Boulder, Colo., USA: Article topics include the Great Unconformity of theRocky Mountain region; new Ediacara-type fossils; the southern Cascade arc(California, USA); the European Alps and the Late Pleistocene glacialmaximum; Permian-Triassic ammonoid mass extinction; permafrost thaw; thesouthern Rocky Mountains of Colorado (USA); "gargle dynamics"; invisiblegold; and alluvial fan deposits in Valles Marineris, Mars. These Geology articles are online athttps://geology.geoscienceworld.org/content/early/recent.
A new kind of invisible gold in pyrite hosted in deformation-relateddislocations
Denis Fougerouse; Steven M. Reddy; Mark Aylmore; Lin Yang; Paul Guagliardo...
Abstract:Mining of "invisible gold" associated with sulfides in gold ores representsa significant proportion of gold production worldwide. Gold hosted insulfide minerals has been proposed to be structurally bound in the crystallattice as a sulfide-gold alloy and/or to occur as discrete metallicnanoparticles. Using a combination of microstructural quantification andnanoscale geochemical analyses on a pyrite crystal from an orogenic golddeposit, we show that dislocations hosted in a deformation low-angleboundary can be enriched in Ni, Cu, As, Pb, Sb, Bi, and Au. The cumulativetrace-element enrichment in the dislocations is 3.2 at% higher compared tothe bulk crystal. We propose that trace elements were segregated during themigration of the dislocation following the dislocation-impurity pair model.The gold hosted in nanoscale dislocations represents a new style ofinvisible gold.
A quantification of the effect of diagenesis on the paleoredox recordin mid-Proterozoic sedimentary rocks
Alec M. Hutchings; Alexandra V. Turchyn
Abstract:Iron speciation in ancient sedimentary rocks is widely used to reconstructoceanic redox conditions over geological time, specifically to assess theextent of oxic, euxinic (anoxic containing sulfide), and ferruginous(anoxic containing iron) conditions. We explore how post-depositionalsedimentary processes can skew particular geochemical signals in the rockrecord. One such process is when aqueous sulfide--including that produced inthe sediment column--reacts with sedimentary iron, converting non-sulfide,highly reactive iron minerals to iron-sulfide minerals; this can lead toincreased preservation of iron as pyrite and an overestimation of seaflooreuxinia. We show that sedimentary rocks with higher (>5 wt%) total ironcontent are more buffered to this effect and thus are a more reliableindicator of true water-column euxinia. When considering this effect in thegeological past, we estimate that true euxinia in the mid-Proterozoic mayhave been as much as fourfold less than previously thought--more in linewith other recent paleoredox proxies not based on iron minerals. Marineiron and sulfate concentrations were more equivalent inProterozoic-Neoproterozoic oceans, suggesting this time period wasparticularly susceptible to this post-depositional alteration, explainingthe extent of euxinia suggested for this geological interval.
How fast do submarine fans grow? Insights from the Quaternary Golofans, 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 QuaternaryGolo deep-sea fans, offshore Corsica, France, give new insights into ratesof submarine fan growth. Average vertical deposition rates for units thatrepresent the Late Pleistocene glacial periods are 0.1-0.5 m/k.y.Glacial-age deposits are sand rich; in contrast, post-glacial deposits lacka significant sand fraction and are dominated by carbonate-rich mud. As aresult, seismically constrained volumetric rates of deposition for glacialperiods with low sea level and a subaerially exposed shelf are ~0.23 km 3/k.y., 2×-5× higher than rates during interglacials when sealevel is high, the shelf is submerged, and sand is trapped inshallow-marine environments. At millennial time scales, variations indeposition rate reflect climate-driven sea-level changes, autogenicavulsion of river channels that extend across the shelf during low sealevel, and autogenic avulsion of submarine channels that shift the locus ofdeposition laterally. Short-term deposition rates range from 8.6 m/k.y. atproximal portions of submarine fans to 0.4 m/k.y. along the distal fringe.Our data show that submarine fans can be dynamic environments withformation and evolution of levee-confined channels and lobe complexes in 10 3-104 yr, comparable to the time scales needed toform fluvial channel belts and delta lobes.
