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 (< 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, backthmsting 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 blocks), 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.

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 >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.

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.

Abstract Reconstruction of the Western Himalaya requires three subduction systems operating beneath the Spong arc, Dras–Kohistan arc and the Asian continent during the Late Cretaceous–Paleocene. The timing of the closure of the Neo-Tethys Ocean along the Indus Suture Zone (ISZ) in Ladakh and south Tibet has been proposed to be as old as c. 65 Ma and as young as c. 37 Ma. The definition of the India–Asia collision can span >15 myr from the first touching of Indian continental crust with Asian crust to the final marine sedimentation between the two plates. There is good geological evidence for a Late Cretaceous–Early Paleocene phase of folding, thrusting and crustal thickening of Indian Plate shelf carbonates associated with obduction of ophiolites. There is no geological evidence of any oceanic ‘Greater Indian Basin’ separating the northern Tethyan and Greater Himalaya from India. There is clear evidence to support final ending of marine sedimentation along the ISZ at 50 Ma (planktonic foraminifera zone P7–P8). There is no evidence for diachroneity of collision along the Pakistan–Ladakh–South Tibet Himalaya. The timing of ultrahigh-pressure metamorphism cannot be used to constrain India–Asia collision, and the timing of high-grade kyanite- and sillimanite-grade metamorphism along the Greater Himalaya can only give a minimum age of collision.