Spontang Ophiolite Research Articles

Geodynamic evolution of the Tethyan lithosphere as recorded in the Spontang Ophiolite, South Ladakh ophiolites (NW Himalaya, India)

The Spontang Ophiolite complex represents the most complete ophiolite sequence amongst the South Ladakh ophiolites and comprises mantle rocks (depleted harzburgites, dunites and minor lherzolites) as well as crustal rocks (basalt, isotropic gabbros, layered gabbros etc.). In the present study, detailed geochemistry (whole rock as well as mineral chemistry) and Sr-Nd isotopic analyses of thirty-six ultramafic- mafic samples have been attempted to constraint the evolution and petrogenetic history of the Tethyan oceanic crust. Major, trace-element and REE patterns of the peridotites and their minerals indicate that the lherzolites experienced lower degrees of partial melting resembling abyssal peridotites (at higher temperatures, TREE = ∼1216 °C) than the harzburgites (6%–8% versus 15%–17%). Elevated εNd(t) and variable 87Sr/86Sr(t) ratios along with REE patterns suggest that the Spontang mafic rocks display N-MORB affinity with negligible participation of oceanic sediments in their genesis are originated from a depleted upper mantle with little contribution from subduction-related fluids. MORB-type Neotethyan oceanic crust is associated with the earliest phase of subduction (of older Jurassic age) through which a younger intra-oceanic island arc (Spong arc) subsequently developed. Harzburgites REE display typical U-shaped patterns, suggesting that these rocks have been metasomatized by LREE-enriched fluids. On the other side, mafic rocks are characterized by heterogeneous (Nb/La)PM and (Hf/Sm)PM and relatively homogeneous εNd(t), indicating interaction of subduction-related melts with the upper mantle during the initiation of subduction, in Early Cretaceous times.

U–Pb dating of detrital zircon grains in the Paleocene Stumpata Formation, Tethyan Himalaya, Zanskar, India

The sediments deposited on the northern margin of Greater India during the Paleocene allow the timing of collision with the Spontang Ophiolite, the oceanic Kohistan–Dras Arc and Eurasia to be constrained. U–Pb dating of detrital zircon grains from the Danian (61–65Ma) Stumpata Formation shows a provenance that is typical of the Tethyan Himalaya, but with a significant population of grains from 129±7Ma also accounting for ∼15% of the total, similar to the synchronous Jidula Formation of south central Tibet. Derivation of these grains from north of the Indus Suture can be ruled out, precluding India’s collision with either Eurasia or the Kohistan–Dras before 61Ma. Despite the immediate superposition of the Spontang Ophiolite, there are no grains in the Stumpata Formation consistent with erosion from this unit. Either Spontang obduction is younger than previously proposed, or the ophiolite remained submerged and/or uneroded until into the Eocene. The Mesozoic grains correlate well with the timing of ∼130Ma volcanism in central Tibet, suggesting that this phase of activity is linked to extension across the whole margin of northern India linked to the separation of India from Australia and Antarctica at that time. Mesozoic zircons in younger sedimentary rocks in Tibet suggest a rapid change in provenance, with strong erosion from within or north of the suture zone starting in the Early Eocene following collision. We find no evidence for strongly diachronous collision from central Tibet to the western Himalaya.

The Indus–Yarlung Zangbo ophiolites from Nanga Parbat to Namche Barwa syntaxes, southern Tibet: First synthesis of petrology, geochemistry, and geochronology with incidences on geodynamic reconstructions of Neo-Tethys

The purpose of this first synthesis is to summarize findings on the Yarlung Zangbo Suture Zone (YZSZ) ophiolites and discuss still remaining problems. The YZSZ studied for almost 30years and is the youngest of the sutures recognized on Tibet Plateau. It is now acknowledged that the YZSZ is a complex assemblage of sedimentary, metamorphic and igneous rocks created during and shortly after the collision between India and Eurasia. The ages of the various lithologies span a time interval from the Jurassic to the Middle Miocene, with some Permian and Devonian exotic blocks from mélange zone. The YZSZ is characterized by ophiolitic complexes and ophiolitic mélange. The ophiolites are of two types: non dismembered and dismembered sections. The non disturbed sections, although tectonically reworked, are observed along the segment from Dazhuqu to Jiding in Xigaze area and Spontang ophiolite. The dismembered sequences are found in various locations such as Nidar, Kiogar, Jungbwa, Saga, Sangsang, Xigugabu, and Luobusa. The incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins. The ophiolites are distributed into two groups of ages: the Luobusa, Zedang and Kiogar sequences being Jurassic-Lower Cretaceous whereas all other sequences are of Lower Cretaceous age. Compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70My from their initial genesis to obduction which occurred around 70–90My ago. Ophiolites differ in terms of petrological and geochemical aspects however, they were all generated in suprasubduction zone and more specifically in arc (few fore-arc) and back-arc settings. Synthesis of more than 700 geochemical analyses show variable mixing of components from N-MORB-type to IAT-CAB and to OIB end-members. The Jurassic ophiolites show the maximum of arc component while the Lower Cretaceous ones show little to strong mixing. In addition, most ophiolites were created in short lived (30My) basins and generated close to the Eurasiatic continental margin. We propose that Ladakh–Tibet ophiolites were generated in a suprasubduction context similar to Mariana arc, interarc and back-arc or Tonga–Lau system. The variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system.