Contrasting northern and southern European winter climate trends duringthe 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 forclimate science as an instance of significantly warmer than preindustrialglobal temperatures. However, LIG climate patterns remain poorly resolved,especially for winter, affected by a suite of strong feedbacks such aschanges in sea-ice cover in the high latitudes. We present a synthesis ofwinter temperature and precipitation proxy data from the Atlantic seaboardof Europe, spanning from southern Iberia to the Arctic. Our data revealdistinct, opposite latitudinal climate trends, including warming wintersseen in the European Arctic while cooling and drying occurred in southwestEurope over the LIG. Climate model simulations for 130 and 120 ka suggestthese contrasting climate patterns were affected by a shift toward anatmospheric circulation regime with an enhanced meridional pressuregradient and strengthened midlatitude westerlies, leading to a strongreduction in precipitation across southern Europe.
A record of syn-tectonic sedimentation revealed by perchedalluvial 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 haveunconstrained subsidence histories. One method for understanding thisrecord of subsidence is through associated alluvial fans, which are sourcedfrom uplifted areas and accumulate in downthrown basins. The source,morphology, and superposition of fan deposits can be used to reconstructfault kinematics, the relative timing of accommodation space formation,and, in turn, the influence tectonic processes had on Martian fanformation. Here we use high-resolution orbital data sets to characterizesediment fan deposits associated with syn-tectonic sedimentation in tworegions of the Valles Marineris canyons: Coprates Chasma and JuventaeChasma. These deposits comprise sediment fans on the current canyon floorand low-gradient surfaces perched several kilometers above the canyonfloor. We interpret the low-gradient surfaces as remnant sediment fandeposits, which originally formed at the former canyon floor and have sincebeen offset due to normal faulting. The preservation of vertically offsetgenerations of sediment fan deposits supports a progressive, basinwardmigration of fault activity into the original hanging wall or repeatactivity along a fault zone. Each episode of faulting was followed by abasinward shift in drainages, which led to fault-scarp degradation andformation of a new generation of fans. Multiple episodes of syn-tectonicsedimentation occurred during the evolution of the basins, with fluvialactivity sporadically active. Our results demonstrate, for the first timeon Mars, that depositional cyclicity was linked to tectonic deformation,possibly representative of regional processes throughout Valles Marineris.
Seismic fault weakening via CO2 pressurization enhanced bymechanical 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 rolein the weakening of carbonate faults by lowering the effective normalstress, which is thermally activated at temperatures above 600-700 °C.However, the mechanochemical effect of low-crystalline ultrafine faultgouge on the decarbonation and slip behavior of dolomite-bearing faultsremains unclear. In this study, we obtained a series of artificial dolomitefault gouges with systematically varying particle sizes and dolomitecrystallinities using a high-energy ball mill. The laboratory-scalepulverization of dolomite yielded MgO at temperatures below 50 °C,indicating that mechanical decarbonation without significant heatingoccurred due to the collapse of the crystalline structure, as revealed byX-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, wherethe 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 presenceof small amounts of low-crystalline dolomite in a fault plane may lead to aseverely reduced shear strength due to thermal decomposition at ~400 °Cwith a small slip weakening distance.
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. Theytend to reduce relief above and scour troughs below the equilibrium linealtitude (ELA). While many studies report this dichotomy, relief-limitingeffects are controversial due to difficulties in quantifying key factorssuch as the initial topography, the timing of glacial occupancy, or rockuplift counteracting glacial erosion. Consequently, effectivity and degreeof glacial erosion remain ambiguous. In geologically and climaticallywell-investigated parts of the European Central Alps, our calculation ofglacial occupation time (GOT) from Quaternary ELA variations allows thequantification of gradual topographic modifications generated by thecumulative impact of cirque erosion over the Quaternary. We show that underlow uplift, relief is effectively limited by glacial and periglacialheadwall retreat, leading to a decline in topographic relief as GOTincreases. Conversely, higher uplift rates seem to induce more persistentvalley glaciation, triggering a positive feedback loop in which steepslopes are protected against erosion and relief increases.
