Northern Margin Of India Research Articles

Cenozoic kinematic history of the Kohistan fault in the Pakistan Himalaya

Detailed mapping suggests a new scenario for the kinematic history of the Late Cretaceous Kohistan arc complex and for the cause of Cenozoic metamorphism and exhumation along the Indian continental margin in Pakistan. The earliest deformational phases resulted in early Cenozoic amphibolite-facies metamorphism, superposed tight to isoclinal F1/F2 folds, and development of strong penetrative rock fabric within the Indian plate in response to southwestward, and then southeastward, overthrusting of ophi-olitic melange in the Indus suture zone and West Pakistan fold belt, respectively. This deformation included the development of the Banna and Malakand thrust faults. Prograde metamorphism ended following this deformational phase, and the early structures were deformed along N-trending, open, upright, F3 folds and domes. This was followed by ESE- to E-directed thrusting of the Kohistan arc complex onto the Indian continent along the dominantly brittle Kohistan fault. Thrusting ended in the late Oligocene–early Miocene when the Kohistan arc assumed its present position along the northern margin of India. South-verging structures began to form on the Indian plate for the first time in the late Oligocene–early Miocene. Deformation had expanded to the foreland by the middle to late Miocene with development of the Panjal-Khairabad thrust. Neogene structures in the metamorphic zone include N-trending, east-side-up, high-angle fault zones that deform or reactivate the Kohistan fault, and NW-trending, east-side-up reverse faults, both of which are seismically active. In the tectonic scenario outlined here, shortening along the Kohistan fault occurred subsequent to metamorphism, and was directed toward the ESE-E throughout its history. The southern termination of the fault is considered to be a right-slip sidewall ramp such that the Kohistan arc never advanced much father south on the Indian plate than its present position. Broad flexure, folding, and erosion in front of and south of the actively advancing Kohistan arc complex contributed to exhumation of the Indian plate. This differs from interpretations that state that Kohistan was thrust in a southward direction; that underthrusting beneath Kohistan was the primary cause of early Cenozoic metamorphism on the Indian plate; and that top-side-north extensional reactivation of the Kohistan fault contributed to exhumation.

Early Palaeozoic orogenesis along the Indian margin of Gondwana: Tectonic response to Gondwana assembly

SHRIMP U–Pb dating of zircons from a peralkaline S-type Lesser Himalayan granite from the Kathmandu region, Nepal indicate an age of emplacement of 475 Ma. The granites along with metasedimentary xenoliths show a similar signature of inherited detrital zircon ages ranging from Archaean to early Palaeozoic with prominent late Mesoproterozoic (Grenvillian) and Neoproterozoic (Pan-African) age peaks and have a maximum age of 500 Ma based on the youngest detrital grains. Deformation structures in xenoliths are truncated by the granite and along with the granites are assigned to a Cambro-Ordovician orogenic event, herein termed the Bhimphedian Orogeny that can be traced across the Himalaya from Pakistan to the eastern Himalaya and possibly extends west into Afghanistan. We interpret the orogeny as being related to Andean-type orogenic activity on the northern margin of the Indian continent, following Gondwana assembly. The magmatic arc was associated with andesitic and basaltic volcanism and was active from c. 530 to 490 Ma. The arc activity overlaps with, and is succeeded by, regional deformation, crustal melting and S-type granite emplacement that extended to 470 Ma. Orogenic activity was driven by coupling across the plate margin either during on-going subduction or through accretion of microcontinental ribbons, possibly represented by the Lhasa and Qiangtang blocks. It represents the termination of an orogenic cycle, termed the North Indian Orogen that commenced with Neoproterozoic rifting and passive margin development and terminated with the Bhimphedian Orogeny. This was succeeded by a return to passive margin setting along the northern Indian margin of Gondwana which continued until the Cenozoic Himalayan Orogeny.