Was Late Cretaceous–Paleocene obduction of ophiolite complexes the primary cause of crustal thickening and regional metamorphism in the Pakistan Himalaya?

Abstract Regional metamorphic rocks in the Pakistan Himalaya include both UHP coesite eclogite-facies and MP/T kyanite–sillimanite-grade Barrovian metamorphic rocks. Age data show that peak metamorphism of both was c. 47 Ma. 40 Ar– 39 Ar hornblende cooling ages date post-peak metamorphic cooling of both through 500 °C by 40 Ma, some 20 Ma earlier than for metamorphic rocks in the central and eastern Himalaya. Typically these ages have been explained by obduction of the Kohistan arc onto the Indian plate at about 50 Ma and India–Asia collision. We suggest instead that the earlier metamorphic and cooling ages of the Pakistani Barrovian metamorphic sequence could be partially explained by Late Cretaceous to Early Paleocene crustal thickening linked to obduction of an ophiolite thrust sheet onto the leading edge of the Indian plate, similar to the Spontang Ophiolite in Ladakh. Heating following on from this Paleocene crustal thickening explains peak Barrovian metamorphism within 5–10 Ma of subsequent obduction of Kohistan. Remnants of the ophiolite sheet, and underlying Tethyan sediments, are preserved in NW India and in western Pakistan but not in northern Pakistan. Tectonic erosion removed all cover sequences (including the ophiolites) from the Indian plate basement.

Discussion on subsidence history of the north Indian continental margin, Zanskar–Ladakh Himalaya, NW India Journal , Vol. 162, 2005, pp. 135–146

E. Garzanti, D. Sciunnach & M. Gaetani write: In their recent paper, Corfield et al . (2005) attempted a subsidence analysis of the northwestern Tethys Himalaya stratigraphic succession, and discussed specifically the implications for the obduction of a section of oceanic lithosphere (Spontang Ophiolite) on top of the Indian passive continental margin (Zanskar Shelf). The emplacement age of the Spontang Ophiolite represents a crucial problem in Himalayan geology, which has been lively debated for two decades (e.g., Searle 1986; Kelemen et al. 1988; Garzanti et al . 1987; Guillot et al . 2003). In a series of papers, Mike Searle and co-workers have long proposed and supported the idea that the Spontang Ophiolite was obducted in the Late Cretaceous, making implicit or explicit correlation with the well-studied Semail Ophiolite of Oman (e.g, Searle 1983; 1986; Pedersen et al . 2001; Corfield et al . 2001, 2005). Semail and Spontang, however, are two very different ophiolite complexes. The Semail crust has a supra-subduction geochemical signature and was generated in the mid-Cretaceous (Searle & Cox 1999). Instead, the Spontang basalts have a MORB-like signature and were generated in the mid-Jurassic (Pedersen et al . 2001), similar rather to the other Oman ophiolite exposed on Masirah Island (Gnos et al . 1997). The analogy with the Semail Ophiolite obduction, associated with Late Cretaceous nappe stacking and high-pressure metamorphism of the underthrusted Arabian margin (Searle & Cox 1999), led Searle (2001) to reject the geochronological evidence for the Eocene age of high-pressure metamorphism of Indian-margin rocks (Kaghan and Tso–Morari eclogites; Tonarini et al . 1993; De Sigoyer et al . 2000). Because of the Eocene age of the eclogites, and of the fact that the Spontang Ophiolite lies tectonically on top of Indian margin sediments as …

Subsidence history of the north Indian continental margin, Zanskar–Ladakh Himalaya, NW India