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 mostcomplete Neogene-Quaternary terrestrial paleoclimatic archives.Understanding the CLP's sediment sources is critical to tracing Asianaridification, atmospheric circulation patterns, and Asian monsoonevolution. Commonly, the sediments that compose the Quaternary strata ofthe CLP are considered largely homogeneous, and thus numerous studies haveapplied a uniform source model when attempting to use CLP-derived proxiesas paleoclimate indicators. Here we present large-n detritalzircon U-Pb geochronology data from the Quaternary CLP. These data supportspatial variability in sediment provenance across the CLP. At least threedistinct provenance zones are recognized for Quaternary loess strata:central western, eastern, and northeastern. These zones received sedimentprimarily from their neighboring river systems. This finding conflicts withthe classic views that attribute the Quaternary loess principally to thedeserts north and west of the CLP. We conclude that fluvial processes, andthus precipitation, played an important and previously underemphasized rolein Quaternary dust production in northern China. Furthermore, nonuniformityin CLP Quaternary sediment provenance raises questions about the validityof using paleoclimate information archived in the CLP to make sweepingregional interpretations.
Igneous rock area and age in continental crust
Shanan E. Peters; Craig R. Walton; Jon M. Husson; Daven P. Quinn; OliverShorttle ...
Abstract:Rock quantity and age are fundamental features of Earth's crust thatpertain to many problems in geoscience. Here we combine new estimates ofigneous rock area in continental crust from the Macrostrat database(https://macrostrat.org/) with a compilation of detrital zircon ages inorder to investigate rock cycling and crustal growth. We find that there islittle or no decrease in igneous rock area with increasing rock age.Instead, igneous rock area in North America exhibits four distinctPrecambrian peaks, remains low through the Neoproterozoic, and thenincreases only modestly toward the recent. Peaks in Precambrian detritalzircon age frequency distributions align broadly with peaks in igneous rockarea, regardless of grain depositional age. However, detrital zircon agesdo underrepresent a Neoarchean peak in igneous rock area; young grains andca. 1.1 Ga grains are also overrepresented relative to igneous area.Together, these results suggest that detrital zircon age distributionscontain signatures of continental denudation and sedimentary cycling thatare decoupled from the cycling of igneous source rocks. Models ofcontinental crustal evolution that incorporate significant early increasein volume and increased sedimentation in the Phanerozoic are well supportedby these data.
Explosive caldera-forming eruptions and debris-filled vents: Gargledynamics
Greg A. Valentine; Meredith A. Cole
Abstract:Large explosive volcanic eruptions are commonly associated with calderasubsidence and ignimbrites deposited by pyroclastic currents. Volumes andthicknesses of intracaldera and outflow ignimbrites at 76 explosivecalderas around the world indicate that subsidence is commonly simultaneouswith eruption, such that large proportions of the pyroclastic currents aretrapped within the developing basins. As a result, much of an eruption mustpenetrate its own deposits, a process that also occurs in large,debris-filled vent structures even in the absence of caldera formation andthat has been termed "gargling eruption." Numerical modeling of theresulting dynamics shows that the interaction of preexisting deposits(fill) with an erupting (juvenile) mixture causes a dense sheath of fillmaterial to be lifted along the margins of the erupting jet. This can causean eruption that would otherwise produce a buoyant plume and falloutdeposits to instead form pyroclastic currents as the dense sheath drivespulsing jet behavior. Increasing thickness of fill amplifies the timevariation in jet height. Increasing the fill grain size relative to that ofthe juvenile particles can result in a much higher jet due to poorer mixingbetween the dense sheath and the dilute jet core. In all cases, materialcollapses along the entire height of the dense sheath rather than from thetop of a simple fountain. These gargle dynamics provide strong backing forprocesses that have been inferred to result in intraplinian ignimbrites andsimultaneous deposition from high- and low-energy pyroclastic currents.
