Tsangpo Suture Research Articles

Petrogenesis of the Qiongduojiang Gabbro in Tethys Himalaya and Its Metallogenetic Implication

With the northward subduction and final closure of the Neo-Tethyan oceanic crust, the Indian and Eurasian plates finally collided together and underwent a strong collision orogenic event, resulting in large-scale crust–mantle magmatic interactions. In order to clarify the controversies about tectono-magmatic activities after the Indian–Eurasian continental collision, we report the newly dated Eocene Qiongduojiang gabbro explored in the Tethyan–Himalaya belt, southern Tibet. LA-ICP-MS zircon U-Pb dating shows that the crystallization age of the Qiongduojiang gabbro is 46.1 ± 1.7 Ma. The whole-rock major and trace elements, as well as Rb-Sr, Sm-Nd, and Pb isotopic data results, show that the Qiongduojiang gabbro is apparently depleted in Nd isotopes, is enriched in Pb isotopes, and has maintained a consistent 87Sr/86Sr(t) value. This paper argues that the E-MORB-like Qiongduojiang gabbro originated from asthenosphere upwelling caused by slab breakoff of the Neo-Tethyan oceanic plate. This event caused large-scale magmatic activities, a magmatic mixing process between ancient crust and deep mantle, and wild distribution of Eocene Gangdese plutons along the Yarlung–Tsangpo Suture Zone, and it rendered the subduction-modified Tibetan lithosphere fertile from the Gangdese porphyry Cu deposits.

Early Cretaceous monsoonal upwelling along the northern margin of the Gondwana continent: Evidence from radiolarian cherts

Radiolarian-bearing cherts are widely exposed in the volcanic-sedimentary sequences associated with the Purang ophiolitic mélange in the western segment of the Yarlung Tsangpo suture zone. Highly diverse and well preserved Lower Cretaceous (Valanginian–Hauterivian) radiolarian assemblages that correlate to the Cecrops septemporatus Zone occur in bedded cherts in the Peiersanggangmu section of the southern Purang ophiolitic mélange. Fifty-five species belonging to 38 genera, including characteristic species: Archaeodictyomitra apiarium (Rüst), Cecrops septemporatus (Parona), Hemicryptocapsa capita Tan, Pseudodictyomitra carpatica (Lozyniak), and Svinitzium pseudopuga Dumitrica, have been recognized.Geochemistry of the cherts reveals high SiO2 contents (average: 92.23 wt%), Al/(Al + Fe + Mn) ratios, and low LaN/CeN ratios. Post Archean Australian Shale (PAAS) normalized rare earth element (REE) abundance patterns exhibit moderately positive Ce anomalies (mean Ce/Ce* =1.35) with relative enrichment in light rare earth elements (LREEs). These features are indicative of deposition in an oceanic basin close to a continental margin environment. Lithological characteristics and geochemical results suggest that the paleogeographical location for these radiolarian-bearing bedded cherts closely matches northern regions of the Indian continent, where existed enhanced upwelling activity and surface mixing resulting from monsoon activities.

Genesis of the Abunabu antimony deposits in the Tethys Himalayan metallogenic belt: Evidence from He–Ar and S isotopes of stibnite

Introduction: The Abunabu antimony mining area is located between the Indus–Yarlung Tsangpo suture and the southern Tibetan detachment system. Ore deposits in the mining area provide an excellent opportunity to understand the nature and genesis of antimony mineralisation in the Tethys Himalayan metallogenic belt.Methods: In this study, we analysed the He–Ar and S isotopic compositions of stibnite-hosted fluid inclusions as a basis for investigating the sources of ore-forming fluids in the Abunabu mining area and the Tethys Himalayan metallogenic belt.Results: The analysed stibnites have 4He contents of 0.016 × 10−7–1.584 × 10–7 cm3 STP/g, 40Ar contents of 1.37 × 10−7–2.94 × 10–7 cm3 STP/g, 40Ar/36Ar ratios of 303.8–320.7, and 3He/4He (Ra) ratios of 0.021–0.351. These isotopic features indicate that the ore-forming fluids were primarily metamorphic fluids of crustal origin, with small amounts of magmatic-derived materials and modified air-saturated water with low 40Ar*/4He ratios. The δ34S values of stibnite vary within a narrow range of −4.9‰ to −3.5‰, with a mean value of −4.31‰, indicating a deep magmatic origin.Discussion: On the basis of these results and a compilation of data for sulphide deposits in the metallogenic belt, we infer that compositional variations in the He and Ar isotopes of the ore-forming fluids of each antimony deposit in the Tethys Himalayan metallogenic belt are independent of each other. This suggests that antimony deposits in the belt had similar ore-forming fluid sources and mixing processes and that differences in the metallogenic tectonic setting within the belt emerged only in the later stages of deposit evolution. Our new results and compiled data also show that antimony–gold deposits and lead–zinc–antimony polymetallic deposits in the Tethys Himalayan metallogenic belt differ in their sulphur isotopic compositions and that multiple sulphur sources were involved in each of these types of deposit.