Late Early Triassic climate change: Insights from carbonate carbon isotopes, sedimentary evolution and ammonoid paleobiogeography

The late Early Triassic sedimentary–facies evolution and carbonate carbon-isotope marine record (δ 13C carb) of ammonoid-rich, outer platform settings show striking similarities between the South China Block (SCB) and the widely distant Northern Indian Margin (NIM). The studied sections are located within the Triassic Tethys Himalayan belt (Losar section, Himachal Pradesh, India) and the Nanpanjiang Basin in the South China Block (Jinya section, Guangxi Province), respectively. Carbon isotopes from the studied sections confirm the previously observed carbon cycle perturbations at a time of major paleoceanographic changes in the wake of the end-Permian biotic crisis. This study documents the coincidence between a sharp increase in the carbon isotope composition and the worldwide ammonoid evolutionary turnover (extinction followed by a radiation) occurring around the Smithian–Spathian boundary. Based on recent modeling studies on ammonoid paleobiogeography and taxonomic diversity, we demonstrate that the late Early Triassic (Smithian and Spathian) was a time of a major climate change. More precisely, the end Smithian climate can be characterized by a warm and equable climate underlined by a flat, pole-to-equator, sea surface temperature (SST) gradient, while the steep Spathian SST gradient suggests latitudinally differentiated climatic conditions. Moreover, sedimentary evidence suggests a transition from a humid and hot climate during the Smithian to a dryer climate from the Spathian onwards. By analogy with comparable carbon isotope perturbations in the Late Devonian, Jurassic and Cretaceous we propose that high atmospheric CO 2 levels could have been responsible for the observed carbon cycle disturbance at the Smithian–Spathian boundary. We suggest that the end Smithian ammonoid extinction has been essentially caused by a warm and equable climate related to an increased CO 2 flux possibly originating from a short eruptive event of the Siberian igneous province. This increase in atmospheric CO 2 concentrations could have additionally reduced the marine calcium carbonate oversaturation and weakened the calcification potential of marine organisms, including ammonoids, in late Smithian oceans.

Paleocene–Eocene record of ophiolite obduction and initial India‐Asia collision, south central Tibet

Uppermost Cretaceous to Eocene marine sedimentary sequences occur both to the south and north of the Yarlung Zangbo suture in south central Tibet. They consist of Indian‐margin strata of the northern Tethyan Himalaya and Asian‐margin strata of the Gangdese forearc. Both assemblages are characterized by major changes in depositional environment and sedimentary provenance at ∼65 Ma and an appearance of detrital chromium‐rich spinel of ophiolite affinity (TiO2 generally <0.1 wt%) during the Paleocene. Ophiolitic melange exposed along the suture could have provided a source for detrital spinel. The melange occurs in the hanging wall of a north dipping, south directed mylonitic shear zone which includes a tectonic sliver of mafic schist. Amphibole from the schist yields 40Ar/39Ar ages of ∼63 Ma, which we attribute to cooling during slip along the shear zone and southward obduction of the melange. Melange obduction was coeval with the development of an angular unconformity within the Gangdese forearc basin to the north (between late Maastrichtian time and ∼62 Ma). Upper Paleocene to middle Eocene sandstones in the northern Tethyan Himalaya yield 200–120 Ma U‐Pb detrital zircon ages and 190–170 Ma 40Ar/39Ar detrital mica ages. These detrital grains were most likely sourced from regions north of the Yarlung Zangbo suture, suggesting that onset of India‐Asia collision in south central Tibet is middle Eocene or older in age. Collectively, our results support previous suggestions that oceanic rocks were obducted onto the northern margin of India during latest Cretaceous–earliest Tertiary time. Coeval changes in Gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between India and Asia at ∼65 Ma. Alternatively, in concert with the conventional view of Eocene collision initiation, the obducted oceanic rocks may be of intraoceanic origin, while coeval changes in Gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean‐continent convergence following the demise of the intraoceanic subduction zone.

Evidence for the multiphase nature of the India-Asia collision from the Yarlung Tsangpo suture zone, Tibet

Abstract Recent investigations in southern Tibet enable the testing and refinement of existing models for India-Asia collision. Presently available data indicate that marine deposition continued in the southern central portion of Tibet until at least the end of the Eocene. Sub-duction-related magmatism continued until the Mid-Oligocene, after which rapid uplift of the plateau was initiated. Mass-wasting of sediments into molasse basins did not commence until the latest Oligocene. The implications are that existing models, based on less-precise age constraints, invoking India-Asia collision at 55 Ma, are either flawed, or collision began at a different time. Recent work has produced sufficient data to allow the recognition of two different collisional events along the suture between India and Asia. Features related to each event require separate interpretation, and no collisional continuum should be assumed. In southern Tibet, a collision between the northern margin of India and a southfacing intra-oceanic island arc occurred at around 55 Ma, whereas continent-continent collision between India and Asia did not occur until at least 20 million years later.

