News Release

New GSA Bulletin articles published ahead of print in January and February

Peer-Reviewed Publication

Geological Society of America

Boulder, Colo., USA: The Geological Society of America regularly publishes articles online ahead of print. GSA Bulletin topics studied this month include the nature and dynamics of China and Tibet; the Hiawatha impact crater in Northwest Greenland; the Wyoming impact crater field; the Olds Ferry terrane; the stability of dinosaur communities; the dispersal processes of supercontinents; and the 1986 Borah Peak earthquake. You can find these articles at .

Disequilibrium river networks dissecting the western slope of the Sierra Nevada, California, USA, record significant late Cenozoic tilting and associated surface uplift
Helen W. Beeson; Scott W. McCoy
Abstract: The timing, rates, and spatial patterns of elevation change in the Sierra Nevada, California, USA, have long been the subject of vigorous debate owing to their importance in constraining the tectonic history of western North America and models of orogenesis. Here we present a systematic analysis of multiple measures of fluvial geomorphology along the entire length of the range and interpret these observations using 1-D and 2-D landscape evolution modeling based on the stream power fluvial erosion rule with the rate parameter calibrated from a large data set of millennial-scale erosion rates. We demonstrate that westward-draining rivers in the Sierra Nevada are in a disequilibrium state and that this state is consistent with the transient fluvial response expected from significant down-to-the-west tilting in the last 11 m.y. Assuming rigid-block tilting and using multiple independent measures of tilt magnitude, we find that the magnitude of surface uplift from late Cenozoic tilting appears to have been similar along strike and likely resulted in ∼500−1300 m of surface uplift at the crest (0.3−0.8° tilt) from the Yuba through the Stanislaus rivers (∼39.2−38.2°N) and 1000−1400 m of surface uplift at the crest (0.6−0.9° tilt) from the Tuolumne River south through the Kings River (∼38.2−36.4°N). We show that the transient fluvial response to tilting in the northern Sierra is heavily modulated by heterogeneous lithology and drainage area exchange in a manner that reconciles the high spatial variability in basement incision observed in numerous prior studies. However, we find that heterogeneous lithology alone cannot explain both the transient state and observed patterns in channel steepness, which seem to require late Cenozoic changes in tectonic forcing. Beyond the regional implications of a short-lived rapid pulse of late Cenozoic surface uplift along the entire length of the range, our results demonstrate that a range-wide approach in which river networks are analyzed both in planform and profile can elucidate tectonic histories despite heterogeneous lithology and ongoing changes in network topology.
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Influence of hydrostratigraphy on the distribution of groundwater arsenic in the transboundary Ganges River aquifer system, India and Bangladesh
Madhumita Chakraborty; Abhijit Mukherjee; Kazi Matin Ahmed; Alan E. Fryar; Animesh Bhattacharya ...
Abstract: The Ganges River delta complex contains a transboundary aquifer system shared between India and Bangladesh. Although it serves as the main freshwater source for the population inhabiting the delta, the aquifer system is severely contaminated with arsenic (As). This study aimed to determine the control of the delta hydrostratigraphy on the regional-scale depth distribution of As within the aquifer system. We developed the first high-resolution, regional-scale, transboundary hydrostratigraphic model of the Ganges River delta and analyzed the patterns of As distribution as a function of the hydrostratigraphy. Model results indicate that, despite the presence of a single aquifer system across the delta, the hydrostratigraphy is spatially variable and can be architecturally divided into three distinct aquifer subsystems from northwest to southeast: a single, thick continuous aquifer (type I); a vertically segregated, semiconfined aquifer subsystem (type II); and a multilayered, nearly confined aquifer subsystem (type III). Results indicate that the spatial distribution of As is characteristically different in each subsystem. In the type I aquifer, As concentrations tend to be homogeneous at all depths, while in type II and type III aquifers, As concentrations sharply decrease with depth. The intervening aquitards in the type II and type III aquifer subsystems appear to act as natural barriers to infiltration of surficial As or organic matter−rich water to the deeper aquifer zones. This delineation of the regional-scale hydrostratigraphic architecture and resulting understanding of its plausible controls on the depth-distribution of As within the delta should significantly aid in the systematic framing of sustainable management plans for the As-safe aquifers within the Ganges River delta aquifer system.
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A trans-Iapetus transform fault control for the evolution of the Rheic Ocean: Implications for an early Paleozoic transition of accretionary tectonics
Lei Wu; J. Brendan Murphy; William J. Collins; John W.F. Waldron; Zheng-Xiang Li ...
Abstract: The Paleozoic era begins with the final assembly of Gondwana and ends with the amalgamation of the supercontinent Pangea. Although this tectonic progression is generally well documented, one fundamental but under-studied phenomenon during this era is the transition from two-way to one-way (northward) migration of peripheral terranes between Gondwana and Laurentia-Baltica from Cambrian to Ordovician time. The two-way terrane accretion was likely initiated during the opening of the Rheic Ocean at ca. 510 Ma when several Gondwana-derived terranes comprising Carolinia, Ganderia, Avalonia, and Meguma sequentially drifted from the northern margin of Gondwana and eventually collided with Laurentia or Baltica. Over the same time interval, the Laurentia-derived Cuyania terrane (a.k.a., the “Precordillera terrane” is commonly believed to have accreted to the proto-Andean margin of Gondwana, whereas the peri-Laurentian/Dashwoods ribbon continent separated from, then re-accreted to, the Laurentian margin after being trapped during the collision of the Taconic arc. Alternatively, the Cuyania terrane is suggested to have remained attached to the Ouachita Embayment throughout the Cambrian−Ordovician, and then passed onto Gondwana during the collision between the proto-Andean margin and the hypothesized Texas Plateau at the leading edge of Cuyania. Here we explain the enigmatic, pene-contemporaneous migration of these peripheral terranes by a trans-Iapetus transform fault that was likely active between 510 and 450 Ma and extended from the proto-Appalachian margin of Laurentia to the proto-Andean margin of Gondwana. The trans-Iapetus transform fault terminated after the respective accretion of Carolinia and Ganderia to the proto-Appalachian margin and of Cuyania to the proto-Andean margin. We interpret the development of the trans-Iapetus transform during the Cambrian−Ordovician to be a consequence of the global geodynamic transition from the break-up of Rodinia (continents/terranes drifting away from Laurentia) to Pangea amalgamation (continents/terranes drifting toward assembling Laurussia).
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Late Mesozoic volcanism in the southern Xiong’er Mountains, eastern Qinling: Partial melting of the subducted Qinling crust underneath North China
Jun He; He Zhang; Zeqiu Qi; Zhiyi Wang; Fukun Chen
Abstract: Volcanism along the continental margins is a powerful tool with which to probe the orogenic processes and crustal components underlying orogenic belts. In this study, we report the zircon ages and geochemical compositions of volcanic rocks exposed in the Luanchuan area, along the southern margin of the North China block. In previous studies, these volcanic rocks were reported to be products of Paleoproterozoic (ca. 1.75 Ga) volcanism related to rifting in the Xiong’er Mountains. Our zircon age data reveal that most volcanic rocks contain late Mesozoic zircon grains of magmatic origin, suggesting that Late Jurassic to Early Cretaceous (ca. 150−120 Ma) volcanism must have occurred in the southern Xiong’er Mountains, coeval with emplacement of large granitoid plutons along the southern margin of the North China block. These late Mesozoic volcanic rocks mainly consist of trachyandesite, andesite, dacite, and rhyolite. They vary in their major-element compositions but are relatively uniform in their normalized patterns of trace and rare earth elements and have consistently enriched Nd isotopic compositions. Numerous inherited and/or xenocrystic zircon grains of mostly Paleoproterozoic and occasionally Neoproterozoic to early Mesozoic ages were found in all of the volcanic rocks, indicating complex magma sources and/or diverse crustal rocks overlying the magma chambers. The Paleoproterozoic zircon grains originated from the Paleoproterozoic volcanic and basement rocks in the southern North China block. Notably, some volcanic rocks contained early Mesozoic, early Paleozoic, and Neoproterozoic zircon grains of both metamorphic and magmatic origin that indicate derivation from the North and South Qinling blocks, which recorded thermo-tectonic events during the closure of the Shangdan and Mianlue oceans in the early Paleozoic and early Mesozoic. Therefore, we propose that these late Mesozoic volcanic rocks originated from partial melting of crustal rocks and are composed partly of the subducted basement of the Qinling orogenic belt underneath the southern Xiong’er Mountains due to asthenospheric upwelling in an extensional setting.