Prior oil and gas production can limit the occurrence ofinjection-induced seismicity: A case study in the Delaware Basin ofwestern Texas and southeastern New Mexico, USA
Noam Z. Dvory; Mark D. Zoback
Abstract:We demonstrate that pore pressure and stress changes resulting from severaldecades of oil and gas production significantly affect the likelihood ofinjection-related induced seismicity. We illustrate this process in theDelaware Basin (western Texas and southeastern New Mexico, USA), in whichhydraulic fracturing and waste-water injection have been inducing numerousearthquakes in the southernmost part of the basin where there has been noprior oil and gas production from the formations in which the earthquakesare now occurring. In the seismically quiescent part of the basin, we showthat pore-pressure and poroelastic-stress changes associated with prior oiland gas production make induced seismicity less likely. The findings ofthis study have important implications for the feasibility of large-scalecarbon storage in depleted oil and gas reservoirs.
Excess ice loads in the Indian Ocean sector of East Antarctica duringthe last glacial period
Takeshige Ishiwa; Jun'ichi Okuno; Yusuke Suganuma
Abstract:An accurate reconstruction of the Antarctic Ice Sheet is essential in orderto develop an understanding of ice-sheet responses to global climatechanges. However, the erosive nature of ice-sheet expansion and thedifficulty of accessing much of Antarctica make it challenging to obtainfield-based evidence of ice-sheet and sea-level changes before the LastGlacial Maximum. Limited sedimentary records from Lützow-Holm and PrydzBays in East Antarctica demonstrate that the sea level during MarineIsotope Stage 3 was close to the present level despite the global sea-leveldrop lower than -40 m. We demonstrate glacial isostatic adjustment modelingwith refined Antarctic Ice Sheet loading histories. Our experiments revealthat the Indian Ocean sector of the Antarctic Ice Sheet would have beenrequired to experience excess ice loads before the Last Glacial Maximum inorder to explain the observed sea-level highstands during Marine IsotopeStage 3. As such, we suggest that the Antarctic Ice Sheet partly reachedits maximum thickness before the global Last Glacial Maximum.
Across-arc variations in Mo isotopes and implications for subductedoceanic 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 tracingmaterial recycling involved in subduction zone processes. However, thegeochemical behavior of Mo in subducted oceanic crust remains enigmatic. Weanalyzed Mo isotope ratios of arc and back-arc basin lavas from the Marianasubduction zone (western Pacific Ocean), combining newly obtained elementand Sr-Nd-Pb-Li isotope data to investigate subduction zone geochemicalprocesses 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-arcvariations, 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 themagma source. The low δ98/95Mo values in the Central MarianaTrough (-0.65‰ to -0.17‰) with mantle-like Sr-Nd-Pb but slightly low δ 7Li values provide direct evidence for the contribution of deeprecycled oceanic crust to the magma source of the back-arc basin lavas. Theisotopically light Mo magmas originated by partial melting of a residualsubducted slab (eclogite) after high degrees of dehydration and thenpenetrated into the back-arc mantle. This interpretation provides a newperspective with which to investigate the deep recycling of subductedoceanic lithosphere and associated magma petrogenesis.
Tectonic controls on basement exhumation in the southern RockyMountains (United States): The power of combined zircon (U-Th)/He andK-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 NorthAmerica), where Precambrian crystalline basement is nonconformably overlainby Phanerozoic strata, represents the removal of as much as 1.5 b.y. ofrock record during 10-km-scale basement exhumation. We evaluate the timingof exhumation of basement rocks at five locations by combining geologicdata with multiple thermochronometers. 40Ar/39ArK-feldspar multi-diffusion domain (MDD) modeling indicates regionalmulti-stage basement cooling from 275 to 150 °C occurred at 1250-1100 Maand/or 1000-700 Ma. Zircon (U-Th)/He (ZHe) dates from the Rocky Mountainsrange from 20 to 864 Ma, and independent forward modeling of ZHe data isalso most consistent with multi-stage cooling. ZHe inverse models at fivelocations, combined with K-feldspar MDD and sample-specific geochronologicand/or thermochronologic constraints, document multiple pulses of basementcooling from 250 °C to surface temperatures with a major regional basementexhumation event 1300-900 Ma, limited cooling in some samples during the770-570 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 tectoniccontrol on basement exhumation leading up to formation of thePrecambrian-Cambrian Great Unconformity and document the formation ofcomposite erosional surfaces developed by faulting and differential uplift.