Magnetic susceptibility, mineral chemistry, and geothermobarometry of granitoids from Lohit Plutonic Complex, Arunachal Trans‐Himalaya, Northeast India: Implications on emplacement and crystallization conditions of oxidized calc alkaline magmatic arc system

AbstractThe Lohit Plutonic Complex (LPC) of Arunachal Trans‐Himalaya represents the northeast extension of Trans‐Himalayan magmatic arc system located in the north of Indus Tsangpo Suture Zone (ITSZ). Field relation, magnetic susceptibility (MS), and phase petrology on the granitoids of LPC was conducted in order to assess the granite series (magnetite, oxidized vs. ilmenite, reduced types), and physico‐chemical conditions of the LPC granitoid magmas. The studied granitoids are well‐exposed in the Dibang and Lohit valleys, and their MS values indicate a bi‐modal patterns corresponding to ilmenite (reduced) series (71%) and magnetite (oxidized) series (29%) granites. The variation of MS in the LPC granitoids is related to the alteration of ferromagnetic minerals, and later tectonic and deformational processes that acted upon them. The amphiboles from the LPC granitoids are calcic (CaB >1.5, Si = 6.30–7.06 apfu) and exhibit tschermak substitutions typical to their evolution in a calc alkaline, metaluminous (I‐type) felsic magmas. Al‐in‐hornblende rims estimate the emplacement of quartz diorite and granodiorite magmas at shallow (~5 km) and mid (~16 km) crustal depths. Geothermometric results point to a regime of magmatic crystallization (940–837°C for quartz diorite; 882–829°C for granodiorite) sufficiently above the solidus of respective melts. Biotites from LPC granitoids are primary to re‐equilibrated, and transitional between magnesio‐ and ferri‐biotites. Quartz diorite and granodiorite biotites evolved under oxidizing magmas (log ƒO2−14 to log ƒO2−13) in a temperature range of ~750–950°C, typical to their formation in a calc alkaline magma of subduction zone environment. However, the biotites from leucogranite appear to have evolved under a mildly reducing magma environment, most likely attained in a collisional setting. The obtained results suggest that the oxidized nature of calc alkaline, subduction‐related magmatic arc rocks of the LPC is largely modified and reduced by post‐magmatic, and later tectonothermal and deformational events that operated during Himalayan and Trans‐Himalayan orogenesis.

Cenozoic kinematic histories of the Tidding and Lohit thrusts in the northern Indo-Burma Ranges: Implications for crustal thickening and exhumation of Gangdese lower arc crust along the Indus-Yarlung suture zone

Crustal thickening has been a key process of collision-induced Cenozoic deformation along the Indus-Yarlung suture zone, yet the timing, geometric relationships, and along-strike continuities of major thrusts, such as the Great Counter thrust and Gangdese thrust, remain inadequately understood. In this study, we present findings of geologic mapping and thermo- and geochronologic, geochemical, microstructural, and geothermobarometric analyses from the easternmost Indus-Yarlung suture zone exposed in the northern Indo-Burma Ranges. Specifically, we investigate the Lohit and Tidding thrust shear zones and their respective hanging wall rocks of the Lohit Plutonic Complex and Tidding and Mayodia mélange complexes. Field observations are consistent with ductile deformation concentrated along the top-to-the-south Tidding thrust shear zone, which is in contrast to the top-to-the-north Great Counter thrust at the same structural position to the west. Upper amphibolite-facies metamorphism of mélange rocks at ∼9−10 kbar (∼34−39 km) occurred prior to ca. 36−30 Ma exhumation during slip along the Tidding thrust shear zone. To the north, the ∼5-km-wide Lohit thrust shear zone has a subvertical geometry and north-side-up kinematics. Cretaceous arc granitoids of the Lohit Plutonic Complex were emplaced at ∼32−40 km depth in crust estimated to be ∼38−52 km thick at that time. These rocks cooled from ca. 25 Ma to 10 Ma due to slip along the Lohit thrust shear zone. We demonstrate that the Lohit thrust shear zone, Gangdese thrust, and Yarlung-Tsangpo Canyon thrust have comparable hanging wall and footwall rocks, structural geometries, kinematics, and timing. Based on these similarities, we interpret that these thrusts formed segments of a laterally continuous thrust system, which served as the preeminent crustal thickening structure along the Neotethys-southern Lhasa terrane margin and exhumed Gangdese lower arc crust in Oligocene−Miocene time.