Stratigraphic and sedimentological constraints on the age and tectonic evolution of the Neotethyan ophiolites along the Yarlung Tsangpo suture zone, Tibet

Abstract Ophiolitic rocks distributed along the Yarlung Tsangpo suture zone in southern Tibet are the few remaining fragmentary remnants of many thousands of kilometres of the ocean space that formerly existed between India and Eurasia. Portions of mid-Jurassic and mid-Cretaceous intra-oceanic island arcs can be recognized amongst those rocks that have been studied in detail. Complete suprasubduction zone ophiolite successions are preserved in the Dazhuqu terrane, which crops out both east and west of Xigaze. Radiolarians in inter-pillow cherts and immediately overlying sedimentary rocks indicate a Barremian ophiolite generation event. Palaeomagnetic data show that this ophiolite formed at equatorial latitudes south of the Lhasa terrane before its south-directed emplacement onto the northern margin of India. Highly refractory ultramafic rocks in the Luobusa ophiolite appear to be of Mid-Jurassic age and are potentially related to intra-oceanic island arc remnants in the nearby Zedong terrane. Ophiolitic massifs along the suture in western Tibet are thrust southwards onto northern India and record Late Jurassic ocean-floor development. Miocene north-directed back-thrusting associated with India-Asia collision has further complicated interpretation of regional geology. The ophiolitic rocks of the Yarlung Tsangpo suture zone provide evidence for the former existence of multiple oceanic island arc segments within Neotethys and suggest that consumption of the oceanic space between India and Asia was more complicated than has been predicted by existing models.

New constraints on the India–Asia collision: the Lower Miocene Gangrinboche conglomerates, Yarlung Tsangpo suture zone, SE Tibet

Lower Miocene conglomerates crop out along the length of the Yarlung Tsangpo suture zone on the southern margin of the Lhasa terrane. These conglomerates, known by various local names, are correlated herein as the Gangrinboche conglomerates. All units exhibit broadly similar stratigraphic histories and a basal depositional contact upon an eroded surface of rocks of the Lhasa terrane is ubiquitous. At most localities the tops of sections are either removed by erosion or truncated by north-directed thrusts. These conglomeratic molasse units developed in response to the India/Asia collision and record aspects of its development. In all units initial clast derivation was from the Lhasa terrane on the northern margin of the Yarlung Tsangpo suture zone. Up-section the first appearance of clasts derived from terranes within the suture zone and the northern margin of India, all of which lie to the south of any outcrops of Gangrinboche conglomerates, is observed. Although these units were previously thought to be Eocene, analysis of fossil and structural constraints indicates Early Miocene deposition. As development of the Gangrinboche conglomerates records a significant phase in the evolution of the India–Asia collision understanding of their age and stratigraphic evolution has wider implications for regional tectonic models.

Radiolarian evidence for the stratigraphy and palaeo-oceanography of the deep-water passive margin of the Indian Plate (Karamba Formation, Indus Suture Zone, Ladakh Himalaya)