Reorienting the West African craton in Paleoproterozoic-Mesoproterozoicsupercontinent Nuna
Zheng Gong; David A.D. Evans; Nasrrddine Youbi; Abdelhak Ait Lahna; UlfSöderlund ...
Abstract:The location of the West African craton (WAC) has been poorly constrainedin the Paleoproterozoic-Mesoproterozoic supercontinent Nuna (also known asColumbia). Previous Nuna reconstruction models suggested that the WAC wasconnected to Amazonia in a way similar to their relative position inGondwana. By an integrated paleomagnetic and geochronological study of theProterozoic mafic dikes in the Anti-Atlas Belt, Morocco, we provide tworeliable paleomagnetic poles to test this connection. Incorporating our newpoles with quality-filtered poles from the neighboring cratons of the WAC,we propose an inverted WAC-Amazonia connection, with the northern WACattached to northeastern Amazonia, as well as a refined configuration ofNuna. Global large igneous province records also conform to our newreconstruction. The inverted WAC-Amazonia connection suggests a substantialchange in their relative orientation from Nuna to Gondwana, providing anadditional example of large-magnitude cumulative azimuthal rotationsbetween adjacent continental blocks over supercontinental cycles.
New Ediacara-type fossils and late Ediacaran stratigraphy from thenorthern 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 arepivotal in understanding the early evolution of animals on the eve of theCambrian explosion and useful in late Ediacaran biostratigraphy. They havebeen discovered on almost all major paleocontinents, except the North Chinaand Tarim blocks, as well as on a series of northwestwest-oriented cratonicfragments between the two blocks, including the Olongbuluke terrane of theQaidam block, where the terminal Ediacaran successions developed. We reporta newly discovered terminal Ediacaran biotic assemblage, the Quanjishanassemblage, containing Ediacara-type fossils from the Zhoujieshan Formationof the Quanji Group in the Olongbuluke terrane, Qaidam Basin, northwesternChina. The Quanjishan assemblage is dominated by the non-biomineralizedtubular taxon Shaanxilithes, which has the potential to be aterminal Ediacaran index fossil, and by the iconic frondose rangeomorph Charnia, which represents the only unambiguous Ediacara-typefossil discovered in northwestern China. The co-occurrence of Charnia and Shaanxilithes from the Quanjishan assemblagelikely constrains the depositional age of the Zhoujieshan Formation to beterminal Ediacaran (ca. 550-539 Ma) and the immediately underlyingHongtiegou diamictites to be late Ediacaran, probably representingpost-Gaskiers glacial deposition. The occurrence of post-Gaskiers Ediacaranglaciation and similarities between the late Ediacaran-early Paleozoiclithostratigraphic and biostratigraphic sequences in the Olongbuluketerrane of the Qaidam block and the North China block suggest that thesetwo blocks may have been located close to each other during this timeperiod, and situated in the middle to high latitudes instead of theequatorial region.
Slab-derived sulfate generates oxidized basaltic magmas in the southernCascade arc (California, USA)
Michelle J. Muth; Paul J. Wallace
Abstract:Whether and how subduction increases the oxidation state of Earth's mantleare two of the most important unresolved questions in solid Earthgeochemistry. Using data from the southern Cascade arc (California, USA),we show quantitatively for the first time that increases in arc magmaoxidation state are fundamentally linked to mass transfer of isotopicallyheavy sulfate from the subducted plate into the mantle wedge. Weinvestigate multiple hypotheses related to plate dehydration and meltingand the rise and reaction of slab melts with mantle peridotite in thewedge, focusing on electron balance between redox-sensitive iron and sulfurduring these processes. These results show that unless slab-derived silicicmelts contain much higher dissolved sulfur than is indicated by currentlyavailable experimental data, arc magma generation by mantle wedge meltingmust involve multiple stages of mantle metasomatism by slab-derivedoxidized and sulfur-bearing hydrous components.