Asthenospheric mantle metasomatized by subducted marine sediments: Li isotopic evidence from Dagze mafic rocks, southern Tibet

Lithium (Li) isotopes show large isotopic fractionation during low temperature processes and have been widely used to trace subduction components at convergent margins. To identify recycled subducting components in the asthenospheric mantle, we report the lithium concentrations and isotopic compositions of the ca. 90 Ma Dagze mafic rocks from the eastern Gangdese Arc, southern Tibet, along with their corresponding whole-rock analyses and Sr-Nd-Pb isotope data. They mainly comprise basalts and diabases with high MgO (7.2–8.3 wt%) and low K 2 O (0.02–0.68 wt%) contents. These rocks show slightly variable initial 87 Sr/ 86 Sr ratios (0.7048 to 0.7057), relatively low and homogeneous Pb isotope ratios ( 206 Pb/ 204 Pb = 18.2948–18.6462, 207 Pb/ 204 Pb = 15.5587–15.6132, 208 Pb/ 204 Pb = 38.3843–38.8352) and positive ε Nd (t) values (3.0–4.9), similar to ophiolites from the Indus-Yarlung Tsangpo suture (IYTS). However, they are remarkably depleted in high field strength elements (HFSE; such as Nb and Ta) and slightly enriched in light rare earth elements (LREE) [(La/Yb) N = 2.8–4.4], both of which imply their mantle affinities with distinctive arc signatures. The δ 7 Li values of these rocks range from −2.0 to +2.0‰ (average = −0.7‰), with the majority of samples falling outside the depleted mantle range (+3.4 ± 1.4‰). They exhibit high Li contents (13.2 to 34.1 ppm) and Li/Y ratios (0.8 to 2.2), which are comparable with marine sediments. The negative correlation between Li/Y ratios and δ 7 Li values also suggests the addition of subducted marine sediments. The modeling of δ 7 Li values in combination with Pb-Sr-Nd isotopic ratios suggests that ~1 to 5% of marine sediments have been added into the mantle source of the Dagze mafic rocks. Combined with a previous study, we suggest that the Dagze mafic rocks were derived from the upwelling asthenospheric mantle metasomatized by subducted marine sediments. • Reporting the negative δ 7 Li values of the Dagze mafic rocks. • Demonstrating the lithium isotopic heterogeneities in the asthenospheric mantle. • Modeling ~1 to 5% of marine sediments to the mantle source of the Dagze mafic rocks.

Subduction Evolution Controlled Himalayan Orogenesis: Implications from 3-D Subduction Modeling

Himalayan orogenesis remains enigmatic in terms of Tibetan Plateau geodynamics originating from the Cenozoic India–Eurasian continental collision. India underthrusts below Tibet to the Yarlung–Tsangpo suture, which has been identified as the northernmost boundary for underplating. However, the way in which the historical evolution of continental subduction induces plateau uplift and the way it controls the variation in uplift between outboard and inboard areas is still unclear. To interpret the evolutionary mechanisms involved in the Himalayan growth history, we constructed different 3-D dynamic models at important stages to address these questions related to the formation of the Himalayas on the basis of paleoenthalpy evidence encoded in fossil leaves from recently documented assemblages in southern Tibet. The results show that (1) the effect of crustal thickening was the predominant factor in the early evolution from the Paleocene to the early Eocene, which resulted in a moderate growth rate. (2) The consecutive slab break-off eastward from the western syntaxis and the associated slab rebound significantly accelerated orogenesis from the late Eocene to the Oligocene. The upwelling asthenospheric flow was a key control of increasing crustal buoyancy, which resulted in the fastest growth of the Himalayas during the early Miocene. (3) Thereafter, the gradually enhanced monsoon and surface erosion during accompanying the increasing mountain height resulted in a slowdown of the orogenic rate, which counterbalanced the buoyant force produced by asthenospheric flow driving continuous Himalayan growth.

Provenance analysis of Cenozoic conglomerates: Insights from Lower Cretaceous radiolarians in chert clasts from the Xigaze area, southern Tibet

Conglomerates, exposed on either side of the Yarlung Tsangpo suture zone (YTSZ) in southern Tibet, have attracted wide attention in elucidating uplift and erosion histories of the Himalayan-Tibetan orogen. However, the provenance of these conglomerates remains controversial. Although radiolarian-bearing chert clasts within these conglomerates have received little focus, identification of the radiolarian assemblages they contain could shed light on the provenance of these sedimentary units. We present the first report of Upper Jurassic to Lower Cretaceous (upper Kimmeridgian–lower Barremian) radiolarian assemblages recovered from chert clasts within the Liuqu and Gangrinboche conglomerates in the Xigaze area. To extract radiolarian fossils from independent clasts in the conglomerates, a detailed and efficient experimental process is illustrated. The assemblages are well correlated to those in the YTSZ and Tethyan Himalaya, showing typical Tethyan characteristics. The lithology of chert clasts and ages of constituent radiolarian faunas suggest that the chert clasts in Liuqu and Gangrinboche conglomerates were derived from the Bainang terrane. This interpretation implies that Early Cretaceous accretionary complexes in the YTSZ had been exposed and eroded before deposited as clasts in the Cenozoic Liuqu and Gangrinboche conglomerates.