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The late Holocene Nealtican lava-flow field, Popocatépetl volcano, central Mexico: Emplacement dynamics and future hazards
Israel Ramírez-Uribe; Claus Siebe; Magdalena Oryaëlle Chevrel; Dolors Ferres; Sergio Salinas
Abstract: Popocatépetl, one of the most hazardous volcanoes worldwide, poses significant threats for nearby populations in central Mexico. Therefore, it is important to reconstruct its eruptive history, including estimates of lava-flow emplacement times and their rheological properties. These studies define possible future eruptive scenarios and are necessary to mitigate the risk. Stratigraphic studies of the cal 350−50 B.C. Lorenzo Plinian pumice sequence indicate that effusive activity (Nealtican lava-flow field) occurred shortly after explosive activity, reflecting drastic changes in the eruptive dynamics. It was likely due to the efficient degassing of the magma during the Plinian phase and a decrease of magma ascent and decompression rates. Magma mixing, fractional crystallization, and a minor crust assimilation are the processes controlling the differentiation of the Nealtican lavas. We used lava chemical and mineralogical composition to estimate lava-flow viscosities, and used high-resolution elevation data to estimate emplacement times. Results indicate that lava viscosities of andesites and dacites ranged from 109 to 1012 Pa·s and emplacement durations were between ∼1 and ∼29 years, depending on the flow unit and morphological method employed. Considering the entire volume of emitted lava (4.2 km3) and a mean output rate of ∼1 m 3/s to ∼15 m3/s, we estimated that the effusive phase that produced the Nealtican lava-flow field may have lasted ∼35 years. This eruption had a considerable impact on pre-Hispanic settlements around the volcano, whose population exodus and relocation probably contributed to the rise of important cities in central Mexico, such as Teotihuacán and Cholula.
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Mesozoic intracontinental ductile shearing along the Paleozoic Shangdan suture in the Qinling Orogen: Constraints from deformation fabrics and geochronology
Shengsi Sun; Yunpeng Dong; Chao Cheng; Dengfeng He; Bo Zhou ...
Abstract: The Qinling Orogen, located between the North and South China Blocks, records subduction-collisional orogeny along the Paleozoic Shangdan and Triassic Mianlue sutures, and Mesozoic intracontinental orogenesis, which all played an important role in building the present tectonic framework and topography. The strike-slip Shagou ductile shear zone that overprints the Paleozoic Shangdan suture between the North and South Qinling Belts is crucial for understanding the Mesozoic intracontinental deformation in the Qinling Orogen. The microstructures, asymmetrical fabrics, and kinematic vorticities (0.81−0.95) suggest sinistral simple shear. The quartz c-axis patterns from felsic mylonites exhibit Y-maximum fabrics, indicating the activity of prism <a> slip, while those from amphibole-rich mylonites display both Y-maximum and Z-maximum fabrics showing the combined activity of prism <a> and basal <a> slip systems. In the mafic mylonites, the plagioclase fabrics were induced by combined (010)[100] and (001)[100] slip systems, while the amphibole fabrics were probably related to anisotropic growth or passive rotation of rigid clasts. Equilibrium P−T conditions of 4.28−6.12 kbar and 646−727 °C estimated from geothermobarometry suggest that the main deformation occurred under amphibolite facies conditions at middle−lower crustal depths. Zircon U-Pb ages constrain their protolith rocks to have crystallized at 816 ± 25 Ma and 726−718 Ma, while the intense mylonitization and sinistral shearing occurred at ca. 200−187 Ma. A U−Pb zircon age of 132 ± 3 Ma from a granitic dike cutting the foliation and an amphibole 40Ar/39Ar plateau age of 119.0 ± 0.9 Ma from mylonites together suggest that the Shagou shear zone evolved through decompression and exhumation stages in the time period of 132−119 Ma.
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Exhumation of plutons controlled by boundary faults: Insights from the kinematics, microfabric, and geochronology of the Taibai shear zone, Qinling Orogen, China
Chao Cheng; Shengsi Sun; Yunpeng Dong; Bin Zhang; Zhao Guo
Abstract: The Taibai granitic plutons lie between the Taibai ductile shear zone to the north and the Shangdan suture to the south. The deformation mechanism of the ductile shearing is crucial to understanding the exhumation processes of the multiple plutons that formed after the Late Mesozoic period. Geological investigations, microstructures, and kinematic vorticity calculations indicate that the Taibai shear zone deformed in response to pure shear-dominated (54%−65%) transpression and top-to-NW shear sense as a result of NE−SW oblique contractional tectonics. The quartz crystallographic preferred orientations of the prism <a> slip system, the grain boundary migration, and sub-grain rotation dynamic recrystallization of quartz—combined with the plagioclase−hornblende thermometer—constrain the main deformation temperatures to a range of 400−650 °C, which suggests amphibolite to greenschist facies conditions. In addition, it is extremely likely that the mylonites experienced late-stage, lower temperature deformation as demonstrated by the sporadic bulging recrystallization, the quartz basal <a> slip system, and the two-feldspar geothermometer calculation. The samples collected from the weakly deformed mylonitic granite pluton and the undeformed quartz-feldspathic dike that intruded into the mylonites yield zircon U−Pb ages of 129 ± 1 Ma and 115 ± 1 Ma, respectively. This information, with the lower intercept ages of ca. 120 Ma obtained from the mylonite samples, suggests that the ductile shearing probably occurred from ca. 129 Ma to 115 Ma. Combined with the regional geological data, these findings suggest that the Taibai shear zone and the Shangdan suture accommodated the oblique upward extrusion of the Taibai plutons during Early Cretaceous time.
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How similar was the 1983 Mw 6.9 Borah Peak earthquake rupture to its surface-faulting predecessors along the northern Lost River fault zone (Idaho, USA)?
Christopher B. DuRoss; Richard W. Briggs; Ryan D. Gold; Alexandra E. Hatem; Austin J. Elliott ...
Abstract: We excavated trenches at two paleoseismic sites bounding a trans-basin bedrock ridge (the Willow Creek Hills) along the northern Lost River fault zone to explore the uniqueness of the 1983 Mw 6.9 Borah Peak earthquake compared to its prehistoric predecessors. At the Sheep Creek site on the southernmost Warm Springs section, two earthquakes occurred at 9.8−14.0 ka (95% confidence) and 6.5−7.1 ka; each had ∼1.9 m of vertical displacement. About 4 km to the southeast, across the Willow Creek Hills, two ruptures at the Arentson Gulch site on the northernmost Thousand Springs section occurred at 9.0−14.7 ka and 6.1−7.5 ka with ∼1.9 m of vertical displacement each. We synthesize these and previous paleoseismic results into a model of five postglacial (<15 ka) ruptures along a ∼65 km reach of the northern Lost River fault zone. Our results show that the Borah Peak earthquake (34 km; 0.9 m mean displacement) was unique compared to previous ruptures that had both longer and shorter rupture lengths (∼25−38 km), more displacement (mean of ∼1.3−1.4 m), and equal or greater magnitude (Mw 6.9−7.1) than that in the 1983 earthquake. These ruptures support a hypothesis of variable rupture length and displacement on the northern Lost River fault zone and show that predecessors to the 1983 rupture have passed unimpeded through the Willow Creek Hills. Our work demonstrates that normal faults are capable of producing variable spatial-temporal patterns of rupture that, together with comparisons of fault geometry and historical rupture length, improve our understanding of fault segmentation and help inform models of earthquake rupture probability.
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Late Miocene−Quaternary seismic stratigraphic responses to tectonic and climatic changes at the northeastern margin of the South China Sea
Xingxing Wang; Feng Cai; Zhilei Sun; Qing Li; Ang Li ...
Abstract: Tectonic and climate evolution could be well archived in deep-water stratigraphy. Based on newly acquired high-resolution two-dimensional (2-D) multichannel seismic profiles and multibeam bathymetry, together with Ocean Drilling Program/International Ocean Discovery Program (ODP/IODP) data, this study investigated the late Miocene−Quaternary deep-water seismic stratigraphy, sedimentary evolution, and responses to regional tectonic and climatic variations at the northeastern margin of the South China Sea. The late Miocene−Quaternary stratigraphy consists of three units (i.e., SU-1, SU-2, and SU-3 from bottom to top) that are dated to 10.5−6.5 Ma (stage 1), 6.5−0.9 Ma (stage 2), and 0.9 Ma−present (stage 3), respectively. SU-1 is dominated by sheeted drifts with slight thickness variation, but SU-2 and SU-3 mainly consist of mounded, lenticular contourite drifts and channel-like moats. This suggests that the bottom currents intensified from stage 1 to stage 2, which was probably caused by the sill uplifting at the Luzon Strait under the influence of tectonic collision between the Luzon arc and Eurasia since ca. 6.5 Ma. SU-2 and SU-3 are separated by a basinwide high-amplitude seismic reflection, across which the average sedimentation rate shows a dramatic increase from ∼28 m/m.y. in SU-2 to ∼144 m/m.y. in SU-3. The high sediment supply since 0.9 Ma could be linked to the mid-Pleistocene climate transition, which resulted in abundant rainfall that promoted the Taiwan orogen to contribute more sediments to the South China Sea. This study indicates that the deep-water stratigraphy recorded the tectonic collision around the sole deep-water gateway (i.e., Luzon Strait) and mid-Pleistocene climate transition event at the northeastern South China Sea margin, which has important implications for understanding the relationships among the tectonic, paleoclimatic, paleoceanographic, and deep-water sedimentary processes in the largest marginal basin of the western Pacific Ocean.