Long-term Aptian marine osmium isotopic record of Ontong Java Nuiactivity
Hironao Matsumoto; Rodolfo Coccioni; Fabrizio Frontalini; Kotaro Shirai;Luigi Jovane ...
Abstract:The early to mid-Aptian was punctuated by episodic phases of organic-carbonburial in various oceanographic settings, which are possibly related tomassive 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 proxiesfor determining the timing of voluminous submarine volcanic episodes.However, available Os isotopic records during the age are limited to anarrow interval in the earliest Aptian, which is insufficient for thereconstruction of long-term hydrothermal activity. We document the early tomid-Aptian Os isotopic record using pelagic Tethyan sediments deposited inthe Poggio le Guaine (Umbria-Marche Basin, Italy) to precisely constrainthe timing of massive volcanic episodes and to assess their impact on themarine environment. Our new Os isotopic data reveal three shifts tounradiogenic values, two of which correspond to black shale horizons in thelower to mid-Aptian, namely the Wezel (herein named) and Fallot Levels.These Os isotopic excursions are ascribed to massive inputs of unradiogenicOs to the ocean through hydrothermal activity. Combining the new Osisotopic record with published data from the lowermost Aptian organic-richinterval in the Gorgo a Cerbara section of the Umbria-Marche Basin, it canbe inferred that Ontong Java Nui volcanic eruptions persisted for ~5 m.y.during the early to mid-Aptian.
Anisotropy-revealed change in hydration along the Alaska subductionzone
Colton Lynner
Abstract:Megathrust earthquake behavior in subduction zones is controlled by avariety of factors including the hydration state of the subducting slab.Increased hydration reduces the occurrence of great, damaging earthquakesby diminishing the strength of the material along the interface betweentectonic plates. Understanding variations in hydration in subductions zonesis necessary for properly assessing the overall hazard posed by eachregion. Fortunately, seismic anisotropy is strongly dependent uponhydration of the subducting crust and lithosphere. I present shear-wavesplitting measurements that illuminate changes in anisotropy, and thereforehydration, of the subducting Pacific plate beneath the Alaska subductionzone (northern Pacific Ocean). Variations in shear-wave splitting directlycorrelate to changes in the behavior of great, megathrust earthquakes. Mymeasurements show that the Shumagin seismic gap is characterized by ahydrated subducting slab, explaining the long-term lack of greatearthquakes. Observations in the immediately adjacent Semidi segment, whichexperiences great events regularly, indicate a far less hydrated slab.These results are driven by the preferential alignment of paleo-spreadingfabrics of the Pacific plate. Where fabrics are more closely aligned withthe orientation of the trench, outer-rise faulting and plate hydration isenhanced. These results highlight the importance of changes in preexistingslab structures and subsequent hydration in the production of great,damaging earthquakes.
Out-of-phase Late Pleistocene glacial maxima in the Western Alpsreflect 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 theEuropean Alps indicate a synchronous Late Pleistocene glacial maximumduring Marine Isotope Stage (MIS) 2, in phase with global ice volumerecords. However, strong controversy remains for the western foreland,where scarce and indirect dating as well as modeling studies suggestglacial maxima out of phase with the rest of the Alps. New luminescencedating brings the first direct Late Pleistocene glacial chronology for thewestern Alpine foreland and reveals two major glacier advances of similarmaximum extent, at ca. 75-60 and ca. 40-30 ka, coinciding with MIS 4 andlate MIS 3. We propose that asynchrony in glacial maxima between thewestern and the northern and southern Alpine forelands results from aprogressive spatial reorganization of the atmospheric circulation over theNorth Atlantic in response to Northern Hemisphere ice-sheet fluctuations.While such a feedback mechanism has emerged from general circulationmodels, our Late Pleistocene paleoglacial reconstruction permits trackingof the spatiotemporal evolution of moisture advection patterns over WesternEurope.