New radiolarian biostratigraphical information sheds light on the Mesozoic sedimentary, tectonic and palaeoceanographic evolution of a south-Tethyan margin in northern India. Radiolarian cherts studied here form part of deformed deep-water successions within the Indus Suture Zone (Ladakh Himalaya); these are defined as the Karamba Formation and interpreted as the stratigraphically higher and palaeogeographically more distal parts of a passive margin (Lamayuru Group). This formation includes radiolarites, pelagic carbonates, redeposited limestones, ferruginous shales, sandstones, and abundant mafic extrusive rocks.Radiolarian assemblages of early and late Middle Jurassic, mid-Cretaceous (mid-Albian to Cenomanian) and Late Cretaceous (Santonian) age were obtained from four measured sequences. Results indicate: (i) long, intact Jurassic-Cretaceous successions, rather than disrupted mélange, are present in the Karamba Formation; (ii) extensive basic volcanism took place during the Middle Jurassic and can be correlated with an extensional phase affecting the NW Indian continental margin; (iii) non-calcareous radiolarian-rich sediments accumulated during the Middle Jurassic, reflecting the influence either of a broad Jurassic equatorial upwelling zone, or a more localised coastal upwelling system.After drowning of the Zanskar clastic shelf in the late Albian, sedimentation on the distal shelf and upper slope was restricted by bottom current activity. Late Albian-Cenomanian radiolarites of the Karamba Formation are contemporaneous with stratigraphie gaps and glauconite-phosphorite deposition on the Zanskar shelf. A well-developed eastward boundary current off the northern margin of India may have persisted until near the end of the Late Cretaceous. During the late Albian-Cenomanian, current activity and coastal upwelling were probably strong and radiolarian-rich sediments accumulated on the distal margin beneath the CCD. Pelagic limestones accumulated during the Turanian over the entire margin, possibly related to eustatic sea-level rise and consequent reduction in bottom current activity. Intense oceanic circulation apparently was renewed during the Coniacian-Santonian, as indicated by sedimentary hiatuses in the Zanskar shelf succession and accumulation of carbonate-free radiolarian ooze in the Karamba Formation.

Palaeomagnetism of Cretaceous to Tertiary sediments from southern Tibet: evidence for the extent of the northern margin of India prior to the collision with Eurasia

Palaeomagnetic studies have been carried out on mid-Cretaceous to latest Palaeocene sediments from the Tethyan Himalaya on the northern margin of the Indian Plate near the localities of Gamba (28.3°N, 88.5°E) and Duela (28.0°N, 89.2°E) in southern Tibet. A total of 61 sites (more than 700 cores) have been sampled mainly from limestones (Zongshan Formation, Jidula Formation Member II, Zongpu Formation), sandstones (Jidula Formation Member III) and graywackes (Gamba Group Member II), with ages well defined from detailed lithological and biostratigraphic analyses. Carriers of characteristic remanences are magnetite in limestones and graywackes, and hematite in sandstones. Reliable palaeodirections were obtained from 35 sites (356 specimens). Positive fold tests, antiparallel normal and reverse polarities, a northward trend of the palaeolatitudes with decreasing age, and absence of any metamorphic overprint indicate a primary origin for the characteristic remanences. The results have been compared with expected palaeodirections according to available data for the Indian subcontinent based on its present-day geometry. The late Maastrichtian sites (fourteen sites, 156 specimens) and the middle-Late Palaeocene sites (fourteen sites, 113 specimens) yield palaeolatitudes of 5.7°S and 4.0°N for the sampling localities in southern Tibet, respectively. These palaeolatitudes are much further north than expected and imply that crustal shortening between the Tethyan Himalaya and the Indian Shield since the time of India-Eurasia collision has occurred on the order of 1500 km at the longitude of Gamba/Duela. Comparison of our observed palaeolatitudes with palaeolatitudes for the Lhasa block during Cretaceous and Palaeocene times indicate that initial contact of the northern Indian margin with the Lhasa block was already established at 65-60 Ma with suturing completed at about 55-50 Ma.

Late Permian and Early Triassic evolution of the Northern Indian margin: carbon isotope and sequence stratigraphy