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Rifting of the Indian passive continental margin: Insights from the Langjiexue basalts in the central Tethyan Himalaya, southern Tibet
Chao Wang; Lin Ding; Fulong Cai; Liyun Zhang; Zhenyu Li ...
Abstract: The Triassic tectonic setting of the Tethyan Himalaya in southern Tibet remains controversial and is key to revealing the mechanism of Neo-Tethys Ocean opening and the breakup history of Gondwana. This paper reports 227−225 Ma mafic volcanic rocks interbedded within the Tethyan Himalaya sequence in southern Tibet, which were formed in a typical passive continental margin setting. The basalts are tholeiitic with high TiO 2 (3.20−3.38 wt%) and moderate MgO (4.05−5.40 wt%) contents and exhibit enrichment in light rare earth elements and weak negative Nb, Ta, Eu, and Ti anomalies. These geochemical compositions, combined with uniform whole-rock εNd(t) (+1.50 to +2.70) values, indicate that the magmas were derived by low-degree melting of a deep-seated garnet source that was heterogeneously modified by an oceanic island basalt-type of component. Given the other geological evidence, we propose that the magmatic evolution of the Tethyan Himalaya during the late Paleozoic−Mesozoic was generally a lithospheric thinning process, that is, it evolved from a fully developed continental rift system during the Late Permian to wane during the Triassic. This interpretation further supports the hypothesis that the Neo-Tethys Ocean opened in a passive pattern and that the breakup of Gondwana in the late Paleozoic was initiated by lithospheric thinning. Therefore, the subsequent magmatism was related to the passive upwelling of normal asthenospheric materials.
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The newly discovered ca. 1.35 Ga metamafic rocks in the Oulongbuluke Block, NW China, and its record for transition from the Columbia to Rodinia supercontinent
Pei Lv; Shengyao Yu; Yinbiao Peng; Sanzhong Li; Yongjiang Liu ...
Abstract: The Oulongbuluke Block is an integral part of the Columbia and Rodinia supercontinents, but the lack of rock records from the transitional period between the Columbia and Rodinia supercontinents during the mid−late Mesoproterozoic has impeded our understanding of the tectonic relationship of the Oulongbuluke Block, which lies between the Columbia and Rodinia supercontinents. In this contribution, we present a systematic petrographic, geochemical, and zircon U-Pb-Hf investigation of newly discovered metamafic rocks in the Oulongbuluke Block. The results show that the metamafic rocks have a protolith age of ca. 1.35 Ga and an arc-related metamorphic age of ca. 1.11−1.09 Ga. The metamafic rock samples are geochemically characterized by relatively high FeOT/MgO and FeOT and low SiO2, MgO, and K2O + Na 2O, which shows tholeiitic affinity. These metamafic rocks exhibit slight light rare earth element (LREE) depletion and flat heavy rare earth element (HREE) content with no obvious Eu anomalies and slightly negative Nb, Sr, and Zr anomalies. These conditions are similar to those of enriched mid-oceanic-ridge basalt (E-MORB) and normal mid-oceanic-ridge basalt (N-MORB). The metamorphic rocks studied also have positive zircon ε Hf(t) values (2.96−7.04). Hence, the protoliths of the metamafic rocks may have been produced by variable degrees of melting of spinel-phase lherzolite mantle in a mid-oceanic ridge setting that was probably induced by a mantle plume. The presence of metamafic rocks indicates that the Oulongbuluke Block experienced the final breakup of the Columbia supercontinent at ca. 1.35 Ga, and the ca. 1.11−1.09 Ga arc-related metamorphism coincided with the convergence of the Rodinia supercontinent. The tectonic setting of the Oulongbuluke Block changed from a mid-oceanic ridge setting to an arc setting during the mid−late Mesoproterozoic, which was likely a response to the transition from the Columbia supercontinent to Rodinia supercontinent.
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Late Paleozoic to Mesozoic tectonic transition in northeastern Eurasia: Constraints from two island arc magmatic belts in eastern NE China
Gong-Yu Li; Jian-Bo Zhou; Long Li; Zhuo Chen; Hong-Yan Wang
Abstract: Since the Paleozoic, the tectonic evolutionary process of eastern Eurasia has been affected by at least three paleo-ocean regimes: the Paleo-Asian Ocean, the Mongol-Okhotsk Ocean, and the Paleo-Pacific Ocean. However, the tectonic transition among these ocean regimes is not well understood. Recently, the Heilongjiang Ocean was proposed to play an important role during the transition from the Paleo-Asian Ocean regime to the Mongol-Okhotsk Ocean (and later the Paleo-Pacific Ocean) regime. Here we report on a new combined study of petrological, geochemical, and geochronological data of biotite−plagioclase gneisses from the Qinglongcun Complex in the west Jiamusi-Khanka Block to better understand the tectonic evolution during this transition period. The results show that the protoliths of these biotite−plagioclase gneisses are medium-K calc-alkaline rhyolites that were derived from a volcanic island arc environment related to paleo-oceanic subduction that occurred at ca. 260 Ma. Integrated with previous studies of the Permian arc-affinity rocks within the Jiamusi-Khanka Block, we identified two island arc magmatic belts with ages of ca. 290 Ma and ca. 260 Ma at the east and west margins of Jiamusi-Khanka Block, respectively. The east island arc magmatic belt was related to subduction of the Mongol-Okhotsk Ocean during the Early−middle Permian, which provided a driving force for the opening of the Heilongjiang Ocean. Subduction of the Heilongjiang Ocean during the middle−Late Permian resulted in the west arc island magmatic belt. These two belts provide key evidence for understanding the tectonic transition from the Paleo-Asian Ocean to the Mongol-Okhotsk Ocean to the Paleo-Pacific Ocean during the late Paleozoic to early Mesozoic.
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Hydrogeochemical evolution of formation waters responsible for sandstone bleaching and ore mineralization in the Paradox Basin, Colorado Plateau, USA
Ji-Hyun Kim; Lydia Bailey; Chandler Noyes; Rebecca L. Tyne; Chris J. Ballentine ...
Abstract: The Paradox Basin in the Colorado Plateau (USA) has some of the most iconic records of paleofluid flow, including sandstone bleaching and ore mineralization, and hydrocarbon, CO2, and He reservoirs, yet the sources of fluids responsible for these extensive fluid-rock reactions are highly debated. This study, for the first time, characterizes fluids within the basin to constrain the sources and emergent behavior of paleofluid flow resulting in the iconic rock records. Major ion and isotopic (δ18Owater; δDwater; δ18OSO4; δ34SSO4; δ34S H2S; 87Sr/86Sr) signatures of formation waters were used to evaluate the distribution and sources of fluids and water-rock interactions by comparison with the rock record. There are two sources of salinity in basinal fluids: (1) diagenetically altered highly evaporated paleo-seawater-derived brines associated with the Pennsylvanian Paradox Formation evaporites; and (2) dissolution of evaporites by topographically driven meteoric circulation. Fresh to brackish groundwater in the shallow Cretaceous Burro Canyon Formation contains low Cu and high SO4 concentrations and shows oxidation of sulfides by meteoric water, while U concentrations are higher than within other formation waters. Deeper brines in the Pennsylvanian Honaker Trail Formation were derived from evaporated paleo-seawater mixed with meteoric water that oxidized sulfides and dissolved gypsum and have high 87Sr/ 86Sr indicating interaction with radiogenic siliciclastic minerals. Upward migration of reduced (hydrocarbon- and H2 S-bearing) saline fluids from the Pennsylvanian Paradox Formation along faults likely bleached sandstones in shallower sediments and provided a reduced trap for later Cu and U deposition. The distribution of existing fluids in the Paradox Basin provides important constraints to understand the rock record over geological time.
