News Release

New Geology articles published online ahead of print in August

Peer-Reviewed Publication

Geological Society of America

Boulder, Colo., USA: Article topics and locations include galactic spiral arms and crust production on early Earth; the role of metamorphic mica; seawater during Snowball Earth; a novel hybrid pyroclastic lithofacies found on Tenerife (Canary Islands) and Pantelleria (Italy); and the 2001 Kokoxili earthquake in Tibet. These Geology articles are online at .

Trench retreat recorded by a subduction zone metamorphic history
Jie Dong; Marty Grove; Chunjing Wei; Bao-Fu Han; An Yin ...
Abstract: Upper amphibolite-facies metamorphism in subduction zone rocks may occur under exceptional tectonic settings. Differentiating competing mechanisms for its occurrence requires carefully integrated, high-resolution thermobarometric and geochronologic studies of mélange rocks with well-defined field relationships. We present new pressure, temperature, and age data from the classic Cretaceous Catalina Schist in southern California (USA) that allow us to establish a plausible model for its high-temperature metamorphic history. Our results indicate that garnet-amphibolite blocks in the structurally highest amphibolite-facies mélange preserve evidence of three stages of tectonic evolution: (1) prograde lawsonite eclogite-facies metamorphism that peaked at 2.4–2.7 GPa with temperatures >580 °C during fixed-trench subduction (120–115 Ma); (2) post-peak epidote eclogite-facies metamorphism followed by amphibolite-facies metamorphism at 1.4–1.3 GPa with temperatures of 740–790 °C during trench retreat (115–105 Ma); and (3) isothermal decompression (1.3 GPa to <1.0 GPa at temperatures of ~780 °C) and cooling during trench advance and slab-flattening subduction (ca. 105–100 Ma). Our model implies the presence of a continuous Cordilleran subduction system in the Cretaceous, which had varying tectonic regimes through episodes of trench retreat/advance and slab shallowing/steepening that, in turn, dictated the development of the Cordilleran arc system.
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Transient fault creep on the Xidatan (Tibet) fault driven by viscoelastic relaxation following the 2001 Kokoxili earthquake
Dezheng Zhao; Chunyan Qu; Xinjian Shan; Roland Bürgmann; Han Chen ...
Abstract: Recent geodetic observations of shallow fault creep have illuminated increasingly complex, time-dependent slip behaviors, including quasi-steady creep and temporary accelerations, termed slow-slip events. We documented two decades of deformation on the Xidatan fault on the Tibetan Plateau measured by radar interferometry during 2003–2010 and 2015–2020 CE, to probe the temporal evolution of shallow creep and illuminate the underlying mechanisms. The geodetic observations reveal an ~80-km-long fault section with temporally decaying creep along the Xidatan fault, one of the current seismic gaps along the Kunlun fault. The transient creep is likely driven by postseismic deformation processes, dominated by viscoelastic relaxation after the 2001 Kokoxili earthquake, rather than triggered by the coseismic rupture. The transient creep behavior, indicating rate-strengthening frictional properties of the fault, contradicts the inference of locking along the Xidatan fault, based on geodetic imaging before the Kokoxili earthquake and on historical ruptures. We propose that, during the interseismic period, the frictionally unlocked shallow portions of the fault are located in the stress shadow cast by the deeper locked asperities, but they creep at resolvable rates when exposed to transient stress and stressing rate increases. We argue that stress interactions in the triple junction of the Kusai Hu, Xidatan, and Kunlun Pass faults promote complex slip behaviors throughout the earthquake cycle.
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Time constraints on hydrocarbon migration and caprock formation recorded by calcite spar in a Carboniferous–Permian carbonate-evaporite succession, Finnmark Platform, Barents Sea
Malcolm S.W. Hodgskiss; Nivedita Thiagarajan; Yue Wang; Niels Rameil; Harald Brunstad ...