The Northern part of Great-India underwent an early rifting phase in the late Paleozoic, just at the end of the large scale Gondwanian glaciation. The beginning of the rifting processes is marked by large hiatus and discontinuities (para- conformities) between the early or middle Paleozoic sedimentary succession and the discontinuous middle-late Permian Traps and transgressive sediments. The Northern Indian passive margin consists of the present High and Lower Himalaya and a small part of the Indian craton and their sedimentary cover. The Permian rift shoulder is located in the Higher Himalaya, with part being in the underthrusted Lower Himalaya. The rim basin (landward of the shoulder) is well developed in the Pottawar- Salt Range area. From the rifting to the beginning of the drifting stages (early late Permian to late early Triassic time), the sedimentary evolution is characterised by three transgressive- regressive (T-R) second order cycles, two in the late Permian and one in the early Triassic. The break-up of the rift occurred during the second cycle (late Dzhulfian).In the Salt Range area, these three T-R cycles have been subdivided in eight third order sequences, five sequences for the upper Permian and three for the lower Triassic.At the end of Permian, hiatuses, gaps and local erosion of part of the margin are direct consequences of a first order relative sea-level fall; this is also the time of the largest extinction event of the Phanerozoic that deeply affected the carbonate productivity and the stratal patterns. With the following worldwide sea-level rise, a rapid and large scale transgression occurred in the early Triassic, well dated and recorded on the whole margin. High rate thermal subsidence gave way to generalized pelagic deposits about 2 My after the transgression.Profiles of whole rock inorganic carbon and oxygen isotopes from Guryul Ravine and Palgham sections in Kashmir, Nammal Gorge and Landu sections in Trans Indus Ranges (Pakistan), Thini Chu section in Kali Gandaki Valley, Central Nepal are presented in connection with the sequence stratigraphic analysis. The upper Permian record of high positive δ13C values are closely correlated with the second order T-R cycles and the third order sequences. The results presented in this study confirm the drastic drop of δ13C from the high positive values that characterised the upper Permian to lower values in the lower Triassic time. Stratigraphic correlation problems in the lower Triassic using carbon isotope geochemistry are briefly discussed. A positive δ13C excursion of 4–5% near the Smithian - Spathian substages boundary is observed for the first time. The δ18O values of samples from all the sections display major variations suggesting that the oxygen isotope record has been significantly affected by meteoric diagenesis, deep burial diagenesis or/and monsoon signature.

Typology of detrital zircon as a key to unravelling provenance in rift siliciclastic sequences (Permo-Carboniferous of Spiti, N India)

AbstractDetrital zircon populations from Carboniferous to Permian sandstones from the Lozar Section of Spiti, northern India, were analyzed with the typology method in order to obtain complementary information on the source areas of the sediments. Zircon grains were subdivided into several groups and subgroups, according to degree of abrasion and morphological features.First appearance of detrital zircons with distinct typologic signature within successive stratigraphic intervals provided useful data about the tectono-magmatic evolution of the northern Indian margin during Late Paleozoic rifting of Gondwana and initial opening of Neotethys. The base of the studied sequence (Lower Carboniferous Thabo Fm.) is characterized by a largely cratonic provenance, seemingly from the Indian Shield to the South. In the Upper Carboniferous Chichong Fm., first occurrence of typical zircons from anatectic granites and increasing abundance of granitoid detritus suggest rapid uplift and unroofing of anatectic rocks of pro...

Origin and Evolution of Turkish blocks during the Late Triassic from magnetostratigraphic data

A palaeomagnetic and magnetostratigraphic study has been carried out on lower Carnian and Norian rocks (Late Triassic) from four different sections of two different units of the Upper Antalya Nappes in the Taurus region of southwestern Turkey, the Bakirli Dag unit and the Kaymaz Dag unit. The origin of these nappes is uncertain and under debate. This study provides well defined directions of probable primary origin and good magnetostratigraphic correlations between the different sections, which allows us to propose a palaeogeographic interpretation. The Bakirli Dag unit of the Upper Antalya Nappes were located at a palaeolatitude of 6.9°N ± 1.5° during the early Carnian and at 10.2°N ± 1.0° during the latest Carnian-middle Norian. This slight northward movement is accompanied by a large clockwise rotation (29.5° ± 6.7°) which may correspond to an important breakup-phase of the north African margin during middle-late Carnian time. The palaeo­magnetic results from the Kavur Tepe section of the Kaymaz Dag unit, previously considered to belong also to the Upper Antalya Nappes, show a palaeolatitude of 17.4°S ± 1.1° during Norian time. This result surprisingly indicates that this unit was located in the vicinity of the northern Indian margin during the Late Triassic.

