Geological Society of America Bulletin最新文献

{"title":"Multistage melt/fluid modification of lithospheric mantle beneath the circum-cratonic orogenic belt: Evidence from the Tuoyun peridotite xenoliths","authors":"Xiao Bian, Yuping Su, Jian‐Ping Zheng, Jian Wang, Xi Chen, Liang Zhou, Bi’an Dong, Tianyi Niu","doi":"10.1130/b37552.1","DOIUrl":"https://doi.org/10.1130/b37552.1","url":null,"abstract":"The juvenile and moderate refractory mantle beneath the circum-cratonic orogenic belt is traditionally believed to be hotter and thinner than the ancient refractory cratonic mantle; it is thus more unstable and subject to modification by melts/fluids. Understanding these modification processes would help to elucidate the evolution of Earth’s continents. Peridotite xenoliths carried by the Tuoyun Cenozoic lamprophyre from the southwestern Tianshan belt show evidence of widespread multistage melt/fluid modification of the unstable circum-cratonic orogenic belt mantle. Tuoyun peridotites mainly consist of moderately refractory to fertile lherzolites (Mg# in Ol: 85.5−90.7; Cr# in Sp: 12.7−26.5) and show strong mechanical modification. They can be divided into four groups (A, B, C1, and C2) based on petrography and mineral chemistry. Group A lherzolites show relatively high basaltic components (Al2O3, CaO, TiO2, and FeO) and are enriched in large ion lithophile elements (LILEs) and rare earth elements (REEs), which indicates melt-peridotite reaction processes at high melt/rock ratios. The high modal pyroxene content in Group A suggests that the addition of high-Si melts caused the transition from olivine to pyroxene. Group B lherzolites show high modal pyroxene but relatively depleted incompatible elements, which should be superimposed by later melt extraction. Comparatively, Group C lherzolites exhibit higher modal olivine but lower basaltic components. The clinopyroxene cores of Group C1 are characterized by high (La/Yb)N and low Ti/Eu content, negative high field strength element (HFSE) anomalies, and relatively high 87Sr/86Sr ratios (cores: 0.70331−0.70457), which suggest metasomatism by carbonatite melts originating from recycled sedimentary carbonate. The Group C1 clinopyroxene (spongy rims) and Group C2 clinopyroxene (cores and spongy rims) have positive Sr anomalies, depleted HFSEs, and spoon-shaped REE patterns, which suggest modification by evolved small-volume and volatile-rich silicate melts. In addition, the melt pockets around spinels and the reactive zones of pyroxenes near the lamprophyre reveal the recent incongruent dissolution induced by the host rock. Based on our research and previously reported geological data, we propose that the high-Si melts and carbonatite melts are the products of dehydration and partial melting of the Paleo-Asian oceanic crust, and lithospheric delamination and fracturing (e.g., the Talas-Fergana strike-slip fault) provided the opportunity for small-volume and volatile-rich silicate melts and basaltic melts to modify the peridotites. Multistage melts/fluids and the deformation process are the protagonists in the evolutionary process of the circum-cratonic lithospheric mantle, with important implications for mantle destabilization and multilayered interaction.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"5 6","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141640964","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"P−T evolution of metapelitic and metamafic rocks from Northern Liaoning: Implications on the Neoarchean tectonic regime of the North China Craton","authors":"Minjie Guo, Jian Zhang, J. Qian, Changqing Yin, Peng Gao, Guokai Chen, Changquan Cheng, Juiyen Hsia, Shuhui Zhang","doi":"10.1130/b37531.1","DOIUrl":"https://doi.org/10.1130/b37531.1","url":null,"abstract":"The Archean basement rocks in the Eastern Block of the North China Craton have undergone extensive granulite-facies metamorphism during the late Neoarchean to Paleoproterozoic. However, the geodynamic process and associated tectonic regime of this area remain poorly understood. The Qingyuan area of Northern Liaoning, located in the nucleus of the Eastern Block, plays a crucial role in unraveling the tectonothermal evolution by reconstructing its metamorphic history. Previous studies have mainly focused on the northern part of the area, while the tectonothermal evolution of the southern part remains unknown. In this study, we utilized petrography, phase equilibria modeling, and geochronology to constrain the pressure−temperature−time (P−T−t) paths of representative metapelite and metabasite samples of the area. Our results indicate that both samples have recorded anticlockwise P−T−t paths. The peak condition for the metapelitic rock sample 22QY02-9 is 10.0−11.2 kbar/800−815 °C. U-Pb zircon and monazite dating results indicate a post-peak metamorphic age of ca. 2470−2450 Ma. Metamafic rock sample 22QY04-3 reaches the ultrahigh temperature peak metamorphic condition of 11.5−12.5 kbar/920−950 °C, which is consistent with the results obtained from the ternary-feldspar thermometry calculations. U-Pb zircon dating reveals a cooling age of metamorphism at ca. 2487 Ma. By combining the data from this study with previous research, a sagduction model, operating under a non−plate tectonics regime, appears to be the most promising explanation for the most prevalent geological phenomena in the Neoarchean North China Craton.