Detailed geological field mapping has allowed the restoration of two full stratigraphic sections through the highly deformed Mesozoic and Early Tertiary fold and thrust belt of the north Indian continental margin. The two sections, which are representative of a proximal and a distal facies on the margin, have been backstripped using standard techniques. Profiles of the tectonic subsidence and uplift through the pre-collisional history of the margin have been constructed and compared with the predictions of simple thermal and mechanical models. The pre-collisional history can be explained by a thermal model with an initial age of rifting of 270 Ma and a stretching factor, β , of c . 1.2. This model accounts for the general exponential decrease in the backstripped tectonic subsidence. The model fails, however, to completely explain the subsidence and uplift history of the margin since the late Cretaceous. The history during this time is characterized by uplift at the most proximal location and an increase in subsidence at the most distal location. We attribute these differential vertical movements to flexural loading of the north Indian margin by obduction of the Spontang ophiolite. The best fit model is one in which a 70 km wide wedge-shaped load, tapering from 10 to 0 km thick, is emplaced on rifted lithosphere with an elastic thickness, T e , of 5–10 km. These results, which are in accord with the late Cretaceous timing of obduction and the structure of the Spontang ophiolite, provide new constraints on the T e structure of extended continental lithosphere 120–150 Ma after a rifting event.

The South Ladakh ophiolites (NW Himalaya, India): an intra-oceanic tholeiitic arc origin with implication for the closure of the Neo-Tethys

In the Ladakh–Zanskar area, two groups of ophiolites document closure of the Neo-Tethys Ocean during India–Asia convergence. They are localized north of the Indus Suture Zone and in South Ladakh, respectively. The South Ladakh group includes the Spontang ophiolite, with both crustal and mantle units, the Nidar ophiolite, with mainly crustal units, and the small Karzog ophiolite, with abundant chromitites and gabbros. The mafic rocks of the three South Ladakh ophiolites are tholeiites with similar compositions, displaying slight LREE depletion relative to HREE and enrichment in LILE relative to REE and HFSE. They are also characterized by a low Y content (<20 ppm) and a Ti/V ratio between 10 and 20. Such chemical features and similarity imply that these ophiolites were derived from the same N-MORB-like mantle source slightly enriched in LILE during a metasomatic event in a supra-subduction zone setting. We suggest the South Ladakh ophiolites are relics of an intra-oceanic arc. According to 39Ar/ 40Ar dating on amphiboles from both Spontang and Nidar mafic rocks, this arc developed at around 110–130 Ma. The associated ultramafic rocks show a decoupling between LREE and LILE (both enriched relative to abyssal peridotites) and HREE (which decrease from lherzolites to harzburgites). This contrasting behavior reflects the depletion of residues during partial melting and subsequent metasomatism in a supra-subduction zone setting. REE modeling indicates that the Spontang and Nidar lherzolites could not have been the source of the mafic rocks because their partial melting would have produced magmas more depleted in LREE. The South Ladakh ophiolites are clearly distinct from the northern ophiolites and require the existence of another north-dipping subduction zone. Thus, the closure of the Neo-Tethys was not accommodated by a single but by two subduction zones in the Ladakh–Zanskar area and from Oman to Tibet, respectively.

Tectonic Setting, Origin, and Obduction History of the Spontang Ophiolite, Ladakh Himalaya, NW India

The Spontang ophiolite forms the highest tectonic thrust slice above the Mesozoic–Early Tertiary continental margin of the north Indian plate in the Ladakh‐Zanskar Himalaya. Detailed field mapping, combined with geochemical analysis, has defined two major units: a full ophiolite sequence (Spontang ophiolite) overlain by an upper unit consisting of >500‐m‐thick basalt‐andesite volcanic and volcano‐sedimentary rocks of island arc affinity (Spong arc). The Spontang ophiolite comprises a harzburgitic mantle sequence, gabbroic and ultramafic cumulates, isotropic gabbros, and highly tectonized sheeted dikes feeding pillow lavas, with a few uncommon highly fractionated plagiogranites. A separate lherzolitic peridotite unit, affected by possible transform‐related shearing, was thrust over the harzburgites in the west, possibly during the early stages of subduction initiation beneath the ophiolite. Whole‐rock geochemistry and U‐Pb geochronology show that the ophiolite formed at a normal mid‐ocean ridge spreading center during the mid‐Jurassic and that the intraoceanic island arc sequence was erupted on top of the oceanic basement during the Campanian. The Spong arc is interpreted to have formed above a northward‐dipping subduction zone that was responsible for the obduction of the Spontang ophiolite during the Late Cretaceous–Early Paleocene. Although the Spong arc shows many similarities to the andesitic Dras arc within the Indus suture zone, structural, tectonic, and palaeomagnetic constraints indicate that the Spong arc was a separate intraoceanic island arc. This interpretation requires three northward‐dipping Tethyan subduction zones during the Late Cretaceous, beneath the Spong arc, Dras‐Kohistan intraoceanic arcs, and the southern margin of Asia, similar to the western Pacific region today.