The Himalayan passive margin from Precambrian to Cretaceous times

The Himalaya passive margin should be separated into the Indian Himalaya and the Pakistan Himalaya, divided by the Nanga Parbat-Haramosh basement uplift in the northwestern syntaxes. The Indian passive margin shows a complex history involving splitting of microcontinents off the northern Gondwana margin from early Paleozoic times until the Jurassic. In contrast, the Pakistan Himalaya formed part of the stable Indian shelf until separation from Africa started in the Jurassic. Each margin has a distinct stratigraphic history until the mid-Mesozoic after which they show similar histories until the Tertiary.The Himalaya mountains in India consist of three distinct tectonic units juxtaposed during the Neogene by southward thrusting of the northern Indian Precambrian to Mesozoic passive margin. The Lesser Himalaya on the south forms a late Precambrian passive margin usually directly overlain by late Cretaceous to Tertiary clastics: its stratigraphy is closest to that of the Indian Shield. In contrast, the High Himalaya has a thin Ordovician to Carboniferous shelf sequence deposited after early Ordovician deformation and granite intrusion. Above this, thick Permian and Mesozoic shelf sequences mark the separation of continental blocks off the northern Indian margin and the opening of the Neotethys ocean. The North Himalaya formed the slope and basin of this ocean and consists of reactivated Paleozoic gneiss domes overlain by thick Mesozoic sediments which pass, in the northeastern Himalaya, into an enormously thick sedimentary sequence resting on oceanic crust.Paleomagnetic and structural evidence indicates at least 500 km of southward thrusting along the Main Central Thrust between the Lesser and High Himalaya. This thrusting, together with a further 250 km of erosion at the front of the nappes, has removed the entire inner shelf of the Himalaya passive margin and can explain the startling contrast between the High and Lesser Himalaya stratigraphic sequences. But there are still discrepancies, particularly in the eastern Himalaya, where Precambrian basement is juxtaposed with the North Himalaya Mesozoic slope. These discrepancies can be partly resolved by strike-slip movements roughly parallel to the Himalaya which have removed parts of the northern Indian passive margin. Such faults of the requisite orientation, displacement and age occur in southeast Asia where their cumulative displacements can add up to several thousand kilometres. Similar faults, of undoubted pre-Tertiary age, occur in the Pakistan Himalaya. Here, the passive margin shows a much simpler history of Jurassic to Miocene subsidence.

Different P-T-t paths and leucogranite occurrences along the High Himalayan Crystallines : Implications for subduction and collision along the northern Indian margin

Different P-T-t paths and a heterogeneous distribution of Tertiary leucogranites are recorded along the High Himalayan Crystallines (HHC), in the collided Indian plate. Rapid subduction with eclogite formation during the early-Eocene is apparently restricted to part of the HHC of northern Pakistan, that is the zone of the first collision between the narrow NW Indian spur and the Kohistan island arc. In northern Pakistan rapid exhumation occurred along a cooling path and the crustally derived leucogranites of Tertiary age are rare or absent. East of the spur the delayed India-Eurasia collision occurred at lower rates of convergence and determined less rapid and deep continental subduction. Here exhumation of the HHC occurred at lower rates and at increasing T, giving a relatively warm thickened crust which underwent extensive anatexis during uplift in Oligocene-Miocene times. In eastern Himalayas, anatexis at medium- and low-P produced numerous cordierite-andalusite-bearing leucogranites which emplaced along extensional shear zones at high structural levels during the Miocene.

Palaeomagnetic Results From the Tibetan Sedimentary Series of the Manang Area (North Central Nepal)