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"7 6","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141653807","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Reconstructing paleoenvironments of the Late Cretaceous Western Interior Seaway, USA, using paired triple oxygen and carbonate clumped isotope measurements","authors":"J. Wostbrock, J. Witts, Yang Gao, Catherine Peshek, Corinne E. Myers, Gregory Henkes, Zachary D. Sharp","doi":"10.1130/b37543.1","DOIUrl":"https://doi.org/10.1130/b37543.1","url":null,"abstract":"Fossiliferous carbonate concretions are commonly found in sediments deposited in the Late Cretaceous Western Interior Seaway. Although concretions are diagenetic features, well-preserved fossils from within them have been instrumental in reconstructing the temperature and δ18O value of Western Interior Seaway seawater, which is essential for accurate reconstruction of Late Cretaceous climate. Here, we constrain formation conditions of Late Campanian and early Maastrichtian carbonate concretions by combining triple oxygen isotope measurements with carbonate clumped isotope paleothermometry on different carbonate phases within the concretions. We measured both fossil skeletal aragonite and sparry calcite infill from cracks and within macrofossil voids to evaluate differences between “primary” and “altered” geochemical signals. Based on the two temperature-sensitive isotope systems of the primary fossil shell aragonite, the temperature of the Western Interior Seaway was between 20 °C and 40 °C and was likely thermally stratified during the Campanian. The reconstructed δ18Oseawater values of ∼−1‰ for Campanian Western Interior Seaway waters are similar to those expected for the open ocean during greenhouse climates, while the Maastrichtian Western Interior Seaway may have been more restricted, with a δ18Oseawater value of ∼2‰, which reflects more evaporative conditions. We reconstructed the diagenetic history of the sparry infill and altered fossils using a fluid-rock mixing model. Alteration temperature, alteration fluid δ18O value, and the initial formation temperature were calculated by applying the fluid-rock mixing model to a particle swarm optimization algorithm. We found a different range of initial formation temperatures between the Campanian (25−38 °C) and Maastrichtian (9−28 °C). We also found that alteration in the presence of light meteoric fluids (δ18O ≈ −10‰) is required to explain both the sparry infill and the altered fossil isotopic values. Based on our results, both lithification and alteration of the carbonates occurred soon after burial, and light meteoric fluids support prior findings that high-topographic relief existed on the western margin of the Western Interior Seaway during the Late Cretaceous. As one of the first studies to apply these techniques in concert and across multiple mineralogical phases within samples, our results provide important constraints on paleoenvironmental conditions in an enigmatic ocean system and will improve interpretations of the overall health of ecosystems leading into the end-Cretaceous mass extinction.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"25 2","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141654334","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Dynamic redox conditions in Mesoproterozoic shallow seawater: Constraints from carbonate fabrics and geochemistry","authors":"Longfei Sun, Xiaoying Shi, Xiqiang Zhou, Limin Zhou, Kangjun Huang, Lei Xu, Baozeng Xie, Xinqiang Wang, Dongjie Tang","doi":"10.1130/b37544.1","DOIUrl":"https://doi.org/10.1130/b37544.1","url":null,"abstract":"The delayed eukaryote evolution during the mid-Proterozoic may have been linked to prolonged low oxygen levels during this period. Two short oxygenation events have recently been identified at ca. 1.57 Ga and ca. 1.4 Ga, but the redox conditions during the intervening interval remain poorly constrained. In this study, we conducted mineralogical and geochemical investigations on three sections of the Wumishan Formation (ca. 1.52−1.47 Ga) that were deposited between the two oxygenation events in the Yanliao basin. The results revealed two distinct changes in carbonate mineralogy and precipitation style associated with concurrent changes in carbonate I/(Ca + Mg): a transition from intervals with positive I/(Ca + Mg) anomalies and abundant water-column carbonate mud to intervals with no I/(Ca + Mg) anomalies and abundant seafloor precipitates, suggesting a shift from suboxic to anoxic conditions in shallow seawater. In both of the suboxic intervals, I/(Ca + Mg) values reached 0.5−2.6 μmol/mol, with negative Ce anomalies (Ce/Ce*(SN) = 0.69; data from literature) occurring in the later interval (ca. 1.48 Ga). The results suggest significant redox fluctuations in the shallow seawater during the Mesoproterozoic. However, these pulsed oxygenation episodes may not have been sufficient to sustain the continuous evolution of early eukaryotes.