Early Cretaceous back‐arc basin basalt‐type gabbros in the southeastern Tibetan Plateau: Implications for Neo‐Tethyan oceanic slab subduction

Although progress has been made in defining and explaining the temporal and spatial distributions of Neo‐Tethyan magmatism in the Tibet–Himalaya region, the correlation of magmatic suites in the SE Tibetan Plateau remains poorly constrained. This paper reports zircon U–Pb dating, geochemistry, and Sr–Nd isotope compositions of a concealed gabbroic unit (coverage area of >6.0 km2) in the southern Lancangjiang tectonic zone, western Yunnan, SW China. Zircon 206Pb/238U ages of 143–133 Ma indicate that the unit formed during the Early Cretaceous (ca. 139.7 Ma). The gabbro samples have variable MgO (7.88–16.1 wt%; Mg# = 58–74) and relatively low TiO2 (0.76–1.24 wt%) and total‐alkali (K2O + Na2O = 2.61–4.06 wt%) contents. The samples exhibit weakly fractionated rare earth element (REE) patterns with slight depletion in light REEs and negligible Eu anomalies. In a primitive‐mantle‐normalized element‐variation diagram, the gabbros, display variable enrichment in large‐ion lithophile elements and relatively flat patterns of high‐field‐strength elements. Initial 87Sr/86Sr ratios of 0.7079 to 0.7142 and relatively depleted εNd(t) values of +3.96 to +4.84 are similar to those of the mantle source of back‐arc basin basalts (BABBs) in the central Lhasa Block and mid‐ocean ridge basalt in the Indus–Tsangpo Suture Zone. These various characteristics suggest that the gabbroic unit was derived from a shallow and depleted mantle source by relatively high‐degree partial melting in the spinel stability field. The parental magma underwent slight crustal contamination, as well as fractional crystallization of clinopyroxene. The studied gabbros have distinct BABB‐type geochemical affinities similar to those of the Okinawa BABB. Combining our data with the coeval occurrence of a continental rift basin (the Simao–Lanping Basin) leads us to suggest the existence of a back‐arc rifting setting in the SE Tibetan Plateau during the Jurassic–Cretaceous. This setting can also be geochronologically and genetically correlated to the back‐arc basin developed within the Lhasa Block. These settings and constituent characteristics formed as a result of Neo‐Tethyan oceanic slab subduction during the late Mesozoic.

Geochemical analysis of magmatic rocks from Shyok Suture Zone (SSZ) Trans-Himalaya, NW India: Insights for geodynamic evolution of the terrane

The present study discusses the geochemistry of the Early Cretaceous Shyok and Nubra volcanics from the Shyok Suture Zone (SSZ), Trans-Himalaya. Geochemically, the studied volcanic rocks belong to calc-alkaline magma series and vary in composition from basalt-basaltic andesite-andesite-dacite. The SSZ rocks are enriched in light rare earth elements (LREE: La, Ce, Nd) and large ion lithophile elements (LILE: Rb, Ba, U, K, Pb and Sr) while depleted in high field strength elements (HFSE: Nb, P and Ti), analogous to subduction zone magmatism. Their perturbed LILE concentrations coupled with higher 87Sr/86Sr ratios reflect possible influence of post-crystallization processes. Trace element modelling reflects variable degrees of partial melting (~5–30%) of a metasomatised spinel peridotite mantle source followed by fractional crystallization of clinopyroxene and amphibole along with olivine and plagioclase. The Shyok and Nubra volcanics depict positive εNd(t = 110 Ma) values (+2.3 to +5.2), slightly lower than the contemporary Chalt fore-arc boninites (+6.3 to +8.0) and adjoining intra-oceanic island arc rocks along the Indus Tsangpo Suture Zone (ITSZ) such as Nidar ophiolite (+6.8 to +8.7), Shergol gabbros (+5.1 to +8.4) and Dras volcanics (+5.8 to +7.0), therefore reflecting depleted mantle characteristics. In contrast to the ITSZ rocks, the Shyok and Nubra volcanics have enriched LREE-LILE characteristics, indicating their derivation from isotopically depleted and elementally (LREE-LILE) enriched mantle wedge sources. Thus, based on the geochemical and isotopic signatures of the SSZ rocks and their association with continentally derived sediments, we suggest an Andean-type arc system as opposed to ITSZ rocks and associated oceanic sediments, representing intra-oceanic arc system.

