Northern Margin Of India Research Articles

Single‐Stage Synchronous India‐Asia Collision Model Revealed by Himalayan High‐Pressure Metamorphic Rocks

AbstractDespite a half‐century of intense research, the timing and diachroneity of initial collision between India and Asia remain highly debated, largely due to different definitions of “initial collision” and correspondingly different methods adopted. This study focuses on high‐pressure pelitic granulites of the Eastern Himalayan Syntaxis (EHS) to elucidate their metamorphic evolution and provide new constraints on the timing of initial India‐Asia collision. Petrological examination and phase equilibria modeling show that high‐pressure pelitic granulites have undergone four metamorphic stages, with the peak assemblage of garnet + K‐feldspar + kyanite + biotite ± plagioclase ± rutile + ilmenite + quartz at P‐T conditions of 13.1–15.7 kbar and 790–850°C. Clockwise P‐T paths suggest that the Indian continent underwent tectonometamorphic processes of initial collision and subsequent continent subduction. Zircon and monazite dating results indicate that the metamorphic ages of pelitic granulites range from 60 to 15 Ma, with the oldest ones clustered at 60–58 Ma. The oldest metamorphic ages of high‐ to ultrahigh‐pressure Himalayan metamorphic rocks can provide an upper age limit of the initial collision. Therefore, the initial India‐Asia collision must have occurred before 60–58 Ma in the EHS, roughly consistent with ca. 57 Ma in the western Himalaya and 63–60 Ma in the central Himalaya. Collectively, we conclude that the northern margin of India most likely underwent a single‐stage synchronous collision with the southern margin of Asia initially at around 60 Ma along the entire Yarlung‐Tsangpo suture zone.

Petrogenesis of S‐type Ladakh granite and mafic microgranular enclaves in the southern margin of Ladakh batholith: An evidence of crust–mantle interaction during the collision between Indian and Eurasian plates

AbstractThe previous studies revealed the I‐type Ladakh magmatism in the Andean‐type southern margin of the Ladakh batholith (LB) was related to the subduction of the Neotethyan Ocean and India‐Eurasia collision. However, LB's S‐type granitic magmatism and associated mafic microgranular enclaves (MMEs) are poorly constrained. Here, we present the new data for S‐type Ladakh granite (LG) and associated monzodiorite MMEs in the Andean‐type orogeny in the southern margin of the Eurasian plate. The low SiO2 (47.4–53.9 wt%), high K2O (1.56–3.21 wt%), Mg# (52–65), continental‐arc tracer patterns, and slightly depleted to evolved Sr‐Nd isotopic composition ((87Sr/86Sr)i = 0.7047–0.7166; ℇNd (t = 50 Ma) = (+1.40 to −8.92)) for MME suggest that they were derived from the phlogopite‐bearing deep lithospheric mantle‐source at a depth of 5.4–10.5 km depth with 810–870°C, 1.4–2.8 kbar, and enriched by sediment‐melts addition into the mantle‐wedge from subducting Neotethyan Oceanic slab. The mantle‐derived ascending hot mafic magma mixing with felsic magma of the ancient northern Indian margin‐derived, generates monzodiorite MME by assimilation and magma mixing processes. Plagioclase, amphibole, and biotite chemistry support the magma mixing processes. LG are characterized by high SiO2 (63.4–75.0 wt%), K2O (3.93–5.67 wt%), CaO/Na2O ratio of >0.3, differentiation index (90.27–97.46), normative corundum (1.0–2.8), A/CNK values (1.00–1.18), hypersthene (0.7–5.7), and low Al2O3, MgO, TiO2, Fe2O3. They also exhibit peraluminous, variable tracer elemental abundances, variable (87Sr/86Sr)i ratios (0.6967–0.7191), and high whole rock ℇNd (t = 50 Ma) values of −4.15 to −11.92) and ancient two‐stage Nd model age of 1160 and 1858 Ma. These features suggest that S‐type Ladakh granites were derived from the melting of ancient metagreywacke‐dominated metasedimentary rocks of the northern Indian margin by a large amount of mafic magma underplating after subducted Neotethyan slab‐rollback. The formation of LG and MMEs related to the Andean‐type orogeny in the southern margin of the Eurasian plate.