Abstract: A late Carboniferous to early Permian carbonate and evaporite succession from the Finnmark Platform (southern Barents Sea) contains nodules of anhydrite partially to fully replaced by calcite spar and native sulfur genetically linked to hydrocarbon migration and/or oxidation, analogous to processes observed in salt diapir caprocks in the Gulf of Mexico. In situ U-Pb dating of this calcite spar therefore has the potential to directly date hydrocarbon migration and provide further insight into the geochemical and temperature conditions during this event when coupled with traditional stable and clumped isotope ratios (δ 13C, δ18O, and Δ47). Results indicate calcite formed ca. 50–15 Ma, postdating host-rock deposition by 250–285 m.y. Strongly negative δ13C values in the calcite spar (mean = –15‰) are consistent with a major contribution of carbon from hydrocarbons, and Δ47 paleothermometry indicates a mean precipitation temperature of 46 ± 11 °C. These geochemical results are consistent with the local burial history and suggest protracted hydrocarbon migration and/or oxidation and caprock formation spanning ~35 m.y.
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Onset of long-lived silicic and alkaline magmatism in eastern North America preceded Central Atlantic Magmatic Province emplacement
Sean T. Kinney; Scott A. MacLennan; Dawid Szymanowski; C. Brenhin Keller; Jill A. VanTongeren ...
Abstract: The White Mountain magma series is the largest Mesozoic felsic igneous province on the eastern North American margin. Previous geochronology suggests that magmatism occurred over 50 m.y., with ages for the oldest units apparently coeval with the ca. 201 Ma Central Atlantic Magmatic Province, the flood basalt province associated with the end-Triassic mass extinction and the opening of the Atlantic Ocean. We use zircon U-Pb geochronology to show that emplacement of White Mountain magma series plutons was already underway at 207.5 Ma. The largest volcanic-plutonic complex, the White Mountain batholith, was emplaced episodically from ca. 198.5 Ma to ca. 180 Ma and is ~25 m.y. older than published ages suggest, and all samples we dated from the Moat Volcanics are between ca. 185 Ma and 180 Ma. The Moat Volcanics and the White Mountain batholith are broadly comagmatic, which constrains the age of a key Jurassic paleomagnetic pole. Our data indicate that a regional mantle thermal anomaly in eastern North America developed at least 5 m.y. prior to the main stage of Central Atlantic Magmatic Province flood basalt volcanism and suggest a geodynamic link between the White Mountain magma series and the Central Atlantic Magmatic Province.
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Small impact cratering processes produce distinctive charcoal assemblages
A. Losiak; C.M. Belcher; J. Plado; A. Jõeleht; C.D.K. Herd ...
Abstract: The frequency of crater-producing asteroid impacts on Earth is not known. Of the predicted Holocene asteroid impact craters of <200 m diameter, only ~30% have been located. Until now there has been no way to distinguish them from “normal” terrestrial structures unless pieces of iron meteorites were found nearby. We show that the reflective properties of charcoal found in the proximal ejecta of small impact craters are distinct from those produced by wildfires. Impact-produced charcoals and wildfire charcoals must derive from different heating regimes. We suggest that charcoal with specific reflective properties may help to recognize the meteoritic origin of small craters.
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Orogenic belt resulting from ocean-continent collision
Jun-Hong Zhao; Ting Yang; Wei Wang
Abstract: Orogenic belts have been thought to form through plate convergence, involving subduction of oceanic lithosphere at continental margins (accretionary orogens), which may ultimately lead to ocean closure and continent-continent collision (collisional orogens). Intraplate orogens away from plate margins have been known, but the mechanisms controlling their evolution are poorly understood. The South China craton, including the Yangtze and Cathaysia blocks, underwent a Paleozoic orogenesis that formed a >500-km-wide orogenic belt with widespread granitoids that are unconformably overlain by Devonian cover sequences. The pre-Devonian basement rocks were subjected to strong deformation and greenschist- to amphibolite-facies metamorphism at 460–400 Ma. Paleozoic magmatism was characterized by voluminous crustally derived Silurian granitoids associated with incorporation of ancient crustal materials at 450–440 Ma and addition of juvenile mantle-derived melts at 420–410 Ma. Based on the absence of arc-like magmatism and the existence of ophiolites in the West Cathaysia terrane, geochemical evidence that oceanic crust existed beneath the East Cathaysia terrane, and geophysical evidence of contrasting lithospheres on both sides and two discrete slabs beneath their fault boundary, we propose that this Paleozoic orogenic belt was formed by collision between the two terranes that was driven by far-field forces during the assembly of Gondwana, and the East Cathaysia terrane represents oceanic lithosphere that was overthrusted by the continental crustal materials of the West Cathaysia terrane. Numerical modeling shows that this type of collision can explain the dynamics of the Paleozoic orogenesis in the South China craton and may be a mechanism for some orogens in which subduction-related igneous and metamorphic rocks are lacking.