Discovery of an Entrapped Early Permian (ca. 299 Ma) Peri-Gondwanic Sliver in the Cretaceous Shyok Suture of Northern Ladakh, India: Diverse Implications

GSA Logo HomeJoinGSA StoreDonateContact Toggle search Toggle navigation Sign In Toggle Search Advanced Search HomeJoinGSA StoreDonateContact AboutToggle Scientific DivisionsGeographic SectionsLeadershipCommitteesHonors & AwardsWho We AreDiversityEthicsGSA InternationalGSA Foundation MembershipToggle Join / RenewBenefits and DuesGet InvolvedCampus RepsMember RecognitionCode of Ethics PublicationsToggle JournalsGSA TodayBooksGSA StoreInfo & ServicesGeofacetsMeeting Abstracts EventsToggle Annual MeetingFuture Annual MeetingsSection MeetingsPast Meetings & AbstractsPenrose ConferencesThompson Field ForumsField ExperiencesGeoscience CalendarEvents Code of Conduct Education & CareersToggle GeoCareersGeoscience Job BoardGrants & ScholarshipsField ExperiencesNetworking/MentoringK-12 Educator SupportWebinar LibraryGeologic Time Scale Science PolicyToggle News from The HillGSA Policy RolesPosition StatementsCritical IssuesCongressional Science FellowshipScience Policy FellowshipEngage with Policymakers NewsToggle Science Communication FellowshipScience Communication InternshipNews ReleasesNewsroom (for Journalists)GSA In The NewsAnnual Meeting NewsroomSpeaking of Geoscience BlogGSA ConnectionGeoScene Newsletter Keyword Search Home » Publications » GSA Today Abstract View GSA Today Full Text | PDF Discovery of an Entrapped Early Permian (ca. 299 Ma) Peri-Gondwanic Sliver in the Cretaceous Shyok Suture of Northern Ladakh, India: Diverse Implications Rajeev Upadhyay Dept. of Geology (CAS), Kumaun University, Nainital-263001 (Uttarakhand), India, rajeevntl1@gmail.com Saurabh Gautam Birbal Sahni Institute of Palaeosciences, 53 University Road, Lucknow-226007, India Ram Awatar 7/464 A, Vikas Nagar, Lucknow-226022, India Abstract In a significant breakthrough, we report the first discovery of twenty-six genera and thirty-five species of Early Permian (Asselian–Sakmarian and Artinskian; 299 Ma to 276 Ma) Gondwanic palynomorphs from a tectonically emplaced metasedimentary sliver of Shyok Ophiolitic Mélange of the India-Asia Collision zone of Northern Ladakh, India. These palynofloral assemblages are of peri-Gondwanian (Cimmerian) origin and have a strong affinity with the Gondwana assemblage of peninsular India. Similar palynofloral assemblages are also known from Extra-Peninsular India, Salt Range, Karakoram, Antarctica, Australia, South Africa, and South America. The occurrence of Gondwanic sliver within the Shyok Suture is interpreted as a thin flake of active continental margin of peri-Gondwanic microcontinent/Kshiroda plate, which was sliced off during the subduction/collision process, between Ladakh block and Karakoram–Qiangtang-Lhasa terrane and amalgamated with obducted remnants of the accretionary prism of the nascent Shyok Suture. The Shyok Suture closed during the mid- to Late Cretaceous period. Subsequent syn- and post-collision synkinematic episodes tectonically juxtaposed the peri-Gondwanic sliver in the tectonized zone of Shyok Ophiolitic Mélange. The India-Asia collision, which took place ca. 60–50 Ma with the demise of Neo-Tethys Ocean, along the Indus Tsangpo Suture Zone, modified the geometry of accreted ophiolitic stack of the Shyok Suture. Manuscript received 16 Aug. 2020. Revised manuscript received 23 Sept. 2021. Manuscript accepted 27 Oct. 2021. Posted 19 Nov. 2021. © The Geological Society of America, 2021. CC-BY-NC. https://doi.org/10.1130/GSATG481A.1 Search Google Scholar for Rajeev Upadhyay Saurabh Gautam Ram Awatar Search GSA Today Publications Journals Geology GSA Bulletin Geosphere Lithosphere Environmental & Engineering Geoscience GSA Today Full Issue Archive Features Info for Authors Advertising Geoscience Job Board Books Special Papers Field Guides Memoirs Reviews in Engineering Geology DNAG Digital Maps Map & Chart Series Memorials GSA Store Info & Services About Our Publications Editors Author Info Manuscript Submission Data Repository Ethical Guidelines Copyright Subscriptions How to Access Publications Advertising Data Policy Open Access Geofacets Meeting Abstracts Facebook Twitter Instagram YouTube LinkedIn Wordpress HomeAdvertisingFAQsSearchPrivacy PolicyJobs at GSA © 2021 The Geological Society of America, Inc.

Detrital Zircon Provenance of the Cenozoic Sequence, Kotli, Northwestern Himalaya, Pakistan; Implications for India–Asia Collision

This study reported the detrital zircon U-Pb geochronology of the Cenozoic sequence exposed in Kotli, northwestern Himalaya, Pakistan, which forms part of the Kashmir foreland basin. The U-Pb detrital age patterns of the Paleocene Patala Formation show a major age cluster between ~130–290 Ma, ~500–1000 Ma and ~1000–1500 Ma, which mainly resembles the lesser and higher Himalayan sequence. However, the younger age pattern (~130–290 Ma) can be matched to the ages of the ophiolites exposed along the Indus–Tsangpo suture zone. In addition, two younger grains with 57 Ma and 55 Ma ages may indicate a contribution from the Kohistan-Ladakh arc. The detrital zircons in the upper Tertiary sequence show the increased input of younger detrital ages <100 Ma, with more pronounced peaks at ~36–58 Ma, ~72–94 Ma and ~102–166 Ma, indicating the strong resemblance to the Asian sources including the Kohistan–Ladakh arc, Karakoram block and Gangdese batholith. This provenance shift, recorded in the upper portion of Patala Formation and becoming more visible in the upper Tertiary clastic sequence (Kuldana and Murree formations), is related to the collision of the Indian and Asian plates in the northwestern Himalayas. Considering the age of the Patala Formation, we suggest that the Indian and Asian plates collided during 57–55 Ma in the northwestern Himalayas, Pakistan.