Reconstruction of the South Qiangtang–Zhongba–Tethyan Himalaya continental margin system along the northern Indian Plate: Insights from the paleobiogeography of the Zhongba microterrane

• The Zhongba microterrane drifted away from the Indian Plate during the Early Permian. • The South Qiangtang–Zhongba–Tethyan Himalaya system developed along the Indian Plate. • The Lhasa Block was exotic to the northern Indian margin system during the Permian. Information regarding the paleobiogeography of the Qinghai–Tibet Plateau can improve our understanding of its tectonic evolution. The Zhongba microterrane has been recognized as a small stratigraphical terrane within the western Yarlung–Zangbo suture zone. Paleobiogeographically, the Zhongba microterrane exhibited a moderate relationship with the Tethyan Himalaya throughout the Permian because they shared several typical Gondwanan/bipolar cool-water brachiopods and bivalves. However, several Cathaysian warm-water brachiopods, bryozoans, fusulines and compound corals appeared in the Zhongba microterrane as early as the Late Cisuralian but were not present in the Tethyan Himalaya until the latest Lopingian, indicating that the Zhongba microterrane drifted away from the northern Indian Plate no later than the Late Cisuralian. Additionally, Early Triassic marine faunas observed in the Zhongba microterrane were characterised by Tethyan ammonoid genera, along with several cosmopolitan/widely distributed ammonoids and bivalves, but Gondwanan/bi-temperate elements observed in the Tethyan Himalaya were lacking in the Zhongba microterrane, suggesting that the Zhongba microterrane had been biogeographically decoupled from the northern Indian Plate during that time. Additionally, both the Zhongba microterrane and Tethyan Himalaya exhibited an abrupt faunal variation from benthic brachiopod- to nektonic ammonoid-dominated assemblages across the P/T boundary, which is thought to be related to harsh and inhospitable oceanic environmental conditions during the Permian–Triassic mass extinction. Our results, together with published paleontologic, paleomagnetic, magmatic, and sedimentary studies, indicated that the Lhasa Block was exotic to the South Qiangtang–Zhongba–Tethyan Himalaya continental margin system along the northern Indian Plate during the Permian.

Subduction initiation of the Proto-Tethys Ocean that facilitated climate change and biodiversification

The effect of plate interactions on surface processes, environmental changes of the earth and their contributions to biodiversification during Gondwana assembly has been hotly debated in the last decades. The late Neoproterozoic-early Paleozoic was a key period in the tectonic assembly of Gondwana, global climate change and metazoan evolution. During this interval, the amalgamation of Greater Gondwana was succeeded by the subduction initiation of oceanic slabs in the Proto-Tethys Ocean. Here, we use whole-rock geochemical data, detrital zircon U-Pb and Hf isotopic of Cambrian and Ordovician sedimentary rocks from the Baoshan block in the Southeastern Tibetan Plateau combined with existing data in the literature to constrain the timing of subduction initiation of the Proto-Tethys Ocean at 540-500 Ma. The results show that the Baoshan block, adjacent to the Pinjarra Orogen on the northern margin of India, witnessed the transition from passive to active continental margin in East Gondwana. After the subduction initiation, the source of the sedimentary rocks experienced more intense weathering, causing a higher sediment flux. Contemporaneously, the Cambrian Explosion and stepwise increase in atmospheric oxygen accompanied the Greater Gondwana assembly at the late Neoproterozoic-early Paleozoic boundary. Subduction of the Proto-Tethys Ocean and occurrence of peripheral orogens further increased atmospheric O2 content, transported nutrients to the ocean and reduced the greenhouse effect, which provided major propulsion for biological evolution.

Position of South China and Indochina along northern Gondwana margin during the Ediacaran–Silurian period

The South China Block (SCB) has often been taken as an integral member of Gondwana at its northern margin, but it remains contentious regarding when and how it joined Gondwana, its precise position in Gondwana, and what other continent(s) might have been adjacent to it at the time. This study reports new detrital zircon U-Pb and Hf isotopic data of twelve Ediacaran-Silurian siliciclastic samples from three field regions along the western margin of the SCB. They show two prevailing age populations of 810–790 Ma and 550–520 Ma, and minor ca. 2500 Ma, ca. 1850 Ma, ca. 1200 Ma and ca. 980 Ma peaks in various units. In combination with the regional lithofacies evolution, provenance analysis demonstrates that the uplifted western SCB provided a persistent supply of 810–790 Ma zircons from local Tonian granites to the Ediacaran-Silurian sediments, whereas the 550–520 Ma zircons were probably derived exotically from a magmatic arc. Provenance analysis also reveals that the SCB and Indochina have a close provenance linkage. We interpret that the exotic magmatic arc was along the northern Indian margin with the subducting lower plate carrying SCB-Indochina to eventually collide with Indian Gondwana during Cambro-Ordovician time. The uplift of the western SCB during the time likely reflects the far-field tectonic compression of the collision.