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"49 7","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141654249","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Mantle structure, anisotropy, and dynamics of the Mendocino Triple Junction, northern California, USA","authors":"Ziqiang Yang, Dapeng Zhao, Yunpeng Dong, B. Cheng","doi":"10.1130/b37367.1","DOIUrl":"https://doi.org/10.1130/b37367.1","url":null,"abstract":"We determined a detailed 3-D model of P-wave anisotropic tomography of the crust and upper mantle beneath the Mendocino Triple Junction (northern California, USA) by using a large number of high-quality P-wave travel-time data of local and teleseismic events recorded by the dense seismic network on the North American continent. Our results show the presence of a slab window and the upwelling asthenosphere material through the slab window directly contacts the surrounding plates. A high-velocity anomaly is revealed at depths of ∼300−400 km beneath the triple junction, which reflects a fragment of the Monterey microplate associated with breaking-off of the Farallon plate. Depth-varying seismic anisotropy occurs in the upper mantle and exhibits a circular pattern, which is mainly caused by 3-D mantle flow around the southern edge of the Gorda plate. Subduction-driven corner flow appears in the mantle wedge. After the Farallon plate first contacted the North American plate, as it broke apart and a slab window formed, the Pacific plate captured the plate remnant and moved northwestward together. Then the Monterey plate fractured and sank into the mantle, where it was subjected to shearing forces from both the Pacific and North American plates and moved to its current position.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"40 24","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141655022","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The Mount Distin assemblage: Neoproterozoic metasedimentary rocks in the Nome Complex of Seward Peninsula, Alaska, USA","authors":"Elizabeth L. Miller, Travis L. Hudson, J. Amato","doi":"10.1130/b37430.1","DOIUrl":"https://doi.org/10.1130/b37430.1","url":null,"abstract":"Proterozoic to Paleozoic rocks involved in Jurassic and earliest Cretaceous (Brookian) orogenesis and subsequently extended, metamorphosed at high temperature and intruded by Cretaceous plutons, are widespread on the Seward Peninsula, Alaska, USA. These blueschist and greenschist facies metasedimentary and metaigneous rocks make up the highly deformed Nome Complex. We describe and bracket the age of a distinct Precambrian succession in the Nome Complex, the Mount Distin assemblage (informal). The Mount Distin assemblage consists of marble, impure marble, calc-schist, quartz mica schist, and lesser quartzite that was originally a shelfal succession. Metamafic bodies, originally dikes and/or sills, are minor but common components. Three new U-Pb detrital zircon (DZ) samples are characterized by numerous peaks between 1.5 Ga and 1.0 Ga, with minor ages of 1000−900 Ma and 2000−1500 Ma and only 1−2 grains 3.0−2.5 Ga, indicating maximum depositional ages of 1000−900 Ma. A new U-Pb zircon age for an orthogneiss lens in marble is 661 ± 3 Ma like others previously dated in the Nome Complex. Orthogneiss ages provide an upper age limit for the Mount Distin assemblage of ca. 687 Ma, the oldest reported date. Zircons in three other orthogneiss bodies are interpreted as inherited as they have a range of U-Pb ages like those in their country rocks. The Mount Distin assemblage DZ ages are like those from schist-bearing carbonate sequences in the Central and Schist belts of the Brooks Range and inherited zircon populations in Neoproterozoic plutons on Wrangel Island, Russia. The discovery and documentation of the Mount Distin assemblage explains the previously reported enigmatic lack of country rocks to Neoproterozoic orthogneiss bodies, establishes the continuity of older rocks across the entire extent of the Arctic Alaska−Chukotka microplate, and further confirms this microplate’s Baltica affinities.