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Modern-like elevation and climate in Tibet since the mid-Miocene (ca. 15 Ma)
Qiang Xu; Shun Li; Yan Bai
Abstract: When the modern-like geomorphology and climate pattern of the Himalaya-Tibetan Plateau were established still remains unclear. In this study, we apply paired stable isotope compositions of carbonate (δ18Oc) and leaf wax-derived n-alkanes (δ 2Halk) from the upper Gazhacun Formation in the Namling Basin to reconstruct the middle Miocene elevation and climate of southern Tibet. Depositional age of the upper Gazhacun Formation has been precisely constrained to between 15.5 Ma and 15.1 Ma by zircon U-Pb ages of dacite interlayers. Paired carbonate derived δ18Ow values (−17.9 ± 1.3‰ to −18.3 ± 1.3‰) and leaf-wax derived δ2H w values (−131.5 ± 20‰ to −145.7 ± 20‰) plot on or very close to the global meteoric water line suggesting that these samples experienced little evaporation enrichment and isotopic alternation. Based on these two independent proxies, paleoelevation estimates for the Namling Basin are consistently between 4.6+0.7/−0.8 km and 5.2 +0.7/−0.8 km, supporting a high elevation for southern Tibet in the mid-Miocene. Integrated with published paleoelevation estimates for the Himalaya, central and northern Tibet in literature, a near-present elevation across the whole Himalaya-Tibetan Plateau has already been established since the middle Miocene (ca. 15 Ma). Besides, stable isotopic values across the Himalaya-Tibetan Plateau show that the δ 2Hw values gradually increase northward from the Himalaya to northeastern Tibet, quite similar to that of the present day. This pattern suggests that during the middle Miocene, the Himalaya-Gangdese system may have blocked southerly monsoonal moisture from reaching northern Tibet. Westerlies or local recycling of moisture might be the dominant moisture sources across northern Tibet, with enriched δ18O w and δ2Hw precipitation values that could lead to erroneous paleoelevation estimates over central and northern Tibet.
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Early Paleozoic Cascadia-type active-margin evolution of the Dunhuang block (NW China): Geochemical and geochronological constraints
Jérémie Soldner; Chao Yuan; Karel Schulmann; Yingde Jiang; Pavla Štípská ...
Abstract: Origins of early Paleozoic metabasites (granulites and amphibolites) and their host metasedimentary rocks in the Dunhuang block, NW China, are addressed by new geochronological and geochemical data. The metabasites show back-arc basalt−like geochemical features, marked by high Zr/Nb ratios and Zr-Hf troughs, but they can be classified into two groups based on their dissimilar protolith ages and distinct Nd signatures. Most group I metabasaltic rocks were emplaced before 455 Ma and possess high Ba/Nb ratios (11.46−224), low (Nb/La)PM (0.10−0.71), and negative whole-rock εNd(t) values (−12.7 to −2.7), whereas group II rocks have protolith ages around 445 Ma, low Ba/Nb ratios (0.70−22.93), low (Nb/La)PM (0.78−1.51), and less evolved whole-rock Nd isotopic features (εNd[t]: −2.0 to +2.7). It is proposed that group I metabasites originated from an enriched lithospheric mantle, while group II metabasites were derived from the depleted asthenospheric mantle. The metasedimentary rocks received detritus mainly from the neighboring Cambrian magmatic arc, and they are compositionally similar to active-margin sediments. Metamorphic zircon U-Pb ages ranging 462−422 Ma from the investigated rocks together with prominent magmatism further suggest high-grade metamorphism prevailing during the Late Ordovician−early Silurian. Based on these data, a Cascadia-type evolution is proposed involving an Ordovician−early Silurian suprasubduction stretching of the Cambrian active continental margin, which culminated with mantle upwelling. Recent paleogeographic reconstructions support the evolution and assembly of interior Proto−Tethys-Ran oceanic and continental plates, including the Dunhuang block, between 510 and 440 Ma, followed by Panthalassan subduction beneath the Tarim−North China continental assemblage at 440−430 Ma.
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Initial uplift of the Qilian Shan, northern Tibet since ca. 25 Ma: Implications for regional tectonics and origin of eolian deposition in Asia
Haijian Lu; Shengping Sang; Ping Wang; Zhiyong Zhang; Jiawei Pan ...
Abstract: Sedimentary rocks in northern Tibet record uplift of the Tibetan Plateau and its potential connection with the evolution of the central Asian aridity, therefore offering a typical example of tectonic-climate linkage. The coarse-grained conglomeratic and sandy red beds of the Lulehe Formation (Fm.) in the northern Qaidam Basin (QB), northern Tibet, have long been held as synorogenic sediment accumulation. There is, however, a heated debate on its source area (the Qilian Shan, the east Kunlun Shan, or Qimen Tagh) and initiation age (ca. 52, 25.5, or ca. 21 Ma, respectively). These proposals lead to distinctly different mountain building processes of the giant Qilian Shan during the Cenozoic. One view argues that the Qilian Shan began to uplift substantially as a simultaneous far-field response to the India-Asia collision at 55−50 Ma. In contrast, others claim that significant rise of the Qilian Shan and thus northeastward expansion of the Tibetan Plateau did not occur until 19 or 12 Ma. Based on an updated magnetostratigraphic framework for the Cenozoic sediments in the northern QB, here we conducted structural, paleocurrent, pebble composition, zircon grain shape and surface texture, and detrital geochronological analyses of the Lulehe Fm., in the northern QB. The results indicate that the Lulehe Fm. was produced essentially by an initial rush of lithic clasts derived from the deformed Mesozoic sedimentary cover, which can be attributed to initial rise of the Qilian Shan since ca. 25 Ma. This finding leads additional credence to the argument that the onset of significant uplift of mountain ranges along the periphery of the plateau occurred nearly synchronously from the latest Oligocene through early Miocene. These prolonged elevated Mesozoic sediments covering the fold-thrust belts of the northern QB, on the other hand, may have acted as a sustained source of material for the Miocene eolian deposits in the western Chinese Loess Plateau.
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Secondary cratering on Earth: The Wyoming impact crater field
Thomas Kenkmann; Louis Müller; Allan Fraser; Doug Cook; Kent Sundell ...
Abstract: A large number of small impact structures have been discovered in Wyoming, USA, and we raise the question of how this accumulation occurred. We document 31 crater structures of 10−70 m diameter with corresponding shock features but missing meteorite relics. All craters occur along the outcrops of the uppermost Permo-Pennsylvanian Casper Sandstone Formation and are ∼280 m.y. old. Their spatial arrangement shows clusters and ray-like alignments. Several craters have elliptical crater morphologies that allow the reconstruction of impact trajectories. The radial arrangement of the trajectories indicates that the craters are secondary craters formed by ejecta from a primary crater whose likely position and size are reconstructed. Modeling ballistic trajectories and secondary crater formation indicates that impacts occurred at around 700−1000 m/s and caused small shock volumes with respect to crater volumes. This is the first field of secondary craters found on Earth, and we disentangle its formation conditions.
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Anthropogenic impact on sediment transfer in the upper Missouri River catchment detected by detrital zircon analysis
Kelly D. Thomson; Daniel F. Stockli; Andrea Fildani
Abstract: Downstream sediment transport in river systems is impacted by anthropogenic modifications, such as dams or levees. This study used detrital zircon U-Pb geochronology and sediment mixture modeling to investigate the effects of dams on provenance signatures and sediment transport along the upper Missouri and Yellowstone Rivers. The Yellowstone River is the longest dam-free river in North America, while the upper Missouri River has several major reservoir-forming dams. We present 4777 new individual detrital zircon U-Pb ages from 32 sand samples collected from sand bars of the Missouri and Yellowstone Rivers and their major tributaries. These new data along the Missouri River track downstream modification of detrital zircon U-Pb age signatures due to sediment sequestration in reservoirs, bank erosion, and mixing at stream confluences. In contrast, detrital zircon U-Pb age data evolve more progressively downstream along the Yellowstone River, which displays less anthropogenic influence and muted tributary mixing and dilution. U-Pb age component proportions along the extensively dammed Missouri River vary up to 27% downstream, with amplified changes occurring at river confluences downstream from dams. These dramatic changes are attributed to storage of sediment and preferential capture of heavy minerals (including zircon) in reservoirs, resulting in a reduced sediment load in the river downstream from dams. Consequently, detrital zircon mixture models show a disproportionate provenance contribution from tributaries compared to the trunk river downstream from the dams. Dams not only reduce the sediment flux from a river but also change the locations where sediment is generated by initiating erosion in a river downstream from a dam.