U–Pb zircon ages from the Spontang Ophiolite, Ladakh Himalaya

The Spontang Ophiolite in the Ladakh Himalaya consist of a thrust sheet of upper mantle and oceanic crustal rocks obducted onto the northern passive continental margin of India during the late Cretaceous. Precise isotopic ages of Himalayan ophiolites are presently unavailable. U–Pb dating of zircons from a dioritic segregation in the high-level gabbros of the Spontang ophiolite shows that the complex formed at 177±1 Ma (mid-Jurassic) and we suggest that it represents a fragment of Neo-Tethyan oceanic crust. This Jurassic age contrasts with late Cretaceous ages (91–95 Ma) previously reported for the formation of the Troodos and Oman ophiolite complexes further west along the Neo-Tethyan suture zone and also predates the early to mid-Cretaceous Masirah ophiolite (c. 150 Ma) and its alkaline volcanic seamounts (115–125 Ma). An andesitic rock from an arc sequence overlying the Spontang ophiolite was also dated using as 88±5 Ma. This Late Cretaceous age constrains the minimum age of initiation of subduction beneath the Spontang Ophiolite. The complex was emplaced during the Turonian–Maastrictian, some 80 Ma after its formation when the Indian continental margin collided with a Cretaceous arc about 25 Ma before the India–Asia collision. The lack of a sub-ophiolite metamorphic sheet beneath the Spontang mantle sequence may be explained by the fact that the ophiolite was old and cold at the time of obduction.

Crustal shortening estimates across the north Indian continental margin, Ladakh, NW India

Abstract A structural and lithological map has been produced covering the Spontang ophiolite and the north Indian continental margin from the Indus Suture Zone in the north to the high-grade metamorphic rocks and granites of the High Himalaya in the south. Cross-section balancing techniques have been used to identify, quantify and sequentially restore three major phases of deformation (D1–3) affecting the north Indian continental margin resulting in &gt; 85 km (280%) shortening. D1 in the late Cretaceous involved obduction of the Spontang ophiolite, associated Neo-Tethyan thrust sheets and Mesozoic continental slope deposits onto the outer passive margin. D1 was responsible for 200% shortening by internal folding and duplex formation within stratigraphic units in the outer shelf, but did not affect the innermost parts of the Indian passive margin. Restoration of later structures suggests that the allochthonous thrust sheets were emplaced a minimum of 70 km onto the continental margin. D2 from the early Eocene to Oligocene was the main phase of deformation associated with the collision of India and Asia. Re-thrusting places the Spontang ophiolite and associated mélanges over the Maastrichtian to Lower Eocene neo-autochthonous cover which accommodated 140–160% shortening in the hanging wall. D2 progressed with the propagation of thrusting down section and towards the foreland causing crustal thickening and Barrovian metamorphism. The thick, argillaceous late Cretaceous Kangi La Formation decoupled deformation in higher and lower structural levels in outer shelf areas. D3 backthrusting and break-backthrusting in the late Tertiary formed a pop-up structure at the northern edge of the continental margin associated with a further 190–230% shortening and inversion of structures in the Indus Suture Zone. A major anticlinal structure also initiated across the southern edge of the Indus Suture Zone. South of Spontang reactivation of D2 thrusts as late D2/D3 normal faults was associated with gravitational collapse of the High Himalaya to the south. Extensional movement on these structures was probably concomitant with shortening in the pop-up structure to the north. A reduction in present day and restored thickness of the Tethyan Himalaya and an increase in absolute shortening from east to west probably reflects a variation in the partitioning of deformation across the whole width of the orogen. This may be associated with the indentation of India into Asia to the northwest.

Formation of mélanges in the Indus Suture Zone, Ladakh Himalaya by successive subduction-related, collisional and post-collisional processes during Late Mesozoic-Late Tertiary time