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"100 7","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141683807","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Onverwacht Group, Barberton Greenstone Belt, South Africa: 300 m.y. development of a Paleoarchean stagnant lid","authors":"D. Lowe","doi":"10.1130/b37573.1","DOIUrl":"https://doi.org/10.1130/b37573.1","url":null,"abstract":"The 3.55−3.26 Ga Onverwacht Group, Barberton Greenstone Belt, South Africa and Eswatini, records nearly 300 m.y. of Paleoarchean history dominated by basaltic, komatiitic, and felsic volcanism. It provides the oldest well-preserved record of crustal development and the nature and evolution of associated surface environments, ocean, atmosphere, and biosphere on early Earth. Sedimentary layers within this 10- to 12-km-thick sequence represent a range of proximal to distal sedimentary environments relative to the centers of volcanism. Proximal deposits include coarse felsic breccias, conglomerates, and sandstones and mafic to komatiitic lapillistones that often show evidence for deposition in relatively shallow water. Distal deposits are composed of fine pyroclastic debris, chemical sediments, and biogenic materials deposited under subaqueous conditions during local volcanic quiescence. They show abundant current-produced features and are interpreted to have formed at water depths of a few hundred meters or less under the influence of tidal and/or ocean-circulation currents. Terrigenous clastic sediments formed by the weathering and erosion of older rocks are essentially absent. The Onverwacht Group was deposited under marine conditions on what appears to have been a water world with little evidence of large land areas and no evidence of active tectonism. It is interpreted to represent the upper part of a Paleoarchean stagnant lid overlying but decoupled from an active mantle. The Barberton Greenstone Belt and other Paleoarchean terranes offer a view of this lid over the past 300 m.y. of its development and during the early stages of its fragmentation and disruption.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"17 S3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141681552","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Assessing the lateral and vertical variability of shelf-margin depositional systems and associated forcing mechanisms: A forward modeling approach","authors":"John W. Shepherd, V. Paumard, Tristan Salles, Simon Lang, Annette D. George","doi":"10.1130/b37598.1","DOIUrl":"https://doi.org/10.1130/b37598.1","url":null,"abstract":"Understanding the primary drivers of lateral and vertical variability in the stratal architecture of shelf-margin settings is key to understanding how sediments are partitioned from the shelf to the slope and the basin floor in source-to-sink systems. In this study, we model the 4-D evolution of a shelf margin over a period of 18.5 m.y. using Badlands stratigraphic forward modeling software. The modeled system is analogous to the Hammerhead shelf margin developed in the Bight Basin (southern Australian margin) during the Late Cretaceous, with forcing parameters interpreted from “real world” 3-D seismic data. A series of seven models were designed and tested to investigate potential drivers of shelf-margin variability, which include shoreline process regime (i.e., fluvial, wave, or mixed coastal processes), uplift, rainfall, and source area extent. We find that shoreline processes, which in the context of this study include fluvial and wave processes, may significantly impact shelf-margin architecture although they are less likely to affect the long-term evolution of a shelf margin. The addition of either fluvial or wave processes increases along-strike lateral variability with mixed-process shorelines resulting in the most variability. We propose that these hydrodynamic processes affect sediment supply locally leading to “out-of-phase” supply influencing both shelf-margin architecture and the character of sequence stratigraphic surfaces laterally. Rainfall is also shown to have a much more immediate effect on shelf-margin architecture compared to changes in tectonics (uplift). The results of this study are particularly applicable to the Hammerhead shelf margin and may also be applied to other shelf margins where eustasy is not the primary control on shelf-margin architecture and/or paleoclimatic conditions are poorly constrained.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"32 3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141695270","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Constraints on the subduction and closure of the Hegenshan Ocean: Magmatic and sedimentary records from central Inner Mongolia, China","authors":"Jianzhou Tang, Zhicheng Zhang, Gaoxue Yang, Ke Li, Yan Chen, Cong Ding, Z. Ji, Qi Wang","doi":"10.1130/b37345.1","DOIUrl":"https://doi.org/10.1130/b37345.1","url":null,"abstract":"Petrological, geochronological, and geochemical data from the volcano-sedimentary sequences, granitoids, and ophiolite relics of central Inner Mongolia, China, were used to reconstruct the subduction and final closure of the Hegenshan Ocean. Geochronological dating and compilation reveal four phases (ca. 360−355 Ma, 348−320 Ma, 320−310 Ma, and 310−275 Ma) of magmatism in the Uliastai continental margin. The ca. 356 Ma I-type Halatumiao granodiorite and Amanwusu ophiolite relics are subduction-related, and the Halatumiao granodiorite provides solid evidence of the northward subduction of the Hegenshan Ocean beneath the Uliastai continental margin at ca. 360−355 Ma. The ca. 348−320 Ma and 320−310 Ma volcanic rocks and granitoids constitute two linear magmatic belts roughly parallel to the