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The stability of dinosaur communities before the K−Pg boundary: A perspective from southern Alberta using calcium isotopes as a dietary proxy
Jeremy E. Martin; Auguste Hassler; Gilles Montagnac; François Therrien; Vincent Balter
Abstract: Reconstructing dinosaur trophic structure prior to the Cretaceous−Paleogene (K−Pg) boundary may provide information about ecosystem organization and evolution. Using calcium isotopes, we investigate preserved biogenic isotope compositions in a set of dinosaur teeth from three continental formations from Alberta, Canada, to assess latest Cretaceous food web structure. Tooth enamel δ44/42Ca values are presented for tyrannosaurids (n = 34) and potential large herbivorous prey (n = 42) in the upper Campanian Dinosaur Provincial Park Formation, uppermost Campanian−Maastrichtian Horseshoe Canyon Formation, and upper Maastrichtian−lower Paleocene Scollard Formation, spanning the last ∼10 m.y. of the Cretaceous. The influence of diagenesis is assessed in a subset sample through major and trace elemental concentrations and ultraviolet (UV) Raman spectra, which provides a framework for interpreting calcium isotope values. In the Dinosaur Park Formation, hadrosaurid δ 44/42Ca values are systematically heavier than ceratopsid values, a difference that is interpreted to reflect niche partitioning among megaherbivores. Tyrannosaurid δ44/42Ca values are scattered but on average, they are 44Ca-depleted relative to herbivorous dinosaurs in all three formations. As interpreted from the Dinosaur Park data set, tyrannosaurids may have preferentially fed on hadrosaurids. These analyses offer possibilities for testing whether trophic structure among non-avian dinosaur ecosystems changed several millions of years prior to the K−Pg boundary.
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The Castle Rock and Ironside Mountain calderas, eastern Oregon, USA: Adjacent venting sites of two Dinner Creek Tuff units—the most widespread tuffs associated with Columbia River flood basalt volcanism
Matthew Cruz; Martin J. Streck
Abstract: The Dinner Creek Tuff is an important unit of mid-Miocene rhyolite volcanism contemporaneous to flood basalts of the Columbia River magmatic province. Field mapping along with analytical data of tuff samples identify two calderas, the Castle Rock and Ironside Mountain calderas, as the venting sites of two widespread ignimbrites of the Dinner Creek Tuff. Both calderas lie within the area of the proposed general storage sites of main-phase Columbia River Basalt magmas. The Castle Rock caldera formed during the eruption of the 16.16 Ma Dinner Creek Tuff unit 1. The northwestern boundary of the caldera is roughly defined by the juxtaposition of over 300 m of densely welded rheomorphic intra-caldera tuff and tuffaceous mega-breccia deposits against Mesozoic Weathersby Formation shale and pre-Miocene Ring Butte trachybasalt lavas. Following caldera collapse, fluvial and lacustrine volcaniclastic sediments were deposited on the caldera floor, and outflow tuffs of the Dinner Creek Tuff units 2 and 4 were deposited into the caldera. Aphyric basaltic andesite and icelandite (Fe-rich andesite), which correlate stratigraphically to upper Grande Ronde Basalt lavas, intrude the caldera floor deposits, and lavas are interbedded with sediments and Dinner Creek Tuff unit 4.The Ironside Mountain caldera formed during eruption of the 15.6 Ma Dinner Creek Tuff unit 2, which lies ∼15 km north of the Castle Rock caldera. The caldera is an 11 × 6 km depression wherein over 900 m of intra-caldera, rheomorphic, and partially welded tuff are bound by Weathersby Formation shale and Tureman Ranch granodiorite. Post-caldera collapse, basaltic andesite and icelandite dikes and sills that are also stratigraphically correlative to upper Grande Ronde Basalt lavas intruded into the tuff, mostly along the margins of the caldera, which altered much of the tuff.Mafic lavas within the study area that closely pre- and post-date Dinner Creek Tuff units were correlated with regional units of the Columbia River Basalt Group. Porphyritic and aphyric mafic lava flows underlying Dinner Creek Tuff unit 1 at Castle Rock are correlated with Picture Gorge Basalt and Grande Ronde Basalt. Aphyric basaltic andesite and icelandite that intrude into and overlie the Dinner Creek Tuff units 1 and 2 are westward extensions of fractionated tholeiitic magmas as seen in late-stage Grande Ronde Basalt units such as the Hunter Creek Basalt. Finally, porphyritic basalt lava flows that overlie the Hunter Creek Basalt and volcaniclastic sediments at the Castle Rock caldera are correlative with the 13.5 Ma Tim’s Peak Basalt. At Castle Rock, pre-caldera Columbia River Basalt Group lavas appear to lap onto a mid-Miocene topographic high that stretches northward and westward for tens of kilometers based on stratigraphic data, and it may be related to regional uplift at initial impingement of the mantle upwelling to produce the Columbia River Basalt Group. The Castle Rock and Ironside Mountain calderas exemplify bimodal volcanism of the Columbia River magmatic province. Eruption of rhyolites is closely pre- and post-dated by the eruption of local and regional tholeiitic lavas belonging to the Columbia River Basalt Group. The local eruption of evolved tholeiitic lavas likely concealed calderas, but these lavas also illustrate the close proximity of mafic and rhyolitic magmas at depth at these rhyolite centers. Consequently, the stratigraphy of both the Castle Rock and Ironside Mountain calderas somewhat differs from that of rhyolite calderas dominated by silicic and calc-alkaline intermediate pre- and post-caldera volcanism.
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Late Miocene−Pliocene onset of fluvial incision of the Cauca River Canyon in the Northern Andes
N. Pérez-Consuegra; G.D. Hoke; P. Fitzgerald; A. Mora; E.R. Sobel ...
Abstract: The incision of kilometer-scale canyons into high-standing topography is often used to constrain the surface uplift history of mountain ranges, controlled by tectonic and geodynamic processes. However, changes in climate may also be responsible for canyon incision. This study deciphers the timing of incision of the ∼2.5-km-deep Cauca River Canyon in the Central Cordillera of the Northern Andes using the cooling (exhumation) history of rocks from the canyon walls and a regional analysis of channel steepness in rivers. Ten bedrock samples and one detrital sample were collected on the eastern border of the canyon between 300 m and 2300 m of elevation. Bedrock and detrital AFT data yield ages from 50 to 38 Ma, while two bedrock AHe ages from the valley bottom yield ages of 7−6 Ma. The AHe ages and inverse thermal history models reveal a previously unidentified late Miocene (ca. 7−6 Ma) pulse of exhumation that we interpret as the age of a single incision event that formed the Cauca River Canyon. We conclude that the Cauca River Canyon was carved as a response to rock uplift in the northern Central Cordillera and propagation of an erosion wave into the mountain range starting in the latest Miocene.
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Genetic mechanism of transfer zones in rift basins: Insights from geomechanical models
Jingshou Liu; Haimeng Yang; Ke Xu; Zhimin Wang; Xinyu Liu ...
Abstract: A transfer zone is a kind of structure that is produced to conserve deformation of a fault structure on both sides. Increasing numbers of transfer zones are being identified in rift basins, which are areas of petroleum accumulation and potential exploration targets. This paper provides a numerical simulation method for the genesis and development of transfer zones based on geomechanical modeling. On the basis of three-dimensional (3-D) seismic interpretation, using the Tongcheng fault as an example, the fault activity parameter and fault activity intensity index were established to quantitatively characterize the difference in fault activity on the two sides of a transfer zone. A geomechanical model was developed for a transfer zone in a rift basin, and the structural characteristics and genetic mechanism of a convergent fault were studied using paleostress and strain numerical simulations. Affected by different movements of boundary faults and basement faults, the evolution of the Tongcheng fault can be divided into three stages: (1) during the Funing period, which was the main development period of compound transfer faults, the activity, stress, and strain of the fault blocks on either side of the Tongcheng fault were obviously different; (2) during the Dainan period, which was the development stage of inherited compound transfer faults, the northern part of the Tongcheng area underwent local compression, and the T3 anticline began to form; and (3) during the Sanduo period, the Tongcheng fault experienced right-lateral strike-slip activity, where the activity showed two stages of change, first increasing and then decreasing, and the Tongcheng fault anticline developed. The superposition of multiple complex tectonic movements produced a transfer zone that has both strike-slip and extensional fault properties. The geomechanical model in this paper provides important insights for analyzing the evolution of transfer zones in rift basins.
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A waning Saxothuringian Ocean evidenced in the Famennian tephra-bearing siliceous succession of the Bardo Unit (Central Sudetes, SW Poland)
Grzegorz Racki; Stanisław Mazur; Katarzyna Narkiewicz; Agnieszka Pisarzowska; Waldemar Bardziński ...