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Sublithospheric melt input in cratonic lamproites
Soumendu Sarkar; Andrea Giuliani; David Phillips; Geoffrey H. Howarth; Sujoy Ghosh ...
Abstract: Cratonic lamproites are diamondiferous ultrapotassic rocks that are emplaced through thick continental lithosphere and thought to derive from melting of metasomatized (i.e., geochemically enriched) regions of the subcontinental lithospheric mantle (SCLM). We explored the alternative hypothesis that melts sourced from sublithospheric (i.e., convective mantle) sources dominate the genesis of cratonic lamproites. Supporting evidence includes a robust linear correlation between the Mg/Fe ratios of xenocrystic and magmatic olivine in lamproites worldwide, overlapping the trend observed for kimberlite olivine. This indicates that, similar to kimberlites, primary lamproite melts originate from broadly similar sublithospheric mantle sources before assimilating SCLM material of variable composition. The lamproites are also characterized by a direct correlation between olivine Mg/Fe ratio and bulk-lamproite K2 O/Al2O3, an index of potassium enrichment in the melt that is independent of mantle-xenocryst entrainment and magmatic differentiation. Quantitative modeling indicates that this correlation results from the interaction between carbonate-bearing sublithospheric melt and phlogopite-rich wall rocks in the SCLM. Our data show that cratonic lamproites and kimberlites have similar mantle sources in the convective mantle, with lamproites acquiring their peculiar enrichment in K by interaction with metasomatized SCLM en route to the surface. Modification of sublithospheric melts during transit through the continental lithosphere might represent a common process for the genesis of alkaline mafic/ultramafic magmas.
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The northern Hikurangi margin three-dimensional plate interface in New Zealand remains rough 100 km from the trench
Harold Leah; Åke Fagereng; Ian Bastow; Rebecca Bell; Victoria Lane ...
Abstract: At the northern Hikurangi margin (North Island, New Zealand), shallow slow slip events (SSEs) frequently accommodate subduction-interface plate motion from landward of the trench to <20 km depth. SSEs may be spatially related to geometrical interface heterogeneity, though kilometer-scale plate-interface roughness imaged by active-source seismic methods is only constrained offshore at <12 km depth. Onshore constraints are comparatively lacking, but we mapped the Hikurangi margin plate interface using receiver functions from data collected by a dense 22 × 10 km array of 49 broadband seismometers. The plate interface manifests as a positive-amplitude conversion (velocity increase with depth) dipping west from 10 to 17 km depth. This interface corroborates relocated earthquake hypocenters, seismic velocity models, and downdip extrapolation of depth-converted two-dimensional active-source lines. Our mapped plate interface has kilometer-amplitude roughness we interpret as oceanic volcanics or seamounts, and is 1–4 km shallower than the regional-scale plate-interface model used in geodetic inversions. Slip during SSEs may thus have different magnitudes and/or distributions than previously thought. We show interface roughness also leads to shear-strength variability, where slip may nucleate in locally weak areas and propagate across areas of low shear-strength gradient. Heterogeneous shear strength throughout the depth range of the northern Hikurangi margin may govern the nature of plate deformation, including the localization of both slow slip and hazardous earthquakes.