Erenhot-Hegenshan ophiolite belt, which record two phases of continental arc magmatism in the Uliastai continental margin. Overall, the ca. 360−310 Ma arc magmatism shows landward migration and then oceanward migration in the Uliastai continental margin, which indicates advancing subduction and subsequent slab steepening of the Hegenshan Ocean. By contrast, the ca. 310−275 Ma magmatic rocks are dominated by I- and A-type felsic volcanic rocks, granites, and dikes, which are post-accretionary, extension-related, and pervasive in the Uliastai continental margin and Erenhot-Hegenshan ophiolite belt. A provenance shift was identified between the Benbatu and Amushan formations of the Amanwusu area of the Erenhot-Hegenshan ophiolite belt. The early detritus was derived from the early Paleozoic rocks in the Sonid Zuoqi arc belt, whereas the late detritus originated from the Early Carboniferous ophiolite relics in the Erenhot-Hegenshan ophiolite belt. The provenance shift and emplacement of pervasive extension-related magmatic rocks imply a Late Carboniferous closure of the Hegenshan Ocean. The Late Carboniferous oceanic closure event in the north of the southeast Central Asian Orogenic Belt is also evidenced by the transition of Hf isotopic composition of zircons dated between ca. 360−310 Ma and 310−275 Ma.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"4 6","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141698905","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Structural and metamorphic architecture of the Zanskar Himalaya, Suru Valley region, NW India: Implications for the evolution of the Himalayan metamorphic core","authors":"I.P. Cawood, M. St-Onge, O. Weller, M. P. Searle, D. Waters, T. Ahmad","doi":"10.1130/b37241.1","DOIUrl":"https://doi.org/10.1130/b37241.1","url":null,"abstract":"New 1:50,000-scale geological mapping in the Zanskar Himalaya of NW India, covering 2,400 km2, is integrated with structural and petrographic analysis to document the evolution and key tectonometamorphic relationships within the Himalayan metamorphic core. The integrated dataset constrains the regional three-dimensional geology and relationships between lithostratigraphy, folds, faults, deformation fabrics, metamorphic isograds, and growth of porphyroblasts within the context of five main deformation phases.\u0000 Following the initial collision of India and Asia, NW−SE-oriented deformation is recorded by D1 (greenschist-facies) fabrics and D2 (greenschist- to amphibolite-facies) fabrics. D2 represents the main tectonometamorphic deformation phase associated with crustal thickening and produced the dominant regional penetrative fabric through crenulation and transposition of D1 fabrics. Thrust-sense D2 fabrics were reactivated during D3 as the Greater Himalayan Sequence was exhumed along the normal-sense Zanskar Shear Zone, which is part of the South Tibetan Detachment System. D3 fabrics, associated with movement on the Zanskar Shear Zone, were temporally continuous with crenulation and mesoscale folding, recording progressive kilometer-scale backfolding and backthrusting toward the NE between the Greater Himalayan Sequence−Tethyan Himalayan Sequence and the adjacent Indus Suture Zone. Finally, D4 and D5 are recorded as kilometer-scale open folding of older planar and linear structures.\u0000 The orientation of mineral isograd surfaces ranges from subparallel to oblique with respect to D2 planar structural elements. The growth of pelitic and metabasic peak metamorphic phases from greenschist to upper-amphibolite facies is synchronous with or postdates D2 fabrics. D3 fabrics wrap thermal peak porphyroblasts and realign linear mineral phases. Tectonic thinning adjacent to D3 normal faults is documented by reduced structural spacing of isograds and alignment of isograd surfaces parallel to the faults. D4 and D5 structures modify the trace of all regional metamorphic isograds. Collectively, these observations imply that the thermal peak of metamorphism was reached after the main phase of deformation (D2), and predated movement on the Zanskar Shear Zone (D3). The results document numerous classical elements of collisional orogenesis, including implied clockwise P-T paths, polyphase deformation, and a complete Barrovian metamorphic isograd sequence supplemented by complementary metabasic isograds. The Zanskar Himalaya, unlike other areas of the Himalayan metamorphic core, records metamorphic conditions primarily attained following substantial crustal thickening rather than during subsequent decompression and exhumation. The reduced expression and/or discontinuous nature of exhuming fault systems, which produces variable levels of crustal exposure, may account for this lateral heterogeneity across the mountain belt. Deciphering the complex kinematics of continent","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"48 48","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-06-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141339484","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}