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

Abstract

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.

Crustal–lithospheric structure and continental extrusion of Tibet

“…We define the final India-Asia collision as the timing of the final marine sedimentation (planktonic foraminifera zone P8; shallow benthic zone SBZ10; Green et al 2008) in the suture zone and along the north Indian plate margin (Garzanti et al 1987;Searle et al 1988Searle et al , 1997bRowley 1998;Zhu et al 2005;Green et al 2008). Ophiolite obduction southward onto the passive margin of Indian preceded India-Asia collision and most probably occurred during the Late Cretaceous-Palaeocene (Searle et al 1988(Searle et al , 1997b.…”

“…Kongur, Muztagh Ata) are not offset across the fault (Robinson et al 2007;Robinson 2009a,b). The southeastern end of the Karakoram Fault merges into the Indus-Yarlung Tsangpo suture zone south of Mount Kailas, where north-directed backthrusting occurs along the northern margin of the Indian plate (Searle 1996;Searle et al 1997b) and north-south extensional faulting occurs in the Purang graben (Murphy et al 2000;Murphy & Yin 2003). The Karakoram Fault forms one major strand along the Nubra Valley but in the Tangtse region the fault splays into two branches, the main strand, the Tangtse Fault strand, to the SW and the Pangong Fault strand to the NE, both of which display spectacular mylonites (Fig.…”

The Quaternary glacial history of the Zanskar Range, north-west Indian Himalaya

“…Prior to the India-Asia collision, India's northern continental periphery was a passive margin of the Tethys Ocean, with late Permian and Mesozoic shelf carbonates deposited over Pennian volcanics and Palaeozoic sediments (Searle et aI., 1997). During the Cretaceous, ophiolite slabs were obducted from the margins of the Tethys basin (Searle, 1983;Sinha, 1992;Osmaston, 1994;Searle et al, 1997) and the fore-arc basin of the Dras island arc was closed. Northward subduction of Mesozoic oceanic crust created an Andean style destructive plate margin and the emplacement of the Trans-Himalayan granite batholith (1 in Fig.…”

40Ar–39Ar dating of volcanic rocks of the Shyok suture zone in north–west trans-Himalaya: Implications for the post-collision evolution of the Shyok suture zone

“…These formations could not be traced to the east of Ladakh sector in Lhasa and southern Tibet region and remain a peculiarity of the northwestern Himalaya. However, tectonic models proposing two subduction zones in general assume two sutures along the entire south Asian boundary (Brookfield and Reynolds, 1981;Rai, 1983;Reynolds et al, 1983;Coward and Butler, 1985;Petterson and Windley, 1985;Srimal, 1986;Sharma, 1987;Treloar et al, 1989;Searle et al, 1997;Weinberg and Dunlap, 2000;Dunlap and Wysoczanski, 2002;Robertson and Collins, 2002;Dubois-Cote et al, 2005;Dupuis et al, 2005). This has resulted in various attempts of correlating older sutures present in Lhasa-Tibet block with the Shyok suture (Dunlap and Wysoczanski, 2002;Lacassin et al, 2004).…”

“…Brookfield and Reynolds (1981), and Reynolds et al (1983) suggested that the Shyok suture closed in Miocene after the Late Cretaceous suturing along the Indus suture. However, Coward and Butler (1985), Petterson and Windley (1985), Treloar et al (1989) and Searle et al (1997) proposed that the Shyok suture is older than the Indus suture. Rai (1983) reported upper Cretaceous to Eocene marine fossils from the flysch of the Shyok suture zone.…”