Integrated provenance and tectonic implications of the Cretaceous–Palaeocene clastic sequence, Changla Gali, Lesser Himalaya, Pakistan

This study documented the field relationship and integrated provenance of a clastic sequence exposed at the Mesozoic–Cenozoic boundary located in Changla Gali section, Lesser Himalaya, Pakistan, to provide an insight into Cretaceous tectonics of the northern Indian margin. This boundary sequence is represented by the Early Palaeocene Hangu Formation, which consists of shales in the lower part and sandstone in the upper part. The contact relationship of the Early Palaeocene Hangu Formation with the underlying Late Cretaceous Kawagarh Formation is marked by an angular unconformity. The detrital zircons extracted from the shale and sandstone samples shows a major age cluster, which varies between ~700 and ~1,100 Ma (45%), ~1,600 and ~1,900 Ma (15%), and ~480 and ~590 Ma. Additionally, two minor age clusters of the detrital zircons are identified, that is, ~2,300–2,500 Ma and ~600–700 Ma. The younger detrital zircon grains have ages of 298 ± 4 Ma, 297 ± 4 Ma and 116 ± 3 Ma. This age pattern reflect the major source area as the Indian Plate. The two younger Permian zircon grains may be derived from the Panjal mafic volcanic rocks exposed in the vicinity of the study area. However, a single Cretaceous grain may be attributed to ophiolites, as well as Tethyan Himalayan (TH) volcanic rocks. Similarly, the sandstone petrographic results show that the sandstones are quartz‐rich, which show derivation from the craton interior provenance, which is likely the Indian Plate. However, the trace element data suggest a mixed source consisting of felsic and mafic rocks. The contribution of the mafic source is likely associated with the Panjal mafic rocks exposed along the northern Indian margin. The field relationship shows that the underlying Mesozoic sequence is folded prior to the deposition of the Hangu Formation. This folding suggests that the northern Indian margin experienced a regional compression during the Late Cretaceous time, which folded the Mesozoic sequence before the resumption of sedimentation during the Palaeocene. Furthermore, the detrital zircon provenance suggests that the sediments were mainly derived from the Indian Plate. Combining the results, it can be concluded that the compressional event is likely associated with the Late Cretaceous ophiolite obduction onto the leading edge of the Indian Plate. However, the absence of the major ophiolitic age component in the detrital record may suggest that the ophiolites were emplaced over the northern Indian margin but remained submerged during Early Palaeocene time.

Late Triassic rifting and volcanism on the northeastern Indian margin: A new phase of Neo-Tethyan seafloor spreading and its paleogeographic implications

The stratigraphic record of the northern Indian margin documents tectonic extension and magmatic activity during the Late Triassic. The geodynamic process and the paleogeographic implications of this event remain, however, incompletely understood. This study presents new stratigraphic, petrographic, geochronologic and geochemical data from a mixed carbonate-clastic Upper Triassic succession of the Tethys Himalaya in southern Tibet (Tela Formation). The new finding of volcanic detritus with bimodal mafic/felsic composition in the Tela Formation indicates nearby volcanic activity constrained by detrital zircon UPb dating as latest Carnian to early Norian in age (229–223 Ma). The alkalic signature of mafic volcanic rock fragments, showing LREE enrichment and HREE depletion, indicates a within-plate extensional setting. The igneous activity in northeastern India compares with coeval volcanism in northwestern Australia, suggesting that a rift-related volcanic belt developed along the northern margin of eastern Gondwana in the Late Triassic. This volcanic belt controlled the accumulation of thick turbidites of the Langjiexue Group in northeastern India and induced accelerated tectonic subsidence and increased sedimentation rates all along the Tethys Himalayan zone as far west as the Zanskar-Spiti Synclinorium. This major rifting phase is suggested to have initiated a new phase of sea-floor spreading in the Neo-Tethys Ocean that torn off the Lhasa Block from eastern Gondwana.

Timing of the Meso-Tethys Ocean opening: Evidence from Permian sedimentary provenance changes in the South Qiangtang Terrane, Tibetan Plateau

Timing of the opening of the Meso-Tethys Ocean, represented by the Bangong–Nujiang Suture Zone on the Tibetan Plateau, remains controversial. Further research is required to understand the breakup of the northern Gondwana margin and the tectonic evolution of the Tethyan realm. In this study, we present petrography, U-Pb dating and Hf isotopic data for detrital zircons from Upper Carboniferous–Upper Permian strata in the South Qiangtang Terrane on the Tibetan Plateau. These data, together with data from previous literature, indicate a youngest detrital zircon age peak of ca. 550 Ma for Upper Carboniferous–Lower Permian strata. This is far older than the depositional age of ca. 300 Ma and indicates a source in the stable Gondwana Continent. Upper Permian strata yield younger ages (490–247 Ma) with peaks at ca. 460, 355, 290 and 260 Ma, indicating a source in the active South Qiangtang Terrane. Combined with the unconformity between the Lower and Upper Permian strata in the western South Qiangtang Terrane, we conclude that a significant change in sedimentary provenance occurred at 280–260 Ma. This provenance change might have resulted from the 300–279 Ma rifting magmatism on the northern Indian margin of Gondwana (e.g., South Qiangtang). The 300–279 Ma magmatism is interpreted to reflect the early stages of rifting, and the subsequent 280–260 Ma sedimentary provenance change is interpreted as the later stage, both of which established a complete Early–Middle Permian (300–260 Ma) rifting process that marks the opening of the Meso-Tethys Ocean.