Highly localized upper mantle deformation during plate boundaryinitiation 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 (NewZealand) provide the first direct evidence of deformation associated withthe propagation of the Australian-Pacific plate boundary through the regionat ca. 25-20 Ma. Two of 11 sampled xenolith localities contain fine-grained(40-150 mm) rocks, indicating that deformation in the upper mantle wasfocused in highly sheared zones. To constrain the nature and conditions ofdeformation, we combine a flow law with a model linking recrystallizedfraction 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 plateoffset rates (10-20 mm/yr) and an estimated interval during whichdeformation likely occurred (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 massextinction
Xu Dai; Dieter Korn; Haijun Song
Abstract:Ammonoids suffered a diversity bottleneck during the Permian-Triassic massextinction (PTME) and experienced a rapid diversification in the EarlyTriassic. However, the kinds of ammonoids that were more likely to survivethe PTME and that fueled subsequent diversification are still poorly known.We compiled a comprehensive morphological data set and used the nonmetricmultidimensional scaling method to reveal the impact of the PTME on themorphological selectivity of ammonoids. Our results show thatpostextinction taxa occupied a quite different morphospace when comparedwith the pre-extinction assemblages. The survivors were mainly smooth andweakly ornamented forms, while the late Permian species were dominated bycoarsely ornamented forms. Contrary to previously recognized nonselectivepatterns, these results suggest a morphological selectivity of thePermian-Triassic crisis. Newcomers in the Griesbachian were mainlycompressed and smooth forms. This morphological shift from the coarselyornamented ammonoids dominating the Changhsingian to the smooth ammonoidsdominating the Griesbachian possibly suggests an ecological turnover ofammonoids during the PTME.
Permafrost thaw induced abrupt changes in hydrology and carbon cyclingin 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 formethane and carbon dioxide emissions. However, there has been no geologicalrecord of the hydrological and biogeochemical responses of lakes throughoutthe thawing of surrounding permafrost. We use multiple biomarker andisotopic proxies to reconstruct hydrological and biogeochemical changes inLake Wudalianchi in northeastern China during regional thawing of thepermafrost. We show permafrost thawing, as indicated by lignin degradation,initiated rapid lake water freshening as a result of the opening ofgroundwater conduits, and negative organic δ13C excursion due toincreased inorganic and organic carbon fluxes. These hydrological changeswere followed, with an ~5-7 yr delay, by abrupt and persistent increases inmicrobial lake methanotrophy and methanogenesis, indicating enhancedanaerobic organic decomposition and methane emissions from lakes aspermafrost thaws. Our data provide a detailed assessment of the processesinvolved during permafrost thaw, and highlight the importance of lakes inventilating greenhouse gases to the atmosphere.
Erosion of the Himalaya-Karakoram recorded by Indus Fan deposits sincethe Oligocene
Han Feng; Huayu Lu; Barbara Carrapa; Hanzhi Zhang; Jun Chen ...
Abstract:The Cenozoic erosion history of the Himalaya-Karakoram, which is a functionof tectonically driven uplift and monsoon climatic evolution in South Asia,remains elusive, especially prior to the Miocene. Here, we present amultiproxy geochemical and thermochronological analysis of the oldestsamples available from the Arabian Sea, which we used to investigate theerosion history of the Himalayan and Karakoram orogenic system. The IndusFan records rapid and sustained erosion of the Himalayan-Karakorammountains from before 24 Ma (ca. 30) to ca. 16 Ma concurrent with changingprovenance from the Indian (Himalayan) and Eurasian plates. Our data,combined with previous studies of younger Indus Fan deposits, indicate thatthe mid-to-late Cenozoic erosion history of the Himalayan-Karakorammountains is overall consistent with a vigorous monsoonal climate from thelate Oligocene to middle Miocene and with changes in global climate in thelate Miocene, whereas erosion and deposition are relatively insensitive tochanges in sources and rock erodibility. Although tectonic processes wereactive throughout, we suggest that the erosional signatures of theHimalayan-Karakoram mountains from the Indus Fan largely preserve a recordof climate changes since the Oligocene.