Abstract The classic Indus Suture Zone in western Ladakh includes two zones (southern and northern) of highly dismembered rocks, that in the past were widely interpreted as ophiolitic mélanges, created mainly by subduction/accretion processes. The ‘ophiolitic mélange’ was reported to include chaotically distributed blocks of ophiolitic rocks (e.g. serpentinite, gabbro, basalt) and sedimentary rocks (chert, limestone) set in a matrix of deep-sea clastic sediments. This accretionary hypothesis is tested in this paper and found to be inadequate. Units formed simply by oceanic subduction-accretion (i.e. local mud-matrix mélange) are minimal (&lt;1% by volume). In reality, the southern and northern mélange zones are the end products of complex multi-stage tectonic processes, involving subduction (mid-Late Cretaceous to Early Tertiary), initial emplacement (latest Cretaceous), collision (Palaeocene-Eocene) and post-collisional (Late Tertiary) stages. Important components of the mélange as a whole include thrust sheets and broken formation of relatively coherent volcanic-sedimentary successions related to the North Indian passive margin (Karamba and Lamayuru complexes), also mid-Late Cretaceous oceanic arc-type volcanics and volcaniclastic sediments (Dras arc complex). Dismembered serpentinite thrust sheets, cut by swarms of (subduction influenced) diabase dykes, most likely record detached oceanic basement related to the oceanic Dras arc complex. Associated serpentinite was injected along tectonic contacts and into adjacent units during collisional and post-collisional deformation, locally forming serpentinite mélange. Post-collisional, Early-mid-Tertiary non-marine coarse clastic sediments (Indus Group) unconformably overlie various units (serpentinite, volcaniclastics, blueschists) in different locations, and were later thrusted and folded within the southern and northern mélange zones in response to regional Late Tertiary tightening of the suture. As a result, backthrusting reversed the original thrust stacking order, placing the originally most southerly units at the highest structural level. The Indus Suture Zone ‘ophiolitic mélanges’ in western Ladakh are, thus, mainly not ophiolitic (i.e. dismembered oceanic crust), or mélange in entirety (i.e. chaotic block), but are instead dominated by disrupted thrust sheets and broken formation of the Mesozoic Lamayuru-Karamba continental margin and the Upper Cretaceous oceanic Dras arc complex, with only minor preserved remnants of units formed by subduction/accretion (e.g. mud-matrix mélange; oceanic exotics). The southern and northern mélange zones reflect the existence of several N-dipping subduction zones active in latest Cretaceous-Palaeogene times. Although only minor volumes of accreted oceanic material (oceanic lithosphere and trench-type sediments) are preserved within the Indus Suture Zone in western Ladakh, additional accreted material was bulldozed further south onto the Zanskar continental shelf and is now preserved as mélanges beneath the Spontang ophiolite. Similar mélange is also preserved in eastern Ladakh. Subduction zones evolved into a major interconnected shear zone (suture) during Early Tertiary (54–50 Ma) collisional deformation, dismembering upper (oceanic arc) and lower plate (continental margin) units to produce most of the southern and northern mélange zones. After suturing and initial deposition of non-marine coarse clastic cover sediments, inherited weakness zones within the Indus Suture Zone were exploited, increasing disruption during Late Tertiary regional backthrusting. The methods used here in the analysis of the Indus Suture Zone mélange could well prove to be useful elsewhere in the Himalaya and in other orogenic belts. This work also emphasizes the need to distinguish between true mélange v. thrust sheets and broken formation in the field, and also cautions against use of the term ophiolitic mélange unless all the components of a dismembered ophiolite are actually present together.

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The western syntaxis of the Himalaya is one of the most exciting frontiers of continental tectonis studies. The region around the mountain of Nanga Parbat has some of the highest peaks, deepest valleys anf highest uplift, exhumation and erosion rates known on earth. Surrounding regions include the Hindu Kush and Karakoram mountains (Asian plate), the Kohistan island arc and the Ladakh and Zanskar ranges of the western Himalaya (Indian plate). This volume includes 24 papers on all these regions as well as five new fold-out maps of the eastern Hindu Kush, the Spontang Ophiolite region of Ladakh, part of the west margin of the Indian plate, the Indus syntaxis in Pakistan and the Bouguer gravity anomalies in Pakistan.

Photang thrust sheet: an accretionary complex structurally below the Spontang ophiolite constraining timing and tectonic environment of ophiolite obduction, Ladakh Himalaya, NW India

The pre-collisional tectonic evolution of the north Indian continental margin is best recorded in the few ophiolite complexes preserved, the largest of which occurs in the Spontang area of the Himalayas. Structural, sedimentological, palaeontological and geochemical work on the ophiolite and associated allochthonous thrust sheets has been carried out to constrain the timing and tectonic environment of ophiolite obduction. A distinct thrust sheet of accretionary complex rocks has been identified immediately underlying the ophiolite. Accreted units include thrust slices of tectonic melanges and alkaline basaltic lavas capped by limestones ranging from late Permian to late Cretaceous in age, interpreted as remnants of former seamounts. The accretionary complex formed above a north dipping intra-oceanic subduction zone during the Cretaceous, the Spontang ophiolite located in the hanging wall. Beneath the Photang thrust sheet, two further distinct, allochthonous thrust sheets of sedimentary melanges and continental slope deposits have been recognized. The structural relations of the allochthonous thrust sheets with the sediments of the north Indian margin have been mapped in detail and show clear evidence that obduction occurred in the late Cretaceous. At this time the Dras-Kohistan intra-oceanic arc had already collided with the southern Asian margin, over 1500 km to the north. Obduction of the Spontang ophiolite therefore records a separate tectonic episode in the Ladakh Himalaya.