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Extensional mountain building along convergent plate boundary: Insights from the active Taiwan mountain belt
Chih-Tung Chen; Ching-Hua Lo; Pei-Ling Wang; Li-Hung Lin
Abstract: Late brittle extension is a common feature in orogenic belts, and its role in mountain building processes is still the subject of debate. Its timing relationship with crustal thickening, the building of topography, basin infill, and rock exhumation are of key importance in determining whether it is a major factor in orogenic development or merely causes near-surface secondary effects. We examined this question in relation to the active arc-continent collision of Taiwan, studying its structural evolution by integrating new and critical geochronological results for tensile vein filling of hinterland metamorphic terrane with syn-collision deposition records. Acceleration of rock exhumation and molasse deposition was found to be coeval with the initiation of brittle tensile structures at ca. 1.6 Ma, which was long overdue as continental subduction started well before 6.5 Ma in central to northern Taiwan. The topographic mountain of Taiwan was thus constructed when the upper crust of the thickened orogenic prism turned extensional, as orographic elevation and relief are prerequisites for molasses production. Syn-collisional brittle extension is therefore proposed as a possible facilitator of both augmented extrusive exhumation and the formation of orography.
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Critical metal enrichment in crustal melts: The role of metamorphic mica
Barbara E. Kunz; Clare J. Warren; Frances E. Jenner; Nigel B.W. Harris; Tom W. Argles
Abstract: Metals such as Li, Be, V, Co, Nb, In, Cs, Sn, Ta, and W are considered resources that are critical for modern economies. They can be significantly enriched in granites and pegmatites, but the mechanisms of enrichment remain poorly understood. Many metal-enriched granitic magmas form through mica dehydration reactions during high-grade metamorphism. The preferential incorporation of these metals into micas provides a mechanism for concentration and mobilization during crustal melting. Comprehensive data sets of these elements and their partitioning in metamorphic micas across different metamorphic grades are currently lacking. We present the first extensive in situ laser ablation–inductively coupled plasma–mass spectrometry element data set collected from metasediment-hosted muscovite and biotite from three different metamorphic cross sections traversing sub-greenschist- (~400 °C) to granulite-facies conditions (>900 °C). Within the same sample, Li, V, Co, Cs, and Ta concentrations are higher in biotite, whereas Be, In, Sn, and W concentrations are higher in muscovite. Subsolidus micas record only nonsystematic compositional variations between samples. Suprasolidus biotites show systematic depletion in Li, Be, Sn, and Cs and enrichment in V and Co with increasing temperature in the highest-grade (muscovite-absent) samples. Indium and W reach peak concentrations in biotite at 750 °C and 850 °C, respectively. Muscovites record systematic enrichment in In and W and depletion in Be, Sn, and Cs with increasing metamorphic grade. These distinctive trends appear to be independent of tectonic setting (i.e., continental collision and crustal thinning). Our data set highlights the importance of higher-temperature melting (>750 °C), in particular, biotite breakdown reactions, for the release of Li, Be, Sn, Cs, and W into crustal melts.
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Mode of continental breakup of marginal seas
G. Mohn; J.C. Ringenbach; M. Nirrengarten; C. Lei; A. McCarthy ...
Abstract: We investigated the continent-ocean transition (COT) structure of three main marginal seas in the western Pacific Ocean (South China Sea, Coral Sea, and Woodlark Basin) to determine the tectono-magmatic processes acting during continental breakup. The COT formed from the activity of a low-angle normal fault system localizing deformation during final rifting. Extension was contemporaneous with magmatic activity, including volcanic edifices, dikes, and sills in the distalmost parts of these basins. The COT shows a sharp juxtaposition in space and time of continental crust against igneous oceanic crust, and its overall structure differs from that of magma-poor or magma-rich passive-margin archetypes. We propose that this mode of breakup is characteristic of marginal seas due to the high extension rates imposed by kinematic forces of nearby subduction zones. Revealed in the context of marginal seas, this mode of breakup and the resulting COT structures highlight the underestimated diversity of continental breakup mechanisms.