Warm and humid Trans-Himalaya during the late Miocene: plant fossil evidence

Neogene fossil records from the Indus Basin sedimentary rocks (IBSR), deposited in the Indus Tsangpo Suture Zone (ITSZ), are very rare, but are important to understand the history of plant diversity and paleoclimate in the Himalaya. We report fossil wood ascribed to Ebenoxylon siwalicus Prakash from late Miocene sediments of the Karit Formation belonging to ITSZ. The anatomical details of the fossil wood, such as small to medium-sized vessels occluded with tyloses, scanty paratracheal to diffuse-in-aggregate axial parenchyma, 1–3 seriate homo to heterocellular rays, bordered intervessel pits with lenticular apertures and simple perforations, suggest its close affinity with Diospyros Linnaeus of the family Ebenaceae. Further anatomical details suggest a close resemblance with extant D. ehretioides Don and D. macrophylla Blume. The present fossil, along with previously known fossil records of Lagerstroemia (Lythraceae) and palms, indicate that the Trans-Himalaya was warm and humid during the late Miocene, quite different from the modern cool and dry climate in the study area.

Middle Jurassic radiolarians and chert geochemistry, Dajiweng ophiolite, SW Tibet: Implications for Neotethyan Ocean evolution

Well-preserved Middle Jurassic (upper Bajocian to lower Bathonian) radiolarian assemblages from bedded cherts constrain the age of cherts associated with the Dajiweng ophiolite in the northern sub-belt of the western section of the Yarlung Tsangpo suture zone (YTSZ). Sixty-one species belonging to 39 genera are recognized and illustrated. They represent the most diverse Middle Jurassic radiolarian fauna yet documented in this ophiolite and support correlation with UA zones 4–5 of Baumgartner et al. (1995). They are thus, the oldest known radiolarian-bearing strata in the northern sub-belt of the western YTSZ. The fauna exhibits a close degree of similarity to the other coeval assemblages previously reported from middle and eastern segments of the YTSZ, reveal widespread development of Middle Jurassic or even earlier oceanic crust within the Neotethyan Ocean. The geochemistry of these bedded cherts, in particular ratios of Al/(Al + Fe + Mn), Ce/Ce*, and LaN/YbN indicate they accumulated in a transitional environment between a continental margin and an oceanic basin.

Direct dating of the Sinongduo thrust system in southern Tibet: immediate response to India-Asia collision

ABSTRACT The Tibetan Plateau is a product of the continental collision between the Indian and Asian plates along the Indus-Yarlung suture zone that started in the early Cenozoic. This collision initially produced stress at the collisional boundary and later propagated with time on both the Indian plate and Asian plate. Many studies have reported tectonic deformation in the Himalayas representing the immediate response of continental collision on the Indian side near the collisional boundary. However, there have been only few studies on the early Cenozoic shortening structures in the Lhasa block that forms the southernmost part of the Asian plate. In this study, we performed Ar/Ar geochronology of syntectonic muscovite from the Sinongduo thrust system in the central Lhasa block. The results show that the Sinongduo thrust system was active in the Early Eocene. We interpret the Sinongduo thrust system as an immediate response to the India-Asia collision in the Lhasa block. This study reconciles the early Cenozoic India-Asia collision with the lack of tectonic response in the Lhasa block.

Tectonic framework of the high‐pressure metamorphic rocks of the Nagaland Ophiolite Complex, North‐east India, and its geodynamic significance: A review

