2016 magnitude 7.1 earthquake suggest that the crust likely consists of fractured mafic material. Furthermore, the presence of low velocities below the Moho advocates for extensive serpentinization of the mantle, indicating that the Moho reflection is unlikely to be produced by a serpentinization front. The crust to the north of the transform fault likely consists of mafic material, but that in the south appears to be more amagmatic, possibly containing serpentinized peridotite. Our results imply that the transform fault structure is complex and highly heterogeneous, and thus would have significant influence on earthquake rupture and alteration processes.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49097.1/600706/Serpentinized-peridotite-versus-thick-mafic-crust
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