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Did transit through the galactic spiral arms seed crust production on the early Earth?
C.L. Kirkland; P.J. Sutton; T. Erickson; T.E. Johnson; M.I.H. Hartnady ...
Abstract: Although there is evidence for periodic geological perturbations driven by regular or semi-regular extra-terrestrial bombardment, the production of Earth’s continental crust is generally regarded as a function of planetary differentiation driven by internal processes. We report time series analysis of the Hf isotopic composition of zircon grains from the North Atlantic and Pilbara cratons, the archetypes of Archean plate tectonic and non-plate tectonic settings, respectively. An ~170–200 m.y. frequency is recognized in both cratons that matches the transit of the solar system through the galactic spiral arms, where the density of stars is high. An increase in stellar density is consistent with an enhanced rate of Earth bombardment by comets, the larger of which would have initiated crustal nuclei production via impact-driven decompression melting of the mantle. Hence, the production and preservation of continental crust on the early Earth may have been fundamentally influenced by exogenous processes. A test of this model using oxygen isotopes in zircon from the Pilbara craton reveals correlations between crust with anomalously light isotopic signatures and exit from the Perseus spiral arm and entry into the Norma spiral arm, the latter of which matches the known age of terrestrial spherule beds. Our data support bolide impact, which promoted the growth of crustal nuclei, on solar system transit into and out of the galactic spiral arms.
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Paleoclimates inform on a weakening and amplitude-reduced East Asian winter monsoon in the warming future
Shugang Kang; Xulong Wang; Jinhua Du; Yougui Song
Abstract: The East Asian winter monsoon (EAWM) has significant impacts on the weather and climate, and subsequently on the economy and society, in East Asia during boreal winters, and its projection into the future is scientifically and practically significant. However, projections relying on geological EAWM reconstructions, which can compensate for instrumental record limitations, are still lacking and urgently needed. It is more promising to conduct prediction under the background of not only instrumental but also geological changes in the EAWM. We used grain-size records from four high-resolution, chronologically well-defined loess sections on the Chinese Loess Plateau to represent past EAWM intensity and its amplitudes. Our results show that the EAWM is weaker and has lower amplitudes during warm periods than during cold stages at various time scales. Moreover, instrumental records reveal that the EAWM shows a weak level and reduced interannual amplitudes after the mid-1980s under the context of global warming. We propose that the EAWM will experience long-term weakening and reduced (e.g., interannual) amplitudes under 21st century global-warming scenarios.
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Secular variation in seawater redox state during the Marinoan Snowball Earth event and implications for eukaryotic evolution
Weibing Shen; Xiangkun Zhu; Bin Yan; Jin Li; Pengju Liu ...
Abstract: The ocean is hypothesized to have been anoxic throughout the Marinoan “Snowball Earth” event, from ca. 649 to 635 Ma, with potentially catastrophic implications for the survival of eukaryotic life. However, the precise nature of ocean redox chemistry across this critical interval, and hence the factors that governed the persistence of eukaryotes, remains unknown. We report records of pyrite iron and sulfur isotopes, combined with Fe speciation, for glaciogenic diamictites from the Nantuo Formation of South China. These data provide constraints on seawater redox state across the Marinoan glaciation, and they reveal that the redox state of the ocean fluctuated in concert with waxing and waning extents of glaciation, to include intervals of expanded oxygenation. The input of meltwater-derived oxygen provides a potential explanation for the persistence of eukaryotes through the Cryogenian “Snowball Earth” events, which ultimately paved the way for subsequent intervals of rapid biological innovation.