Rift-related sedimentation and volcanism of the north-Indian margin inferred from a Permian–Triassic exotic block at Lamayuru, Indus suture zone (Ladakh Himalaya) and regional comparisons

Stratigraphical, petrographical and geochemical work on an important limestone exotic within the Indus suture zone, near Lamayuru, reveals the following history of events: (i) Late Permian deposition on a shallow-water carbonate platform open to the Neotethys Ocean; (ii) Late Permian–Early Triassic break-up, tilting, fissuring and subsidence; (iii) Early Triassic mantling by ammonite-bearing, pink pelagic carbonate and chlorite-rich volcaniclastic sediment; (iv) Deposition of Fe, Mn- and/or Mn-rich oxide-sediments, as crusts and fissure fills. The metal-rich oxides were precipitated from hydrothermal solutions related to nearby volcanism; (v) Late Triassic? eruption of pillowed and massive basalts of within-plate type on an irregular seafloor. Taking account of evidence from similar exotic units associated with Upper Cretaceous melange beneath the Spontang Ophiolite further south in the Zanskar Mountains (e.g. Photoskar unit), the Lamayuru exotic is interpreted as part of a small carbonate platform formed somewhere between the Zanskar/Lamayuru passive continental margin and a Permian–Triassic Neotethyan spreading axis to the north. The carbonates probably capped a volcanic seamount that later collapsed and was covered by pelagic sediments and basic extrusives. The Lamayuru exotic was later detached from its igneous basement and tectonically emplaced, probably in the latest Cretaceous, and was also influenced by Early Tertiary suturing of Neotethys, and mid–late Tertiary backthrusting.

Structure of the North Indian continental margin in the Ladakh–Zanskar Himalayas: implications for the timing of obduction of the Spontang ophiolite, India–Asia collision and deformation events in the Himalaya

The collision of India and Asia can be defined as a process that started with the closing of the Tethyan ocean that, during Mesozoic and early Tertiary times, separated the two continental plates. Following initial contact of Indian and Asian continental crust, the Indian plate continued its northward drift into Asia, a process which continues to this day. In the Ladakh–Zanskar Himalaya the youngest marine sediments, both in the Indus suture zone and along the northern continental margin of India, are lowermost Eocene Nummulitic limestones dated at ∼54 Ma. Along the north Indian shelf margin, southwest-facing folded Palaeocene–Lower Eocene shallow-marine limestones unconformably overlie highly deformed Mesozoic shelf carbonates and allochthonous Upper Cretaceous shales, indicating an initial deformation event during the latest Cretaceous–early Palaeocene, corresponding with the timing of obduction of the Spontang ophiolite onto the Indian margin. It is suggested here that all the ophiolites from Oman, along western Pakistan (Bela, Muslim Bagh, Zhob and Waziristan) to the Spontang and Amlang-la ophiolites in the Himalaya were obducted during the late Cretaceous and earliest Palaeocene, prior to the closing of Tethys.The major phase of crustal shortening followed the India–Asia collision producing spectacular folds and thrusts across the Zanskar range. A new structural profile across the Indian continental margin along the Zanskar River gorge is presented here. Four main units are separated by major detachments including both normal faults (e.g. Zanskar, Karsha Detachments), southwest-directed thrusts reactivated as northeast-directed normal faults (e.g. Zangla Detachment), breakback thrusts (e.g. Photoksar Thrust) and late Tertiary backthrusts (e.g. Zanskar Backthrust). The normal faults place younger rocks onto older and separate two units, both showing compressional tectonics, but have no net crustal extension across them. Rather, they are related to rapid exhumation of the structurally lower, middle and deep crustal metamorphic rocks of the High Himalaya along the footwall of the Zanskar Detachment. The backthrusting affects the northern margin of the Zanskar shelf and the entire Indus suture zone, including the mid-Eocene–Miocene post-collisional fluvial and lacustrine molasse sediments (Indus Group), and therefore must be Pliocene–Pleistocene in age. Minimum amounts of crustal shortening across the Indian continental margin are 150–170 km although extreme ductile folding makes any balancing exercise questionable.