Raising the West: Mid-Cenozoic Colorado-plano related to subvolcanicbatholith assembly in the Southern Rocky Mountains (USA)?
Peter W. Lipman
Abstract:The Southern Rocky Mountains of Colorado, United States, have the highestregional elevation in North America, but present-day crustal thickness(~42-47 km) is no greater than for the adjacent, topographically lower HighPlains and Colorado Plateau. The chemistry of continental-arc rocks of themid-Cenozoic Southern Rocky Mountain volcanic field, calibrated tocompositions and Moho depths at young arcs, suggests that paleocrustalthickness may have been 20%-35% greater than at present and elevationsaccordingly higher. Thick mid-Cenozoic Rocky Mountain crust and highpaleo-elevations, comparable to those inferred for the Nevadaplano fartherwest in the United States from analogous volcanic chemistry, could beconsistent with otherwise-perplexing evidence for widespread rapid erosionduring volcanism. Variable mid-Cenozoic crustal thickening and uplift couldhave resulted from composite batholith growth during volcanism,superimposed on prior crustal thickening during early Cenozoic (Laramide)compression. Alternatively, the arc-crustal thickness calibration may beinappropriate for high-potassium continental arcs, in which case otherpublished interpretations using similar methods may also be unreliable.
Labyrinth patterns in Magadi (Kenya) cherts: Evidence for earlyformation from siliceous gels
Kennie Leet; Tim K. Lowenstein; Robin W. Renaut; R. Bernhart Owen; AndrewCohen
Abstract:Sedimentary cherts, with well-preserved microfossils, are known from theArchean to the present, yet their origins remain poorly understood. LakeMagadi, Kenya, has been used as a modern analog system for understandingthe origins of nonbiogenic chert. We present evidence for synsedimentaryformation of Magadi cherts directly from siliceous gels. Petrographicthin-section analysis and field-emission scanning electron microscopy ofcherts from cores drilled in Lake Magadi during the Hominin Sites andPaleolakes Drilling Project in 2014 led to the discovery of two-dimensionalbranching "labyrinth patterns" in chert, which are a type of fractal"squeeze" pattern formed at air-liquid interfaces. Labyrinth patternspreserved in chert from Lake Magadi cores indicate invasion of air alongplanes in dewatering gels. These patterns support the precipitation ofsilica gels in the saline-alkaline Lake Magadi system and syndepositionaldrying of gels in contact with air as part of chert formation. Recognizingcherts as syndepositional has been critical for our use of them for U-Thdating. Identification of labyrinth patterns in ancient cherts can providea better understanding of paleoenvironmental and geochemical conditions inthe past.
Serpentinized peridotite versus thick mafic crust at the Romancheoceanic transform fault
Emma P.M. Gregory; Satish C. Singh; Milena Marjanović; Zhikai Wang
Abstract:The crust beneath transform faults at slow-spreading ridges has beenconsidered to be thin, comprising a thin mafic layer overlyingserpentinized peridotite. Using wide-angle seismic data, we report thepresence of a Moho at ~6 km depth and a low-velocity anomaly extending downto 9 km beneath the 20-km-wide Romanche transform valley floor in theequatorial Atlantic Ocean. The low crustal velocities above the Moho couldbe due to either highly serpentinized mantle peridotite or fractured maficrocks. The existence of clear Moho reflections and the occurrence of alarge crustal-depth rupture during the 2016 magnitude 7.1 earthquakesuggest that the crust likely consists of fractured mafic material.Furthermore, the presence of low velocities below the Moho advocates forextensive serpentinization of the mantle, indicating that the Mohoreflection is unlikely to be produced by a serpentinization front. Thecrust to the north of the transform fault likely consists of maficmaterial, but that in the south appears to be more amagmatic, possiblycontaining serpentinized peridotite. Our results imply that the transformfault structure is complex and highly heterogeneous, and thus would havesignificant influence on earthquake rupture and alteration processes.