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Magma pressurization sustains ongoing eruptive episode at dome-building Soufrière Hills volcano, Montserrat
J. Hickey; K. Pascal; M. Head; J. Gottsmann; N. Fournier ...
Abstract: Dome-building volcanoes, where long-term eruptive episodes can be interspersed with periods of intra-eruptive repose, are particularly challenging for volcanic hazard assessment. Defining the end of eruptive episodes is vitally important for the socioeconomic recovery of affected communities but highly problematic due to the potential for rapid transition from prolonged, seemingly low-risk repose to dangerous effusive or explosive activity. It is currently unclear what constitutes the end of repose and an eruptive episode. We show that analysis of surface deformation can characterize repose and help define an eruptive episode. At Soufrière Hills volcano, Montserrat, the long-term post–2010 deformation at 12 continuous GPS stations requires the pressure in the magma system to have increased with time; time-dependent stress relaxation or crustal creep cannot explain the deformation trends alone. Continued pressurization within the magmatic system during repose could initiate a renewed eruption, qualifying as sustained unrest and therefore continuation of the eruptive episode. For Soufrière Hills volcano, persistent magma pressurization highlights the need for sustained vigilance in the monitoring and management of the volcano and its surroundings, despite the last eruptive activity ending in 2010. Our results show promise for application to other dome-building volcanoes.
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Simultaneous fall and flow during pyroclastic eruptions: A novel proximal hybrid facies
Natasha Dowey; Rebecca Williams
Abstract: The deposits of Plinian and subplinian eruptions provide critical insights into past volcanic events and inform numerical models that aim to mitigate against future hazards. However, pyroclastic deposits are often considered from either a fallout or pyroclastic density current (PDC) perspective, with little attention given to facies exhibiting characteristics of both processes. Such hybrid units may be created where fallout and PDCs act simultaneously, where a transitional phase between the two occurs, and/or due to reworking. This study presents analysis of a novel hybrid pyroclastic lithofacies found on Tenerife (Canary Islands) and Pantelleria (Italy). The coarse pumice block facies has an openwork texture and correlates with distal Plinian units, but it is cross-stratified and relatively poorly sorted with an erosional base. The facies is proposed to record the simultaneous interaction of very proximal fallout and turbulent PDCs, and it reveals a fuller spectrum of hybrid deposition than previously reported. This work highlights the importance of recognizing hybrid deposition both in the rock record and in hazard modeling.
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Preserved intercratonic lithosphere reveals Proterozoic assembly of Australia
Yongjun Lu; Michael T.D. Wingate; Robert H. Smithies; Klaus Gessner; Simon P. Johnson ...
Abstract: The Proterozoic assembly of Australia, the understanding of which is critical for reconstructing Proterozoic supercontinents, involved amalgamation of the West Australian (WAC), North Australian (NAC), and South Australian cratons (SAC). However, the basement between these Archean to early Proterozoic lithospheric blocks is mostly buried beneath younger basins; hence, its composition and age and the timing of Proterozoic assembly remain uncertain. In situ zircon U-Pb-O-Hf analyses of igneous rocks from drillholes that intersected basement beneath the northwestern Canning Basin reveal the presence of a substantial domain of juvenile Proterozoic lithosphere, the Percival Lakes province, between the WAC and NAC. Although isotopically distinct from the neighboring WAC and NAC, the Percival Lakes province is strikingly similar to other juvenile Proterozoic tectonic elements between the WAC, NAC, and SAC. Combining isotope and seismic data, we interpret the Percival Lakes province as part of an ~1700 × 400 km Proterozoic lithospheric domain that lacks evidence of Archean provenance but consists mainly of reworked remnants of Mesoproterozoic oceanic crust that survived WAC-NAC-SAC convergence. The apparent absence of Archean lithosphere between the cratons implies they never directly collided or that complete collision was prevented by impingement of three-dimensional promontories in the converging lithospheric blocks. Instead, the Percival Lakes province and other Proterozoic elements between the WAC, NAC, and SAC consist of oceanic lithosphere extracted from Earth’s mantle in the Proterozoic. Our results imply that WAC-NAC convergence was younger than Columbia amalgamation at ca. 1.8 Ga and that Proterozoic Australia formed during the earliest phases of Rodinia assembly at ca. 1.3 Ga.
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