India in the Nuna to Gondwana supercontinent cycles: Clues from the north Indian and Marwar Blocks

Evolution of the Indian Block can be traced through Earth9s Phanerozoic and Precambrian supercontinent cycles. The Paleoproterozoic tectonostratigraphic record of the North Indian Block and the Aravalli Delhi Fold Belt in the Nuna supercontinent assembly shows a close link with the events in the Cathaysia Block of South China. Accretion of the two terranes is documented by 1.97 to 1.92 Ga continental arc igneous rocks and 1.91 to 1.81 Ga syn- and post-collisional magmatism in the Lesser Himalaya, along with 1.88 to 1.86 Ga granulite metamorphism in both continental blocks. The connection between the North Indian Block and the Cathaysia Block continued through Nuna dispersal and was followed by the accretion of a series of terranes/microcontinents along the western margin of this united North India-Cathaysia Block during Rodinia assembly (ca. 1.0 Ga). This is recorded by accretion of the Marwar Block to the North Indian Block and Yangtze Block to the Cathaysia Block. Long-lived active continental margins continued along Marwar (NW India), Yangtze, Madagascar and the Seychelles until ca. 720 Ma that jointly occupied a peripheral or even independent paleoposition in the Rodinia reconstructions. The eastern margin of India sutured with the Western Australia-Mawson blocks along the Kunnga Orogen during the final assembly of Gondwana in the early Paleozoic, whereas microcontinental blocks including south Qiangtang and north Lhasa, were accreted to the northern margin of Gondwana in the vicinity of India. The collision of this ensemble of blocks with Africa (western Gondwana) is marked by the East African Orogen/Mozambique Belt, extending through central east Africa, Madagascar, South India and Antarctica. However, further north, India was separated from the Arabian-Nubian Shield by an embayment of the proto-Tethys that remained integral until the breakup of Gondwana. The accretion of Laurussia to Gondwana in the mid-Paleozoic during the assembly of Pangea corresponds with lithospheric extension along the northern margin of India (Gondwana) and separation of several continental blocks including South China, south Qiangtang, and north Lhasa, which then drifted northward across the Paleo-Tethys to collide with the Asian segment of Pangea in the Permo-Triassic.

Implications for supercontinent reconstructions of mid-late Neoproterozoic volcanic – Sedimentary rocks from the Cathaysia Block, South China

The tectonic affinity of mid - late Neoproterozoic magmatism in the South China Block provides a first order constraint on its role in the break-up of Rodinia and the subsequent amalgamation of Gondwana. The Yingyangguan area in the western margin of Cathaysia Block preserves a magmatic and sedimentary record of this period. U-Pb zircon age data indicate magmatism at around 750 Ma, followed by accumulation of a succession of tuff and carbonate rocks at ca. 670–660 Ma. All samples show evidence for ductile deformation and metamorphism. The geochemical characteristics of mid-Neoproterozoic mafic and intermediate rocks suggest crustal assimilation and fractional crystallization play an important role in magma evolution, and they were derived from a source dominated by modified lithospheric mantle sources previously metasomatized by slab-derived fluids. Analyzed samples, as well as contemporaneous mafic rocks in neighboring areas, have high Zr/Y ratios, and plot in the within-plate basalt field at ca. 750 Ma. In addition, zircons with ages between 1200 and 950 Ma and 850–770 Ma mostly fall within the arc-related field on discriminating diagrams based on zircon trace-element compositions, whereas, most ca. 750 Ma and 700–600 Ma zircons plot in the within-plate field. This evolving tectonic record of magmatic activity suggests a transformation from a convergent plate margin setting on the periphery of Rodinia to one involving within plate extension at ca. 750 Ma. Furthermore, the detrital zircons from samples of the Yingyangguan Group present abundant late Mesoproterozoic to early Neoproterozoic (1200–950 Ma) ages. These ages match the record of North India and Indo-Antarctica, and imply a peripheral setting for South China with respect to the supercontinent and most likely adjacent to India from at least 750 Ma. Thus, we suggest that the Yangtze Block accreted to Cathaysia on the northern margin of India in the early Neoproterozoic, and that the active plate boundaries located along the northern and western margins of South China continue to ca. 750 Ma. After this time, the tectonic setting within the South China Block transformed into a stable within-plate siliciclastic depositional environment that continued to receive detritus from East Gondwana until the early Devonian.