Early stages of foreland basin evolution in the Lesser Himalaya, N India

Abstract In this paper we report preliminary results of an integrated sedimentological and structural study of the Late Cretaceous-Early Tertiary facies exposed in the Lesser Himalaya. The Precambrian-Palaeozoic basement of the Indian plate was initially transgressed in Late Cretaceous times (after c. 100 Ma), with the deposition of shallow shelf carbonates, (50 m thick Singtali Formation). These are depositionally overlain by green ferruginous mudstones and thin limestones of the 150 m thick Palaeocene-Eocene ( c. 65–40 Ma) Subathu Formation. During Late Eocene-Oligocene times ( c. 40–30 Ma), clastic sediment input increased and red lenticular sandstones, thinner planar-bedded sandstones, mudstones and caliche of the 350 m thick Dagshai Formation were deposited. A semi-arid, meandering fluvial/floodplain setting is envisaged, with thick channelised and overbank sands. Palaeocurrent evidence indicates a general SE progradation. Our detailed field mapping shows that where the entire Subathu and Dagshai Formations are intercalated, this is the result of post-depositional tectonics, rather than primary intertonguing as described in some previous reports. The relative abundance of sandstones greatly increased in upper Dagshai times, continuing into the Early-Mid-Miocene ( c. 30–10 Ma) when the 250 m thick Kasauli Formation was laid down. This formation is characterized by lenticular and planar-bedded grey sandstones, rich in plant material including occasional logs (but without caliche) interbedded with minor grey mudstones. A rapidly prograding, braided fluvial environment is proposed, with the petrography (e.g. presence of garnets) suggesting erosion and derivation from deeper levels of the Himalayan mountain belt to the north. The climate had by then changed from semi-arid to humid, possibly in response to the onset of the monsoon, initiated when the mountain belt had reached sufficient height to interfere with the jet-stream. During Late Miocene-Quaternary times (younger than c. 15 Ma), the Tertiary foreland basin was deformed and incorporated into three structural levels of a southward migrating thrust stack. At the lowest level, thick successions of the Tertiary Subathu, Dagshai and Kasauli Formations are structurally underlain by fluvial sediments (Siwalik Gp.) and overlain by the Krol Precambrian-Palaeozoic sedimentary nappe. At the intermediate structural level, Early Tertiary sediments unconformably overlie the stratigraphy within this nappe and comprise, often thin (&lt;50 m), successions of mainly Subathu and Dagshai Formation sediments. The highest structural level sediments form small imbricates, highly dismembered units and melange (including evaporites) beneath the major metamorphic Jutogh Nappe. We interpret the Late Cretaceous-Palaeocene Singtali Formation as pre-collisional transgressive sediments related either to flexural downwarping of the Indian plate due to the obduction of the Spontang ophiolite onto the north Indian plate margin, or to extensional tectonics related to the detachment and drift of India from Gondwana and Neo-Tethyan subduction beneath Asia. This would have been coupled with the Late Cretaceous global eustatic sea-level high stand. The marine Subathu Formation is interpreted as having been deposited between initial and terminal India-Eurasia continental collision, with the overlying fluvial/floodplain Late Eocene-Oligocene Dagshai and Miocene Kasauli Formations as southward prograding foreland basin successions related to progressive stages of India-Eurasia continental collision.

Collision tectonics of the Ladakh-Zanskar Himalaya

The collision of the Indian Plate with the Karakoram-Lhasa Blocks and the closing of Neo-Tethys along the Indus Suture Zone (ISZ) is well constrained by sedimentologic, structural and palaeomagnetic data at ca. 50 Ma. Pre-collision high P— low T blueschist facies metamorphism in the ISZ is related to subduction of Tethyan oceanic crust northwards beneath the Jurassic-early Cretaceous Dras island arc. The Spontang ophiolite was obducted south westwards onto the Zanskar shelf before the Eocene closure (Dl). The youngest marine sediments on the Zanskar shelf and along the ISZ are Lower Eocene, after which continental molasse deposition occurred. After ocean closure, thrusting followed a SW-directed piggy-back sequence (D2). This has been modified by late-stage breakback thrusts, overturned thrusts and extensional normal faulting associated with culmination collapse and underplating. The ISZ and northern Zanskar shelf sequence are affected by late Tertiary redirected backthrusting (D3), which also affects the Indus molasse. A 50 km wide ‘pop-up’ zone with divergent thrust vergence was developed across the Zanskar Range. Balanced and restored cross sections indicate a minimum of 150 km of shortening across the Zanskar shelf and ISZ. Post-collision crustal thickening by thrust stacking resulted in widespread Barrovian metamorphism in the High Himalaya that reached a thermal climax during Oligocene-Miocene times. Garnet-biotite-muscovite + tourmaline granites were generated by intracrustal partial melting during the Miocene within the Central Crystalline Complex. Their emplacement on the hangingwall of localized ductile shear zones was associated with SW-directed thrusting along the Main Central Thrust (MCT) zone and concomitant culmination collapse normal faulting along the Zanskar Shear Zone (ZSZ) at the top of the slab. Metamorphic isograds have become inverted by post-metamorphic SW-verging recumbent folding and thrusting along the base of the High Himalayan slab. Along the top of the slab, isograds are the right way up but are structurally and thermally telescoped by normal faulting along the ZSZ. 1