Abstract: A tephra-rich cherty-clayey Famennian succession within the major Brzeźnica olistostrome in the Bardo Mountains, Central Sudetes, SW Poland, preserves a record of the lost ocean later incorporated into the Variscan orogenic belt. Fluctuating but mostly oligotrophic regimes and low primary production levels were influenced by weak upwelling below the perennial oxygen minimum zone, which controlled the interplay between biosiliceous and siliciclastic deposition in the oceanic basin, with episodic oxygen deficiency. The Hangenberg Black Shale has been identified in this oceanic setting based on its characteristics described worldwide (including mercury enrichments). A tectonic uplift of the sediment source area near the Devonian-Carboniferous boundary, recorded in the distinguishing provenance signal of old continental crust, was paired with a global transgression, anoxia, and volcanic episode in an interglacial interval. Assuming paleogeographic affinity with the Bavarian facies of the Saxothuringian terrane, we interpret the allochthonous sediments as part of an accretionary prism that was gravitationally redeposited into the late orogenic basin in front of advancing Variscan nappes. The oceanic basin parental to the Bardo pelagic succession is therefore thought to represent a tract of the waning Saxothuringian Ocean in the Peri-Gondwanan paleogeographic domain that was eventually subducted beneath the Brunovistulian margin of Laurussia. The sediments of the Bardo Ocean basin also include a distal record of Famennian explosive volcanic activity that was likely related to a continental magmatic arc whose remnants are preserved as the Vrbno Group of the East Sudetes.
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Mass extinction or extirpation: Permian biotic turnovers in the northwestern margin of Pangea
Sangmin Lee; G.R. Shi; Hans A. Nakrem; Jusun Woo; Jun-Ichi Tazawa
Abstract: The Capitanian (Middle Permian) mass extinction event, prior to and separate from the end-Permian mass extinction, has been suggested as a severe biotic crisis comparable to the big five mass extinctions of the Phanerozoic. However, there is still controversy about its global significance. In particular, this purportedly disastrous event in the Capitanian was mostly documented in the eastern Tethys, especially South China and Japan, whereas its extent in higher latitudinal regions remains unclear. A few recent studies have reported biostratigraphic and chemostratigraphic evidence for the Capitanian extinction at the northwestern marginal shelf of Pangea, including in the Kapp Starostin Formation in Spitsbergen. However, we here report a different result from these previous studies based on a study of abundant brachiopod fossils collected from eight geological sections that represent the same formation in western and central Spitsbergen, Arctic Norway. Our biostratigraphic investigation recognizes a total of five brachiopod assemblages from the type section of the Kapp Starostin Formation at Festningen in Spitsbergen. The most striking biotic change in species composition is observed at the interval between the lowermost Vøringen Member (late Artinskian) and its overlying member (Kungurian) of the Kapp Starostin Formation in Spitsbergen, which makes it much earlier than the Capitanian. A similar faunal shift at the same stratigraphic interval is also observed from bryozoan-based biostratigraphic data. This faunal turnover could be linked to a significant climatic shift (cooling) along the northwestern margin of Pangea during the Artinskian−Kungurian. Specifically, it is inferred that a climatic perturbation (cooling) likely drove the extirpation (emigration) of marine faunas out of Spitsbergen and dispersal eastward into some lower latitudinal and climatically more habitable areas. Our result indicates that the Capitanian interval in Spitsbergen does not record a catastrophic event that corresponds to the Capitanian mass extinction in Tethyan regions but rather marks gradual faunal transitions throughout the Middle to Late Permian. This faunal transition, driven by continuous cooling, was accompanied by major changes in regional lithology, which suggest a degree of local environmental control, especially in the changes of substrate and water depth, on the composition of the benthic faunas. The Wegener Halvø and Schuchert Dal Formations (Lopingian) in central East Greenland contain a diverse brachiopod fauna that is comparable to that of the post-Vøringen Member in Spitsbergen. This implies that the brachiopods in the northwestern marginal shelf of Pangea did not suffer a severe mass extinction in the Capitanian; instead, many of them migrated southward with the development of the Zechstein seaway.
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Paleozoic−Mesozoic dispersal of Gondwana: Insights from detrital zircon geochronology of Lesser Himalaya strata, eastern Nepal
Upendra Baral; Lin Ding; Megh Raj Dhital; Kumar KC; Shun Li
Abstract: Detrital zircon geochronology has rapidly evolved into a powerful tool for reconstructing the assembly and dispersal processes of supercontinents. Currently, the dispersal history of Gondwanaland remains highly controversial. Here we focus on detrital zircon geochronology of the Gondwana (Carboniferous−Permian Kokaha Diamictite and Jurassic−Cretaceous Sapt Koshi Formation) and post-Gondwana (Miocene Tamrang Formation) sequences of the Lesser Himalaya in eastern Nepal. Detrital zircon U-Pb dating results show that the Carboniferous−Permian sequence peaks at 544 Ma, 890 Ma, 1178 Ma, and 1752 Ma. Likewise, the Jurassic−Cretaceous sequence peaks at 531 Ma, 947 Ma, 1176 Ma and 1806 Ma along with a much younger peak at 123 Ma. Similarly, the Miocene sequence peaks at 526 Ma, 987 Ma, and 1740 Ma. Comparing these newly obtained ages with those of surrounding regions, we confirm that during the Carboniferous−Permian, the Indian continent was still connected to Gondwana. The U-Pb age distribution of the Kokaha Diamictite coincides with that of the Tethys Himalaya, which further suggests the possibility that this unit either shared the same provenance with or was recycled from the Tethys Himalaya. During the early Mesozoic, the Indian plate rifted from Gondwana and drifted northward, as evidenced by Triassic−Jurassic, rift-related magmatism along the Indian continental margin. Remarkably, there were significant inputs from the Rajmahal Basalt during deposition of the Jurassic−Cretaceous sequence. Besides, the Miocene sequence records a large number of zircons that closely resemble those of the Greater Himalaya, which implies that the Greater Himalaya may have already been uplifted and eroded by then.
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Ordovician−Silurian back-arc silicic magmatism in the southernmost Appalachians
Clinton I. Barineau; Douglas A. Sagul; Paul A. Mueller
Abstract: Ordovician and Silurian siliciclastic, volcanic, and plutonic rocks in the southern Appalachian eastern Blue Ridge and western Inner Piedmont, formerly interpreted as components of an obducted Taconic arc terrane, are recognized as parts of an Ordovician paired arc/back-arc system that formed on the seaward edge of the Laurentian plate. New crystallization ages from metamorphosed plutonic-volcanic components intruding and intercalated with metasedimentary back-arc basin lithologies of the Wedowee-Emuckfaw-Dahlonega back-arc basin confirm rapid deposition (hundreds of meters per million years) in an Ordovician−Silurian extensional basin that received input from continentally derived Mesoproterozoic crust and bimodal volcanic components. U-Pb (206 Pb/238U) zircon ages from 12 samples of Zana Granite yield minimum crystallization ages between 459 Ma and 430 Ma, while zircon grains from six samples of Kowaliga Gneiss have minimum crystallization ages between 452 Ma and 435 Ma. Zircons from a probable metavolcanic unit within the Wedowee Group yield a 206Pb/238U age of 454 ± 3 Ma. Mesoproterozoic depleted mantle model ages and negative initial Ɛ Hf values of zircons from the Ordovician−Silurian metaigneous rocks suggest a significant Mesoproterozoic component in their genesis. Formation of these silicic plutons was likely associated with the intrusion of mantle-derived mafic melts and decompression melting of Mesoproterozoic lower crust as part of a tectonically thickened crustal section with a contribution from sub-continental mantle lithosphere during back-arc extension. Silicic magma intruding thick successions of bimodal volcanic rocks and intercalated sedimentary rocks shortly after deposition is typical of back-arc extension and contraction phases above B-type subduction zones (i.e., Benioff oceanic subduction) in Lachlan-style orogens, which is consistent with the accretionary orogenic nature of the southern Appalachian Taconic orogen.