Paleocene latitude of the Kohistan–Ladakh arc indicates multistage India–Eurasia collision

We report paleomagnetic data showing that an intraoceanic Trans-Tethyan subduction zone existed south of the Eurasian continent and north of the Indian subcontinent until at least Paleocene time. This system was active between 66 and 62 Ma at a paleolatitude of 8.1 ± 5.6 °N, placing it 600-2,300 km south of the contemporaneous Eurasian margin. The first ophiolite obductions onto the northern Indian margin also occurred at this time, demonstrating that collision was a multistage process involving at least two subduction systems. Collisional events began with collision of India and the Trans-Tethyan subduction zone in Late Cretaceous to Early Paleocene time, followed by the collision of India (plus Trans-Tethyan ophiolites) with Eurasia in mid-Eocene time. These data constrain the total postcollisional convergence across the India-Eurasia convergent zone to 1,350-2,150 km and limit the north-south extent of northwestern Greater India to <900 km. These results have broad implications for how collisional processes may affect plate reconfigurations, global climate, and biodiversity.

Provenance of the Hangu Formation, Lesser Himalaya, Pakistan: Insight from the detrital zircon U-Pb dating and spinel geochemistry

Clastic rich Hangu Formation of the Early Paleocene exposed at the apex of Hazara-Kashmir syntaxis, is significant in providing evidence of the regional Mesozoic–Cenozoic unconformity and nature of detritus supplied. The current study focuses on U-Pb age dating of the detrital zircons along with spinel geochemistry to interpret tectonic implications on provenance. The Hangu Formation is composed of coarse grained sandstone, arenaceous limestone, carbonaceous shale and laterite around the Paras region and rests unconformably over Mesozoic Samana Suk Formation. The probability density plots of the detrital zircons of the Hangu Formation exhibit strong similarity to the Tethyan Himalayan detrital record. The younger Mesozoic detrital zircons exhibit resemblance to Tethyan Himalayan as well as ophiolitic detrital suites, but are very few compared to their coeval Stumpata, Jidula and Denggeng/Sangdanlin formations. The quantitative comparison reflects strong resemblance to the Tethyan Himalaya, whereas, the ophiolitic component relationship is quite poor. Comparison of spinel geochemical data with Northwestern ophiolites (Chilas Ophiolites) and Tethyan Himalaya exhibits mixed source regime of the Hangu sediments. However, the combined U-Pb dating, sandstone petrography and spinel geochemical data suggest that the detritus of the Hangu Formation is mainly derived from the Tethyan Himalayan source (Indian source). The absence of ophiolitic signature in petrography and U-Pb geochronology may suggest that the northern ophiolites were not exposed to provide detritus to the Early Paleocene Hangu Formation. This may also be interpreted that the ophiolites were emplaced during the Late Cretaceous on the northern Indian margin but uplifted and eroded later with the final India–Eurasia collision, which is clearly recorded in various studies all along the Indian margin. This fact also intimates the ophiolite emplacement as a possible cause of the regional Mesozoic–Cenozoic unconformity.

