Stratigraphy and Tectonics of Ladakh, Eastern Karakoram, Western Tibet and Western Kun Lun

A new geological map on 1:1 million scale has been prepared for the first time, primarily to depict the various stratigraphic and tectonic units of northwestern Himalaya and adjoining Karakoram and Kun Lun orogens covering parts of Zanskar, Ladakh, eastern Karakoram and western Tibet. The map embodies the information published by earlier workers and by the present author.

The Great Himalaya (Higher Himalaya) in the southern part is made up of Precambrian high-grade metamorphic rocks, granites and migmatites. This terrane trends NW-SE and forms the basement of the sedimentary rocks of the Tethys Himalaya. The doubly plunging anticlinal fold of the Tso Morari represents a domal upwarp of the basement and characterized by high Pteclogite and Lower Palaeozoic granites that are emplaced within the Upper Mesozoic sedimentary rocks. The contact between the basement and the overlying Tethyan sedimentary pile is tectonic. The Tethys sequence comprises over 10 km thick sedimentary rocks, predominantly fossiliferous and ranging in age from Late Precambrian to Eocene. The south-hading Zanskar Thrust separates this succession from the highly tectonized and compressed assemblages of the India-Asia collision zone, the Indus-Tsangpo Suture. The ophiolite and tectonic melange squeezed out southwards from this suture forms the Spongtang Klippe that now rests on Jurassic to Eocene Tethyan sediments. In the Indus-Tsangpo Suture the northward-directed intra-oceanic subduction, first occurring in Late Jurassic to Cretaceous, culminated in the closing of the Neotethys around 60-50 Ma. A number of pre-collision accretionary processes shaped the geometry of different marine sedimentary basins within the suture zone.

The vast region across the Indus-Tsangpo Suture embraces the Ladakh Batholith, the Shyok Suture Zone in Ladakh and the Kohistan Arc Complex in northern Pakistan and the Karakoram Batholith. The basaltic to andesitic Dras Volcanics follow the zone of the Indus Suture and represent an island arc that was active between Late Jurassic and Late Cretaceous. The Ladakh block occupies an intermediate position between the Indian Plate and the Tibeto-Karakoram Plate, the Karakoram Fault forming the defining boundary. The Ladakh block comprises mainly the Cretaceous and Tertiary calcalkaline granites. The batholith is unconformably overlain along its southern boundary by the Miocene-Pliocene molassic Indus Group.

The Shyok Suture Zone north of the Ladakh block is interpreted as a suture embodying the rocks of a backarc basin. They are represented by intensely deformed tectonic slices between the Ladakh and the Karakoram Batholiths. The Saltoro limestone within this suture is a reefal complex that grew on volcanic basalts and andesites, with ignimbrites forming the seamount of Middle Cretaceous or older age. Similarity exists between the Shyok Suture of northern Ladakh and the Northern Suture of Kohistan, implying their evolution as a single tectonic unit during Cretaceous-Palaeogene. The Shyok Suture is older than the Indus Suture and was closed sometime between 100-75 Ma, the accretionary processes in the Karakoram having begun prior to the final closure of the Neotethys. The formation of the Nanga Parbat-Haramosh syntaxis separated Kohistan and Ladakh blocks.

The sediments of the eastern Karakoram Tethyan Zone are more or less similar to those of the adjoining Permian- Cretaceous Shaksgam Sedimentary Belt of the Northwestern Karakoram in Sinkiang and to those of western Qiangtang in western Tibet. The Karakoram Batholith granites are calc-alkaline and sub-alkaline in composition and intimately associated with volcanic rocks of the mountain arc of the Chilean type. Further north, the Karakoram Plutonic Complex intrudes the Carboniferous-Permian sedimentary sequence that is divisible into six litho - and tectonostratigraphic units. The occurrence of the 270-265 Ma (Early Permian) marine sediments characterized by Gondwanic plants and palynomorphs within the Chhongtash Formation indicates that the Karakoram was a Peri-Gondwanian microcontinent at latitudes of - 35" S during the Early Permian time, and lying somewhere between the Indian Plate and the Qiangtang- Lhasa microcontinent. The Karakoram microcontinent did not form a part of the Indian Plate and was welded to Asia around 130-120 Ma. Different phases of magmatic activity in the Ladakh and the Karakoram domains have contributed greatly to the welding together of the Indian and Asian plates. The active Karakoram Fault, characterized by -120 km dextral offset, is traceable NW along the Shyok Suture further up to Pamir. It constitutes the geomorphic boundary between the Ladakh, Karakoram and western Tibet blocks.

Kun Lun was the site of a Mid-Palaeozoic collision, and three phases of post-Palaeozoic accretion thickened the sedimentary cover in western Tibet. The left-lateral Altyn Tiigh-Karakash strike-slip fault zone is a major crustal lineament and defines the southern limit of the western Kun Lun that has been uplifting relative to Xbet. Strike-slip motions along the Karakoram and the Altyn Tagh faults were coeval with the overthrusting movements in the Himalaya and in the Kun Lun. Significantly, the northward subduction of India and the southward subduction of the Tarim in the north resulted in the eastward extrusion of Tibet.