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Linkages between nitrogen cycling, nitrogen isotopes, and environmental properties in paleo-lake basins
Liuwen Xia; Jian Cao; Eva E. Stüeken; Wenxuan Hu; Dongming Zhi
Abstract: The linkages between nitrogen cycling, nitrogen isotopes, and environmental properties are fundamental for reconstructing nitrogen biogeochemistry. While the impact of ocean redox changes on nitrogen isotopes is relatively well understood, it is poorly known how nitrogen responds to changes in pH and salinity. To fill the knowledge gap, we explore the effects of these environmental parameters using a well-controlled set of samples from Carboniferous−Paleogene lake sediments in China. Our results show that the threshold of 10−12‰ in δ15N works to distinguish alkaline (pH > 9) from circum-neutral conditions. Elevated Mo levels in the alkaline samples support the idea of NH3 volatilization from a reducing water column in an alkaline setting. For non-alkaline lakes, δ15 N values tend to be higher (up to +10‰) in more saline, anoxic settings, which is attributed to either the expansion of stagnant anoxic waters spurring water-column denitrification or a shift from plant-based toward more microbially dominated ecosystems or both. Our results imply that salinity-induced redox stratification and basicity can alter nitrogen biogeochemical cycling beyond what is shown by the marine nitrogen isotope record alone. This finding will result in an improved understanding of the dynamic controls of δ15N in sediments and lead to better biogeochemical interpretations of paleo-environmental conditions from unknown environmental settings on Earth and beyond Earth.
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Enhanced precipitation in the Gulf of Mexico during the Eocene−Oligocene transition driven by interhemispherical temperature asymmetry
Mingqiu Hou; Guangsheng Zhuang; Brooks B. Ellwood; Xiao-lei Liu; Minghao Wu
Abstract: Studies reveal that the sea-surface temperature (SST) of the Northern Hemisphere decreased at a smaller amplitude than that of the Southern Hemisphere during the EoceneOligocene transition (EOT). This interhemispheric temperature asymmetry has been associated with intensified Atlantic Meridional Overturning Circulation (AMOC) that may have driven enhanced precipitation and weathering in low latitudes and the subsequent drawdown of atmospheric carbon dioxide. However, no quantitative constraints on paleo-precipitation have been reported in low latitudes to characterize the AMOC effect across the EOT. Here, we present the results of high-resolution (ca. 6 k.y. per sample) isotopic and biomarker records from the Gulf of Mexico. Reconstructed precipitation using leaf wax carbon isotopes shows an increase of 44% across the EOT (34.1−33.6 Ma), which is accompanied by a secular increase in SST of ∼2 °C during the latest Eocene. We attribute the enhanced precipitation in the Gulf of Mexico to the northward shift of the Intertropical Convergence Zone that was driven by an enlarged polar-tropic temperature gradient in the Southern Hemisphere and an invigorated AMOC. Our findings link changes in meridional temperature gradient and large-scale oceanic circulation to the low-latitude terrestrial hydroclimate and provide paleohydrological evidence that supports CO2-weathering feedback during the EOT “greenhouse” to “icehouse” transition.
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Emergence of wet conditions in the Mono Basin of the Western USA coincident with inception of the Last Glaciation
Guleed A.H. Ali; Ke Lin; Sidney R. Hemming; Stephen E. Cox; Philipp Ruprecht ...
Abstract: At present, the Basin and Range of the western USA is arid, but geologic studies show evidence of past wetness. The timing of these wetter conditions reveals a close association with glacial conditions. This association has led to the hypothesis of a causal link between glacial climate and regional wetness, but poor age control on the onset of regional wetness thwarts a test of this hypothesis. Here we determine the start of the most recent interval of persistent wetness in the Mono Basin, which is a hydrologically closed depression that sits at the west-central edge of the Basin and Range. The most recent emergence of persistent wetness in the Mono Basin is stratigraphically correlated with the depositional age of Ash 19—a rhyolitic ash bed that represents the oldest tephra of the Wilson Creek Formation and one of the earliest-known products of explosive volcanic activity from the Mono Craters. We constrain the depositional age of Ash 19 by using the U/Th disequilibrium dating method to date carbonates that are younger and older than Ash 19. Our U/Th dating results show that Ash 19 was deposited before the formation of a cross-cutting carbonate bed dated to 69.2 ± 0.3 ka but after an underlying carbonate tufa dated to 67.4 ± 3.5 ka, which suggests that the start of wetness in the Mono Basin was contemporary with the inception of the Last Glaciation—the beginning of Marine Isotope Stage 4—at ca. 70 ka. This finding corroborates the hypothesis of a link between glacial climate and regional wetness.
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Paleo−Pacific plate subduction on the eastern Asian margin: Insights from the Jurassic foreland system of the overriding plate
Liang Qiu; Ruoyan Kong; Dan-Ping Yan; Hong-Xu Mu; Weihua Sun ...
Abstract: The subduction of the paleo−Pacific plate beneath the North China block has been well documented in terms of magmatic activity, geophysical investigations, and numerical modeling, but the timing of its onset along the eastern Asian margin and the tectonic processes involved remain poorly understood. We have now reconstructed the structural evolution and sedimentation of the Shihuiyao−Gangzidian−Yuantai region of the southern Liaodong Peninsula, on the overriding plate at the eastern Asian margin, using geologic mapping, borehole data sets, field structural studies, and zircon geochronology. In the Shihuiyao area, based on geologic mapping and drilling, top-to-the-NW thrust faults truncated earlier top-to-the-NE reverse faults and then were subsequently overprinted by normal faults. In the Gangzidian area, geologic mapping and 40 boreholes allowed us to construct eight cross sections revealing the top-to-the-WNW/W thrust faults from the surface to a depth of at least 0.5 km; the sections show that the thrust faults extend to the basement and that subsequent normal faults dip subvertically. In the Yuantai thrust system, the top-to-the-NW thrusts, expressed as four tectonic windows and a duplex on the map scale, were intruded by late porphyry and diabase sills. Integration of the data from the three study areas allowed us to identify one angular unconformity and at least two phases of later deformation (D1 and D2). The pre-D1 unconformity is marked by a foreland basin that was filled with Middle Jurassic clastic rocks that unconformably overlie the Neoproterozoic and Cambrian basement. The D1 deformation is represented by NE-SW−striking thrust faults that displaced Neoproterozoic or Cambrian strata onto the Middle Jurassic strata. The D2 deformation is defined by kilometer-scale, high-angle normal faults with variable dips and strikes. Although three samples from the Middle Jurassic clastic rocks did not yield ideal maximum deposition ages (MDAs; ca. 246 Ma), a porphyry dike and a diabase sill that intruded the Yuantai thrust system and remain undeformed yielded zircon U-Pb ages of ca. 124 and 117 Ma, respectively. Detrital zircon geochronology yielded a MDA of ca. 129 Ma for Cretaceous deposits in graben structures with hanging walls formed by D 2 normal faults. Thus, the D1 and D2 deformation events can be constrained to the Late Jurassic and Early Cretaceous, respectively (D1 between ca. 174 and 124 Ma, and D 2 after ca. 129 Ma). The provenance of the detritus in the Middle Jurassic Wafangdian Formation suggests that the deposits in the retroarc foreland basin had a source in a thrust sheet of the Paleoproterozoic basement, but the source of deposits in a minigraben (D 2) was possibly the nearby Neoproterozoic rocks. Therefore, we reconstructed the retroarc foreland basin and fold-and-thrust belt of the southern Liaodong Peninsula in terms of a subduction margin and constrained a Toarcian−Aptian (ca. 174−124 Ma) age for the onset of paleo−Pacific plate subduction. We interpret the foreland basin system and the subsequent synconvergent extension to have been the result of slab flattening and rollback during episodic subduction of the paleo−Pacific plate.
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The Numidian sand event in the Burdigalian foreland basin system of the Rif, Morocco, in a source-to-sink perspective
Anas Abbassi; Paola Cipollari; M.G. Fellin; M.N. Zaghloul; Marcel Guillong ...
Abstract: During the Tertiary evolution of the Western Mediterranean subduction system, a migrating foreland basin system developed between the Maghrebian orogenic belt and the adjacent African Craton. However, a comprehensive reconstruction of the foreland basin systems of the Rif Chain is still missing. By integrating field observations with quantitative biostratigraphic data from calcareous nannofossil assemblages, sandstone composition, and detrital zircon U-Pb geochronology from selected stratigraphic successions, we reconstruct the foreland basin system that developed in the early Miocene in front of the growing Rif orogen. The successions analyzed are representative of (1) the classical “Numidian Facies” from the Intrarifian Tanger Unit and (2) the Numidian-like deposits (mixed successions) of the “Mérinides Facies” from the “Maghrebian Flysch Basin” and the “Beliounis Facies” from the Predorsalian Unit. Our petrographic analyses and detrital zircon U-Pb ages show that the quartzarenites of the “Numidian Facies” originated from the African Craton, whereas the sublitharenites and feldspathic litharenites from the Mérinides and Beliounis Facies originated from a cratonic area and the exhuming Rif Chain. Our biostratigraphic analyses suggest a simultaneous arrival of the quartz grains in the Numidian, Mérinides, and Beliounis deposits, which indicates that their deposition occurred at ∼1 m.y. (ca. 20−19 Ma, early Burdigalian) and allows us to delineate the early Burdigalian foreland basin system of the Rif Chain. The foreland depozone received the “Numidian Facies,” the foredeep-hosted ∼2000 m of the “Mérinides Facies” and the Beni Ider Flysch, whereas the wedge-top depozone was characterized by deposition of the “Beliounis Facies.” The Numidian Sandstones and the Numidian-like deposits analyzed in Morocco show the same age as similar deposits from Algeria, Tunisia, and Sicily, which suggests a comparable early Burdigalian tectono-sedimentary evolution along the southern branch of the Western Mediterranean subduction-related orogen.