Transversal folding in Himalaya foothill ranges

The Himalayan foreland in N Pakistan, dissected by Main Frontal Thrust (MFT) and Main Boundary Thrust (MBT) contains spectacular salients and syntaxes. The lateral (N-S) boundaries between these salients and syntaxes around Kalabagh city and east-southeast of Islamabad were believed to host deep-seated lateral ramps with strike slip movements. However, seismic data in these two sectors suggest that there are N-S trending folds and locally east- or west-vergent thrusts that affect the Paleozoic-Paleogene cover of the Indian shield, as well as the Miocene-Pliocene molasse sediments. The proposed lateral ramps cannot be followed on the seismic and on maps either; instead, both maps and seismic data suggest folding, often on a regional scale of harder Paleo-Mesozoic-Paleogene and softer Oligo-Miocene-Pilo-Pleistocene cover. The NE corner of Surghar range is proposed to be formed of relaying thrust sheets with emergent heads composed of Paleozoic-Paleogene and its slightly detached Miocene molasse. These relaying imbricates are taken in a southward flexure generated by a major right lateral shear of a wide zone, where transpressive Riedel shears, en echelon anticlines and southwards flexed earlier thrust faults are the main elements (but a single, through-going Kalabagh fault is missing). The generation of mapped N-S trending folds and east-vergent thrusts preceded the formation of the wide shear zone and southwards flexing.Hazara syntaxis is interpreted as a major antiform that re-folded MBT and Panjal thrust around Oligo-Miocene molasse, itself forming an antiform (BOSSART ET AL. 1988). In our model we propose that the west-vergent Balakot thrust and deeper blind thrusts are in the core of this antiform. In the southern continuation we propose that folds in Miocene molasse continue from eastern Potwar region to western Kashmir and there appears no major break. These structures are also re-folded in a major antiform with N-S axial trend. Map analysis also suggests that N-S trending folds bending earlier main thrusts are occurring in a wide area south of the Indus-Tsangpo suture.Several independent geological and geophysical observations including mapping, seismic analysis, earlier measurements of strain axes and of paleomagnetic declinations suggest that the salients and syntaxes may have been much more linear in the past (although a total linearity is not realistic). It is proposed that the present-day undulating pattern may have been generated by N-S trending folds due to general (and episodic) E-W shortening. If the main fault zones were more linear, the relay pattern along their segments suggests a left lateral shear component along MBT and a mixed, locally left, locally right lateral component along MFT. Earlier (ZEITLER 1985) and now provided low temperature thermochronological ages strongly suggest a rather general episode of E-W shortening between 4-5 Ma for the whole northern Indian margin. However, there should have been original transversal dome formation as early as Oligocene (DIPIETRO ET AL. 2008). It is also clear that longer N-S shortening and shorter E-W shortening episodes should alternate eventually in a very short time frame, since earthquake focal mechanisms (LISA AND KHWAJA 2004, BURG ET AL. 2005) suggest the coexistence of E-W compression and NW-SE compression in Potwar.There are several potential explanations for generating E-W shortening and related structures in a general N-S shortening regime. Possibilities range from fault terminations of thrust faults at high angles in a particular zone (TREOLAR ET AL. 1992) to en echelon folding along a major right lateral E-W fault zone. However, we speculate that E-W shortening could be much more general, suggesting a mechanism that affects the whole of Indian plate. Possibly the best explanation is given by analogue models (REPLUMAZ ET AL. 2012) proposing major, slightly convergent confining boundaries. If applied to the northwards advance of India, the northwards converging boundaries generate secondary E-W shortening and east-or west-vergent orogens parallel to these boundaries.

Sedimentologic and stratigraphic constraints on the orientation of the Late Triassic northern Indian passive continental margin

The orientation and distribution of the northern Indian passive continental margin basin during different epochs constrained the geometric characteristics of the hyperextended northern Indian edge (“Greater India”). Traditional paleogeographic reconstructions envisage the Late Triassic northern Indian passive continental margin as E/W-trending, whereas paleomagnetic data support an ~55° counterclockwise rotation during northward drifting. This case study of Upper Triassic units from the northern Indian margin, based on facies analysis, sandstone petrography, heavy mineral assemblages, U-Pb detrital zircon geochronology, and stratigraphic correlation from the Zanskar Range to southern Tibet, provides new constraints and suggests that the northern Indian passive continental margin might have been N/S-trending in the Late Triassic. Given the dominance of quartz sandstones with zircon, tourmaline and rutile—indicating a provenance from continental blocks and the recycling of older quartz-rich sandstones—and detrital zircon age spectra characterized by a Neoproterozoic detrital zircon U-Pb age peak (921 ± 13 Ma), the Upper Triassic sandstones widespread along the Tethyan Himalayas may have been partly derived from the Eastern Ghats Mobile Belt in eastern India and the Rayner Orogen in eastern Antarctica. The Upper Triassic Quartzite Series comprises feldspatho-quartzose to pure quartzose sandstones with intercalated bioclastic oosparites deposited in backshore, foreshore and shoreface coastal environments. Without considering the subsequent counterclockwise rotation of India, the consistent pattern of the Upper Triassic bidirectional E-W paleocurrent observed throughout the Himalaya suggests that the paleoshoreline was striking near N-S, approximately perpendicular to the regional orientation of the northern Indian margin. Progressive eastward thickening of the sandstone layers and eastward younging of the Quartzite Series suggest diachronous sedimentation. Such stratal patterns and the strike of the Late Triassic paleoshoreline were attributed to the Neo-Tethys transgression from the Zanskar Range to southern Tibet.