In this study, we made a review of the published metamorphic and geochronological data of an array of high‐pressure metamorphic rocks from the Nagaland Ophiolite Complex, North‐east India, and present them in the tectonic framework of two temporally separate subduction systems within the eastern arm of the Neo‐Tethys. The metamorphic rocks are studied in terms of three structural units. The structurally lowermost Mokie–Satuza–Thewati (MST) unit shows a metamorphic zonation from greenschist through pumpellyite–diopside (TMax ~ 335°C at ~6 kbar) to lawsonite blueschist (TMax ~ 340°C at ~11.5 kbar) to epidote eclogite (TMax ~ 550–660°C at ~24 kbar in Mokie to TMax ~ 630°C at ~26–28 kbar in Thewati localities) facies structurally upward. Both the blueschists and eclogites record a clockwise (CW) P–T path of evolution. We relate the low apparent thermal gradient at the metamorphic peak (7–9°C/km), the steep dP/dT slope of the metamorphic field gradient, the general CW metamorphic P–T paths, and the published U–Pb zircon dates in the eclogites (between ca. 205 and ca. 172 Ma) and radiolarian biostratigraphy data in the MST metamorphic sequence with the development of a Late Triassic to Middle Jurassic‐aged cold mature stage of an intra‐oceanic subduction system within the Neo‐Tethys. This is considered to be one of the oldest in the Yarlung–Tsangpo Suture Zone. The metamorphosed uppermost part of the structurally intermediate unit, named the Moya–Salumi (MS) sequence, consists of a structurally upper hornblende‐eclogite (Hbl‐EC) to epidote amphibolite facies subunit and a lower epidote blueschist (EBS) facies subunit. The eclogite subunit is polyphase metamorphosed (Hbl‐EC facies M1 with TMax ~ 625°C at ~13.8 kbar and garnetiferous EBS facies M2 with TMax ~ 540°C at ~14.4 kbar) with both the events recording counterclockwise P–T paths. The monocyclic EBS facies subunit, on the other hand, has a memory of the M2 cycle of metamorphism only. We relate this combined M1–M2 cycles of metamorphism in the MS sequence to a second oceanic subduction beneath the continental Naga Metamorphics, the latter constituting the structurally uppermost unit. The metamorphic pulses together reflect progressive cooling of the second subduction channel from an initially warm stage during subduction infancy to a later cold mature stage of subduction. The different metamorphic rocks in the MST and MS units reveal architectures of two temporally separate accretionary complexes. Their present occurrence as a tectonic collage is likely to be the product of the Eocene collision of the Indian and the Myanmar plates.

An origin of ultraslow spreading ridges for the Yarlung-Tsangpo ophiolites

As relics of ancient ocean lithosphere, ophiolites are the most important petrological evidence for marking the sutures and also play a key role in reconstructing plate configuration. They also provide valuable windows for studying crustal accretion and mantle processes occurring at modern ocean ridges. Abundant ophiolites are distributed along the Yarlung-Tsangpo suture and represent the relics of ocean lithosphere of the Neo-Tethys. They are characterized by an incomplete litho-stratigraphy, of which the mantle section is much thicker than the crustal section. Ocean crustal rocks outcropped in the Yarlung-Tsangpo ophiolites are much thinner than normal ocean crusts (~ 7 km) or even absent. Tectonic settings from which the Yarlung-Tsangpo ophiolites originated remain highly controversial, although an origin of the supra-subduction zone is prevailing. Moreover, their incomplete litho-stratigraphy has been commonly attributed to tectonic dismemberment during the late-stage emplacement after their formation. Nevertheless, such an incompleteness resembles the ocean lithosphere generated at modern ultraslow spreading ridges, such as the Southwest Indian Ridge (SWIR). In this paper, we present several lines of evidence that support the formation of the Yarlung-Tsangpo ophiolites at ultraslow spreading ridges, during which detachment faults were developed. This suggests that the Yarlung-Tsangpo ophiolites might represent the ocean core complexes (OCC) in the Neo-Tethys Ocean. The OCC with high topography in the seafloor were clogged in the trench and preserved as ophiolites through Indo-Eurasia collision. The clogging resulted in the demise of an old subduction and a new subduction was re-initiated beneath the clogged OCC.

The Late Triassic-Jurassic magmatic belt and its implications for the double subduction of the Neo-Tethys Ocean in the southern Lhasa subterrane, Tibet