Structural evolution and sequence of thrusting in the High Himalayan, Tibetan-Tethys and Indus Suture zones of Zanskar and Ladakh, Western Himalaya: Reply

Abstract The timing of motion on major thrusts in the Western Himalaya shows an extremely complex sequence that spans approximately 70 Ma from the latest Cretaceous throughout the Tertiary. Three major phases of thrusting can be distinguished. The earliest phase (T1) is associated with emplacement of Tethyan basin thrust sheets (Lamayuru sediments and Spontang ophiolite) south and south-westwards onto the submerged northern passive margin of India (75-60 Ma). Collision between India and Asia occurred at 50-36 Ma and was followed immediately by the major phase (T2) of crustal shortening involving large-scale south and south-westward directed thrusting of the complete Palaeozoic, Mesozoic and Late Tertiary Tibetan—Tethys zone rocks. Preliminary balanced cross-sections show a minimum shortening of 126 km of these rocks across the Zanskar Range. The late collision phase (T3) involved re-thrusting of the previously stacked pile (breaching or leap-frog thrusting) reversing the earlier stacking order in places, and widespread steepening, overturning and backthrusting along the whole northern margin of the Tibetan—Tethys zone and throughout the Indus suture zone.

Structural evolution and sequence of thrusting in the High Himalayan, Tibetan—Tethys and Indus suture zones of Zanskar and Ladakh, Western Himalaya

The timing of motion on major thrusts in the Western Himalaya shows an extremely complex sequence that spans approximately 70 Ma from the latest Cretaceous throughout the Tertiary. Three major phases of thrusting can be distinguished. The earliest phase (T1) is associated with emplacement of Tethyan basin thrust sheets (Lamayuru sediments and Spontang ophiolite) south and south-westwards onto the submerged northern passive margin of India (75-60 Ma). Collision between India and Asia occurred at 50-36 Ma and was followed immediately by the major phase (T2) of crustal shortening involving large-scale south and south-westward directed thrusting of the complete Palaeozoic, Mesozoic and Late Tertiary Tibetan—Tethys zone rocks. Preliminary balanced cross-sections show a minimum shortening of 126 km of these rocks across the Zanskar Range. The late collision phase (T3) involved re-thrusting of the previously stacked pile (breaching or leap-frog thrusting) reversing the earlier stacking order in places, and widespread steepening, overturning and backthrusting along the whole northern margin of the Tibetan—Tethys zone and throughout the Indus suture zone.

Stratigraphy, structure and evolution of the Tibetan–Tethys zone in Zanskar and the Indus suture zone in the Ladakh Himalaya

ABSTRACTThe Tibetan–Tethys zone of the Zanskar Himalaya shows a complete Mesozoic shelf carbonate sequence overlying metamorphic basement of the Central crystalline complex and Palaeozoic sedimentary rocks. Continental rifting in the Permian produced the alkaline and basaltic Panjal volcanic rocks and by Triassic time a small ocean basin was developed in the Indus-Tsangpo zone. Stable sedimentation continued until the Middle-Late Cretaceous when a thick sequence of tholeiitic to andesitic island arc lavas (Dras arc) were erupted in the basin above a N-dipping subduction zone. The Spontang ophiolite was emplaced southwards onto the Zanskar shelf edge during latest Cretaceous or earliest Tertiary times.Following emplacement of the Spontang ophiolite, deep-sea sedimentation ended abruptly with initial collision between the Indian plate and the Dras island arc. Emplacement of the massive Ladakh (Trans-Himalayan) batholith along the southern margin of Tibet in late Cretaceous-Eocene time occurred by crustal melting as a result of northward subduction of Mesozoic oceanic crust along the Indus subduction zone. Southward-directed thrusting in both Zanskar and Indus zones accompanied ocean closure during the late Cretaceous–Eocene. Late Tertiary compression caused intense folding, overturning and a phase of northward-directed thrusting along the Indus suture zone and the northern margin of the Tibetan–Tethys zone, resulting in a large amount of crustal shortening.