Origin and early evolution of the Lhasa Terrane, South Tibet: Constraints from the Bomi Gneiss Complex

The Precambrian history of the Lhasa Terrane provides important context for understanding the India–Asia continental collision and formation and evolution of the Tibetan Plateau. However, it has not yet been well constrained, due to the scarcity of Precambrian outcrops. In this study, we present geochronological, geochemical, and Lu–Hf isotopic data for granitoid gneisses from the Bomi Complex in the eastern Lhasa Terrane. Our new data, together with those from the literature, reveal three episodes of magmatic activity at ca. 1866–1782 Ma, ca. 1343–1250 Ma, and ca. 824 Ma, and two metamorphic events at ca. 1117 Ma and ca. 625–600 Ma. While the Mesoproterozoic gneiss complexes suggest a transition from early rift-related to late arc signatures, the other two episodes of gneiss formation have arc signatures. We suggest that during the Paleoproterozoic and Middle Neoproterozoic, the Bomi Complex had an affinity with the Lesser and Greater Himalaya terranes along the northern margin of India, respectively. During the Mesoproterozoic and late Neoproterozoic, the Bomi Complex might be correlated with the contact zone between the Indian Archean cratons and the Eastern Ghats Belt in southeastern India. These magmatic and metamorphic events indicate that the Lhasa Terrane was located along the northern margin of the Indian continent during its early evolution. We propose the Precambrian evolution of the Lhasa Terrane was as follows: (1) the Lhasa Terrane and Lesser Himalayan sequence represent a Paleoproterozoic continental arc along the northern margin of the Indian continent; (2) the eastern Lhasa Terrane was affected by the Grenvillian-age Eastern Ghats orogeny during the Mesoproterozoic; (3) the Lhasa Terrane and Greater Himalayan Crystalline Complex formed a middle Neoproterozoic Andean-type orogen along the margin of the Rodinia supercontinent; and (4) the Lhasa Terrane experienced the Pan-African Orogeny, which might have marked the initial assembly of East Gondwana.

The Langjiexue Group is an in situ sedimentary sequence rather than an exotic block: Constraints from coeval Upper Triassic strata of the Tethys Himalaya (Qulonggongba Formation)

The Upper Triassic Langjiexue Group in southeastern Tibet has long been an enigmatic geological unit. It belongs tectonically to the northern Tethys Himalayan zone, but provenance signatures of the detritus it contains are significantly different from those of typical Tethys Himalayan sandstones. Because the Langjiexue Group is everywhere in fault contact with Tethys Himalayan strata, its original paleogeographic position has remained controversial for a long time. According to some researchers, the Langjiexue Group was deposited onto the northern edge of the Indian passive continental margin, whereas others interpreted it as an independent block accreted to the northern Indian margin only during final India-Asia convergence and collision in the Paleocene. This study compares the Langjiexue Group and coeval Upper Triassic strata of the southern Tethys Himalayan zone (Qulonggongba Formation). Our new provenance data indicate that Qulonggongba Formation sandstones contain common felsic volcanic rock fragments, minor plagioclase, and euhedral to subhedral zircon grains yielding Late Paleozoic to Triassic ages. These provenance features compare well with those of the Langjiexue Group. Because the Qulonggongba Formation certainly belongs to the Tethys Himalayan zone, the provenance similarity with the Langjiexue Group indicates that the latter is also an in situ Tethys Himalayan sedimentary sequence rather than part of an exotic block. Volcanic detritus including Late Paleozoic to Triassic zircon grains in both Langjiexue Group and Qulonggongba Formation are interpreted to have been derived from the distant Gondwanide orogen generated by Pan-Pacific subduction beneath the southeastern margin of Gondwana. The Qulonggongba Formation, deposited above marlstones of the lower Upper Triassic Tulong Group, is overlain by India-derived coastal quartzose sandstones of the uppermost Triassic Derirong Formation. Deposition of both the Qulonggongba Formation and the Langjiexue Group were most likely controlled by regional tectonism, possibly a rifting event along the northern margin of Gondwana.

Protolith lithostratigraphy of the Greater Himalayan Series in Langtang, Nepal: implications for the architecture of the northern Indian margin

Abstract Reconstruction of the protolith lithostratigraphy of amphibolite-facies metasedimentary rocks of the Greater Himalayan Series (GHS) in Nepal documents a single, long-lived passive-margin succession that was deposited along the northern margin of the Indian Craton. In the Langtang area, Paleoproterozoic gneisses are unconformably overlain by a succession of upper Neoproterozoic–Ordovician fluvio-deltaic quartzite, basinal pelite and psammitic beds that grade upsection into micaceous semipelite and pelite. U–Pb zircon geochronology yields maximum depositional ages between c. 815 and 460 Ma for the GHS in Langtang. Regional variations in the composition and thickness of the GHS along the length of the Himalaya are attributed to siliciclastic depocentres centred on Zanskar in northern India, Langtang and Everest in central to western Nepal, which contrast with coeval marine carbonate shelf deposition in the Annapurna region. The protolith lithostratigraphy documented for Langtang provides a coherent framework for interpreting subsequent Cenozoic Himalayan deformation, specifically the homogeneously distributed layer-normal shortening (i.e. flattening) and layer-parallel stretching (i.e. transport-parallel stretching) that characterizes the GHS. Within the context of a single protracted northern Indian marginal sedimentary succession, the distinction between the Lesser, Greater and Tethyan Himalaya is structural rather than lithostratigraphic in origin.