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Microporphyritic and microspherulitic melt grains, Hiawatha crater, Northwest Greenland: Implications for post-impact cooling rates, hydration, and the cratering environment
Adam A. Garde; Nynke Keulen; Tod Waight
Abstract: Sand-sized impactite melt grains hand-picked from a glaciofluvial sample proximal to the Hiawatha impact crater in Northwest Greenland contain new information about the crystallization and cooling history of this impact structure, which is concealed by the Greenland Ice Sheet. Of course, the original locations of the individual sand grains are unknown, but this is offset by the substantial number and wide variety of impactite grains available for study. A detailed investigation of 16 melt grains shows that post-cratering crystallization took place under very variable conditions of strong undercooling with temperatures that dropped rapidly from high above their solidus to far below. A distinct event of near-isochemical hydration at above or ∼250 °C is recorded by intense perlitic fracturing and the growth of closely packed mordenite spherulites only 1−3 μm across in felsic melt grains, which was followed by lower temperature hydrothermal alteration along the pre-existing perlitic fractures. The formation of abundant mordenite microspherulites appears to be very rare or not previously recorded in impactite melts and suggests the rapid infilling of the Hiawatha crater by a hydrous source. The infilling did not occur immediately after the impact as in submarine impacts, but soon thereafter, and before the establishment of a low-temperature hydrothermal alteration system common to the waning stage of cooling in many impact structures. These observations and previous documentation of terrestrial organic matter in the impactites are consistent with an impact into a water-rich terrestrial environment, such as through the Greenland Ice Sheet or into a forested, lacustrine−fluvial region.
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Middle−Late Triassic southward-younging granitoids: Tectonic transition from subduction to collision in the Eastern Tianshan−Beishan Orogen, NW China
Qigui Mao; Songjian Ao; Brian F. Windley; Zhiyong Zhang; Miao Sang ...
Abstract: To constrain the closure mechanism and time of the Paleo-Asian Ocean, we report new geochronological and geochemical data for Triassic granites along a NW−SE corridor from Eastern Tianshan to Beishan, NW China. Seven granites have U-Pb ages that young southwards from 245 Ma to 234 Ma in the Kanguer accretionary complex, to 237 Ma to 234 Ma in the eastern Central Tianshan block, to 229 Ma to 223 Ma in the Liuyuan accretionary complex. Granites in the Kanguer accretionary complex formed by fractional crystallization and are peraluminous, high-K, calc-alkaline, and crust-derived. They have very low MgO (Mg# = 6−9), Cr, and Ni contents, and their high εNd(t) (+3.40) and εHf(t) (+4.49 to +11.91) isotopes indicate that the Dananhu arc crust was juvenile. The Huaniushan pluton in the Liuyuan accretionary complex displays the geochemical signatures of both A1- and A2-type granites (Y/Nb = 0.32−3.39). All other granites in the Central Tianshan block and Liuyuan accretionary complex are aluminous A2-types with high K2O+Na2O, Al, rare earth elements (REE), Zr+Nb+Y, Ga, Fe/Mg, and Y/Nb and remarkable depletions of Eu, Ba, Nb, Ta, Sr, P, and Ti. They have a broad range of MgO (Mg# = 9−59), Cr, and Ni contents, Isr (0.70741−0.70945) values, negative εNd (t) (−2.98 to −1.14), and low to moderate ε Hf(t) (−1.22 to +7.78), which suggests a mixture of mantle and crustal components. These 245−223 Ma granitoids show marked Nb-Ta depletions that point to a subduction origin. Notable enrichments in Nd-Hf isotopes of Late Triassic granites are likely an indication of collision. Integration with previous data enables us to conclude that the delamination of an oceanic slab and mantle upwelling induced partial melting of thickened arc crust during a tectonic transition from a multiple supra-subduction margin to a collisional setting in the Late Triassic.
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Geochronology and geochemistry of the Huntington Formation, Olds Ferry terrane, Blue Mountains province, northern U.S. Cordillera: Implications for accreted terrane correlation and assembly
Bryant Ware; Kyle P. Tumpane; Gene A. Kurz; Mark D. Schmitz; Clyde J. Northrup
Abstract: The Olds Ferry terrane is the more inboard of two accreted volcanic arc terranes in the late Paleozoic−early Mesozoic Blue Mountains province of the northern U.S. Cordillera. We present geologic, geochronologic, and geochemical data from the volcano-sedimentary Huntington Formation of the Olds Ferry arc that place the terrane within a firm temporal and tectonomagmatic context, and establish its identity as a fringing arc terrane along the Triassic to Early Jurassic Cordilleran margin. The Huntington Formation is divided into two unconformity-bounded informal members: a Norian (ca. 220 Ma) lower member comprising a sequence of mafic-intermediate volcanics, massive volcaniclastic breccias, and minor carbonates deposited unconformably onto the 237.7 Ma Brownlee pluton and intruded by the 210.0 Ma Iron Mountain pluton; and a Rhaetian through Pleinsbachian (<210−187.0 Ma) upper member composed of massive conglomerates, abundant rhyodacite to rhyolite effusive and pyroclastic flows, and interlayered sandstone turbidites, deposited with angular unconformity onto the lower member. An erosional hiatus and regional tilting produced an angular unconformity separating the Huntington Formation from the overlying basal conglomerates of the late Early to Middle Jurassic Weatherby Formation of the Izee forearc basin transgressive onlap sequence. Huntington Formation volcanic rocks are isotopically enriched relative to depleted mantle and coeval igneous rocks in the outboard Wallowa terrane. A temporal evolution to more radiogenic 87Sr/86Sr ratios (0.7036−0.7057) and εNd values (+5.4 to +3.1) in the upper member volcanics suggests increasing involvement of continental-derived material in their petrogenesis. Precambrian xenocrystic zircons in both lower and upper member volcaniclastic rocks strongly support a proximal location of the Olds Ferry terrane to cratonal North America during much of its history. The chronology and tectonostratigraphic architecture of the Olds Ferry terrane allows its robust correlation to other fringing-arc terranes along the U.S. and Canadian Cordillera.
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Tectonic controls on Quaternary landscape evolution in the Ventura basin, southern California, USA, quantified using cosmogenic isotopes and topographic analyses
A. Hughes; D.H. Rood; D.E. DeVecchio; A.C. Whittaker; R.E. Bell ...
Abstract: The quantification of rates for the competing forces of tectonic uplift and erosion has important implications for understanding topographic evolution. Here, we quantify the complex interplay between tectonic uplift, topographic development, and erosion recorded in the hanging walls of several active reverse faults in the Ventura basin, southern California, USA. We use cosmogenic 26Al/10Be isochron burial dating and 10Be surface exposure dating to construct a basin-wide geochronology, which includes burial dating of the Saugus Formation: an important, but poorly dated, regional Quaternary strain marker. Our ages for the top of the exposed Saugus Formation range from 0.36 +0.18/−0.22 Ma to 1.06 +0.23/ −0.26 Ma, and our burial ages near the base of shallow marine deposits, which underlie the Saugus Formation, increase eastward from 0.60+0.05/−0.06 Ma to 3.30 +0.30/ −0.41 Ma. Our geochronology is used to calculate rapid long-term reverse fault slip rates of 8.6−12.6 mm yr−1 since ca. 1.0 Ma for the San Cayetano fault and 1.3−3.0 mm yr−1 since ca. 1.0 Ma for the Oak Ridge fault, which are both broadly consistent with contemporary reverse slip rates derived from mechanical models driven by global positioning system (GPS) data. We also calculate terrestrial cosmogenic nuclide (TCN)-derived, catchment-averaged erosion rates that range from 0.05−1.14 mm yr−1 and discuss the applicability of TCN-derived, catchment-averaged erosion rates in rapidly uplifting, landslide-prone landscapes. We compare patterns in erosion rates and tectonic rates to fluvial response times and geomorphic landscape parameters to show that in young, rapidly uplifting mountain belts, catchments may attain a quasi-steady-state on timescales of <10 5 years even if catchment-averaged erosion rates are still adjusting to tectonic forcing.
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