The southern Lhasa subterrane (SL), located between the Yarlung-Tsangpo Suture Zone and the Luobadui-Milashan Fault, preserves a complete Late Triassic-Jurassic magmatic record related to Neo-Tethys Ocean evolution. Therefore, the SL is key to understanding the geodynamic processes occurring during the controversial early-stage evolution of this ocean. Here, we present new zircon U–Pb data and a detailed synthesis of geochronological, geochemical, and Sr-Nd-Pb-Hf isotope data from Late Triassic-Jurassic igneous rocks in the SL to constrain the early-stage evolution of the Neo-Tethys Ocean. The data suggest that these igneous rocks can be divided into two groups. Group 1 magmatism lasted from ca. 243 Ma to 145 Ma, with three stages at ca. 243–200 Ma, 199–165 Ma, and 164–145 Ma, and is characterized by high positive εHf(t) and εNd(t) values and low (206Pb/204Pb)t, (207Pb/204Pb)t, and (208Pb/204Pb)t values. Group 2 magmatism lasted from ca. 225 Ma to 160 Ma, with two stages at ca. 225–190 Ma and 189–160 Ma, and is characterized by lower εHf(t) and εNd(t) values and higher (206Pb/204Pb)t, (207Pb/204Pb)t, and (208Pb/204Pb)t values than Group 1. We suggest that the Triassic-Jurassic magmatic belt in the SL was controlled by double subduction; the Group 1 igneous rocks formed in an intraoceanic arc setting, while the Group 2 igneous rocks formed in a continental margin arc setting. An integrated interpretation of magmatism, ophiolites, sedimentary rocks, and metallogenesis within the SL constrains the geodynamic processes that occurred during the early-stage tectonic evolution of the Neo-Tethys Ocean as follows. (1) During the Middle-Late Triassic, the initiation of intraoceanic subduction between Gondwana and the SL produced the ~243–200 Ma magmatism of Group 1. Subsequently, a new continental margin subduction system formed on the southern margin of the SL, which induced the ~225–190 Ma magmatism of Group 2. (2) With ongoing subduction, slab roll-back of the intraoceanic subduction system resulted in the ~199–165 Ma magmatic flare-up of Group 1. Additionally, slab roll-back of the continental margin subduction system resulted in extension of the overriding plate and formed the ~189–160 Ma magmatic flare-up of Group 2. (3) Finally, the slab roll-back associated with intraoceanic subduction transitioned to flat subduction and produced the 164–145 Ma magmatism of Group 1, and porphyry Cu-Au mineralization developed during the stress transition period. In summary, the double subduction system in the early-stage evolution of the Neo-Tethys Ocean controlled the tectonic, magmatic, and metallogenic events in the SL.

Late Pleistocene–Holocene flood history, flood-sediment provenance and human imprints from the upper Indus River catchment, Ladakh Himalaya

Abstract The Indus River, originating from Manasarovar Lake in Tibet, runs along the Indus Tsangpo suture zone in Ladakh which separates the Tethyan Himalaya in the south from the Karakoram zone to the north. Due to the barriers created by the Pir-Panjal ranges and the High Himalaya, Ladakh is located in a rain shadow zone of the Indian summer monsoon (ISM) making it a high-altitude desert. Occasional catastrophic hydrological events are known to endanger lives and properties of people residing there. Evidence of such events in the recent geologic past that are larger in magnitude than modern occurrences is preserved along the channels. Detailed investigation of these archives is imperative to expand our knowledge of extreme floods that rarely occur on the human timescale. Understanding the frequency, distribution, and forcing mechanisms of past extreme floods of this region is crucial to examine whether the causal agents are regional, global, or both on long timescales. We studied the Holocene extreme flood history of the Upper Indus catchment in Ladakh using slackwater deposits (SWDs) preserved along the Indus and Zanskar Rivers. SWDs here are composed of stacks of sand-silt couplets deposited rapidly during large flooding events in areas where a sharp reduction of flow velocity is caused by local geomorphic conditions. Each couplet represents a flood, the age of which is constrained using optically stimulated luminescence for sand and accelerator mass spectrometry and liquid scintillation counter 14C for charcoal specks from hearths. The study suggests occurrence of large floods during phases of strengthened ISM when the monsoon penetrated into arid Ladakh. Comparison with flood records of rivers draining other regions of the Himalaya and those influenced by the East Asian summer monsoon (EASM) indicates asynchronicity with the Western Himalaya that confirms the existing anti-phase relationship of the ISM-EASM that occurred in the Holocene. Detrital zircon provenance analysis indicates that sediment transportation along the Zanskar River is more efficient than the main Indus channel during extreme floods. Post–Last Glacial Maximum human migration, during warm and wet climatic conditions, into the arid upper Indus catchment is revealed from hearths found within the SWDs.

Early Cretaceous radiolarians and chert geochemistry from western Yarlung Tsangpo suture zone in Jiangyema section, Purang county, SW Tibet

Moderately to well-preserved radiolarian faunas belonging to 51 genera and 81 species have been identified in a continuous bedded chert succession in the Jiangyema section in the west segment of the Yarlung Tsangpo suture zone (YTSZ), Purang county, SW Tibet. Based on the occurrence of some characteristic species, especially the evolutionary appearance of age-diagnostic species two radiolarian zones and two subzones are recognized and respectively named as the Cecrops septemporatus zone, Aurisaturnalis carinatus zone, Aurisaturnalis carinatus carinatus subzone and Aurisaturnalis carinatus perforatus subzone. These can be correlated with coeval Hauterivian to upper Barremian radiolarian biozones in both the western Tethys and Japan. They provide a better biostratigraphical framework and reliable age constraints for parts of the Neotethyan Ocean floor that were subducted beneath supra-subduction zone (SSZ) ophiolites in the western part of the YTSZ.Geochemical data indicate that SiO2 contents of bedded cherts range from 79.84 to 94.65% with an average of 89.39%. Comparison of geochemical characteristics on discriminant graphs suggests that the bedded cherts were deposited in a deep oceanic basin near a continental margin. Our results and integration of available radiolarian studies from bedded cherts along the YTSZ suggest that the Neo-Tethys was a deep ocean between the Indian and Eurasian continents in which pelagic sedimentation was on-going until at least the late Barremian.