1.74 Ga crustal melting after rifting at the northern Indian margin: investigation of mylonitic orthogneisses in the Kathmandu area, central Nepal

ABSTRACTMylonitic orthogneisses in the Kathmandu area, central Nepal have been investigated using whole-rock and mineral chemistry, Rb-Sr isotopes, and zircon U-Pb age dating. Zircon REE patterns determined from orthogneisses are characterized by enriched HREE patterns and the prominent Eu anomalies, consistent with a magmatic origin. The U-Pb zircon age dating and Ti-in-zircon thermometry revealed crystallization took place ca. 1.74 Ga at temperatures of 705–765℃; typical of felsic magmatism in the crust. Whole-rock data from most orthogneisses in this study and from similar rocks in previous studies span the ‘syn-collisional’ and ‘post-collisional’ fields on various tectonic discrimination diagrams, while some data also plot in rift-related magmatism fields. The peraluminous compositions, very high Sr isotopic ratios (0.865–3.585) and high Th and U concentrations for all orthogneisses in this study indicate that mylonitic orthogneisses are largely of S-type crustal origins. The new data presented herein, combined with that of previous studies, outline at least two Palaeoproterozoic magmatic episodes: 1) ca. 1.92–1.90 Ga rift-related magmatism derived from mantle melting and 2) 1.84–1.74 Ga crustal melting, resulting from burial of the Indian basement during thermal subsidence after rifting. This two-stage Palaeoproterozoic magmatism in Nepal occurred along the northern passive margin of the Indian basement during and/or after the breakup of the Columbia supercontinent.

Tectonic Implications of Detrital Zircon Ages From Lesser Himalayan Mesozoic‐Cenozoic Strata, Pakistan

AbstractWe carried U‐Pb‐Hf geochronology of the clastic sequence covering upper Mesozoic‐Cenozoic period. The upper Mesozoic sequence is overlain unconformably by the Cenozoic strata, marking regional K‐T boundary, which is mapped as angular unconformity. This angular relationship may strongly indicate a compressional orogenic event that occurred during the Late Cretaceous. Late Cretaceous Kohistan‐Karakoram collision and ophiolite emplacement may account for the development of this compression along the northern Indian margin. The U‐Pb‐Hf isotopic analyses of the upper Mesozoic and lower Cenozoic sequence (Hangu Formation and Lower Patala Formation) are similar to the Tethyan and Lesser Himalayan signatures, which indicate the Indian‐plate provenance. The two younger Cretaceous grains recorded in the Hangu Formation might be derived from the Indian‐plate volcanic rocks and/or Tethyan Himalayan volcanic rocks. The absence of ophiolitic age component in the precollision sequence may indicate that these ophiolites are not exposed until final India‐Eurasia collision. The major shift in provenance is recorded in the upper part of the Patala Formation, where the presence of disconformity and the appearance of &lt;100 Ma detrital zircons indicate the contribution of Eurasian source (i.e., Kohistan‐Ladakh arc; KLA), which is further supported by the ɛHf(t) signatures. Upsection, the contribution from Eurasian source increased in the Early Middle Eocene Kuldana Formation. Therefore, we suggest that the provenance mixing close to Paleocene‐Eocene boundary indicates that the India‐Eurasia collision in the north Pakistan occurred at ∼56–55 Ma and subsequent exhumation increased the contribution of the northern (Eurasian) provenance in the upsection.

Smithian ammonoid faunas from northeastern Nevada: implications for Early Triassic biostratigraphy and correlation within the western USA basin

Intensive sampling of the lower portion of the Thaynes Group within the Palomino Ridge area (northeastern Nevada) yielded abundant and well-preserved Smithian (Early Triassic) ammonoid faunas. Ammonoid taxonomy and a detailed biostratigraphy for this locality are reported herein. One new genus (Palominoceras) and one new species (?Pseudosageceras bullatum) are described. Additionally, based on new data from Palomino Ridge and previous data from neighboring localities in Utah, we porvide here the first quantitative Smithian ammonoid biochronological scheme for the western US basin. This new zonation is based on the Unitary Associations (UA) method. The biochronical sequence comprises five unitary association zones that can be correlated with other localities from the Northern Indian Margin (Salt Range, Pakistan, Spiti, northern India and Tulong, South Tibet). Three unitary association zone (UAZ1, UAZ2 and UAZ3) are defined for the early Smithian, one (UAZ4) spans the entire middle Smithian and one (UAZ5) signifies the first part of the late Smithian. Finally, a hypothetical UAZ6 would represent the second part of the late Smithian.