The remanent magnetization of Palaeozoic and Mesozoic carbonates from the Tibetan Sedimentary Series (TSS) of the Manang area north of the Annapurna Range (north central Nepal) has been investigated. the TSS forms part of the tectonic unit between the Indian-Eurasian plate boundary in the north and the Main Central Thrust (MCT) in the south. Apart from a present field direction, three remanence components have been separated: a normal polarity, post-folding remanence (negative fold test) is carried by pyrrhotite (the core corrected mean direction for five sites with 47 specimens from Silurian/Devonian rocks is decl. = 312.9°, incl. = 58.3°, α95 = 17.6°, k = 19.8). the origin of this component is not understood. A more precisely determined, reversed polarity, post-folding remanence (99 per cent negative fold test) is also carried by pyrrhotite (the core corrected mean direction for 12 sites with 229 specimens from Carboniferous/Triassic rocks is decl. = 196.4°, incl. = -65.9°, α95 = 3.2°, k = 183). One site of Lower Carinan age shows—in addition to the pyrrhotite component—a second, stable remanence direction carried by magnetite. the fold test is 99 per cent positive and there is a high probability that the remanence is of detrital origin (the bedding corrected direction for one site with 40 specimens is decl. = 334.3°, incl. = -54.1°, α95 = 4.2°, k = 30.2, palaeolatitude 34.6°S). The direction of the magnetite component coincides with other Triassic palaeodirection data from the literature and the inclination suggests a palaeoposition of the TSS at the northern margin of India at this time. the northern extent of India in the Triassic cannot be reconstructed from the data because of the strong anticlockwise rotation (48°) of stable India since this time. The inclination of the reverse polarity pyrrhotite component is 19° steeper than expected from the maximum northern position of the area. This suggests a regional northward tilt along the MCT which is supported by steep inclinations found in other areas. Declination data for the reverse polarity pyrrhotite component and for the magnetite component deviate clockwise from expected values for the area by 23°-34° (calculated from the Indian APWP). Utilizing Klootwijk, Conaghan & Powell's (1985) rotational underthrusting model, the minimum magnitude of continental underthrusting between the TSS and stable India at the longitude of central Nepal (84°E) has been calculated from the declination of our reverse polarity pyrrhotite component. A possible range of 520–1100 km results from consideration of different tectonic models.

Crustal growth and two-stage India-Eurasia collision in Ladakh

The post-Palaeozoic history of crustal growth between India and the Karakoram-Lhasa accreted southern margin of Asia begins with the creation of the oceanic crust during the northward drift of the Karakoram-Lhasa block, away from the northern margin of India, after Permo-Triassic rifting. Data suggest initiation of northward subduction and the development of the island arc close to the southern margin of Asia sometime in Upper Jurassic (160 m.y.). The northward retreat of the overriding Eurasian Plate due to movements along transform faults, which subsequently changed into transcurrent faults, resulted in the opening of the Shyok back-arc basin in the Kohistan-Ladakh region during pre-Upper Cretaceous time. The mature and thickened tholeiitic island arc lying between the subducting Indian Plate and the spreading Shyok back-arc basin suffered compression, deformation, emplacement of ophiolitic melanges and metamorphism during the Upper Cretaceous (80 m.y.). The intrusion of the calc-alkaline magma into the deformed rocks of the island arc and the back-arc basin during the Paleocene-Eocene (60-40 m.y.) was followed by explosive calc-alkaline volcanism (40-25 m.y.) resulting in further crustal growth in this part of the Trans-Himalaya. The collision of the Dras island arc with the approaching margins of the Indian and Eurasian plates took place in two stages. In the first stage it collided with the northern margin of India during the Paleocene along the Indus Suture. The final collision of the island arc-accreted northern margin of India with the Eurasian Plate took place along the Shyok Suture during the Early Oligocene (30 m.y.).

Sedimentology, petrography and tectonic significance of the shelf, flysch and molasse clastic deposits across the Indus Suture Zone, Ladakh, NW India

In the Ladakh area of India, a passive Triassic to Lower Cretaceous continental margin is indicated by Indian-shield-derived clastics on the shelf and Atlantic-type turbidites off the continental margin. Mid-Cretaceous initiation of ocean closing is reflected in Pacific-type flysch and associated island are volcanics, which were initially emplaced over the northern Indian continental margin in late Cretaceous times-resulting in the formation of a fore-deep in which flysch and minor continental molasse accumulated briefly during the late Cretaceous. These transient uplifts were, however, rapidly destroyed for by the latest Cretaceous to latest Palaeocene, uniform carbonate sediments were being laid down over the area. With the early Eocene, the development of a second fore-deep, this time filled with very thick flysch and molasse sediment, indicates a major uplift of the northern Indian margin, which we attribute to the development of an Andean-type magmatic arc on the northern edge of the Indian plate. Uplift and molasse sedimentation in this fore-deep continued through the Oligocene and Miocene, when the collision of India and Asia caused extensive deformation of all the sequences and the shift of molasse sedimentation southwards to the Himalaya foothills and Indo-Gangetic plain.