2.3 Major tectonic subdivisions of the Himalaya

One of the most striking aspects of the Himalayan orogen is the lateral continuity of its major tectonic elements. Since Blanford & Medlicott, 1879 and Heim & Gansser, 1939, the Himalaya is classically divided into four tectonic units than can be followed for more than 2400 km along the belt (Fig. 2.2 ). In this work we will stick to these divisions but we introduce a slightly different terminology suggested by Steck (oral comm.). The ancient or frequently used terms are given within brackets.

1) The Subhimalaya forms the foothills of the Himalayan Range and is essentially composed of Miocene to Pleistocene molassic sediments derived from the erosion of the Himalaya. These molasses known as Muree and Siwaliks Formations are internally folded and imbricated. The Subhimalaya is thrust along the Main Frontal Thrust over the quaternary alluvium deposited by the rivers coming from the Himalaya (Ganges, Indus, Brahmaputra...), which demonstrates that the Himalaya is still a very active orogen.

2) The Lesser Himalaya, LH is mainly formed by Upper Proterozoic to Lower Cenozoic detrital sediments from the passive Indian margin intercalated with some granites and acid volcanics (1840± 70 Ma, Frank et al., 1977). These low-grade sediments are thrust over the Subhimalaya along the Main Boundary Thrust (MBT). The Lesser Himalaya often appears in tectonic windows (Kishtwar or Larji-Kulu-Rampur windows) within the High Himalaya Crystalline Sequence.

3) The Central Himalayan Domain, CHD (or High Himalaya) forms the backbone of the himalayan orogen and encompasses the areas with the highest topographical relief. It is commonly separated into four zones.

The High Himalayan Crystalline Sequence, HHCS (approximately 30 different names exist in the literature to describe this unit. The most frequently found equivalents are Greater Himalayan Sequence, Tibetan Slab and High Himalayan Crystalline) is a 30 km thick, medium- to high-grade metamorphic sequence of metasedimentary rocks which are frequently intruded by granites of Ordovician (~ 500 Ma) and Lower Miocene (~ 22 Ma) age. Although most of the metasediments forming the HHCS are of Upper Proterozoic to Lower Cambrian age, much younger metasediments can also be found in several areas (Mesozoic in the Tandi syncline and Warwan region, Permian in the Tschuldo slice, Ordovician to Carboniferous in the Sarchu Area). It is now generally admitted that the metasediments of the HHCS represent the metamorphic equivalent of the sedimentary series forming the base of the overlying Tethys Himalaya. The HHCS forms a major nappe which is thrust over the Lesser Himalaya along the Main Central Thrust (MCT).

The Tethys Himalaya, TH is an approximately 100 km large synclinorium formed by strongly folded and imbricated, weakly metamorphosed sedimentary series. Several nappes, termed North Himalayan Nappes (Steck et al. 1993) have also been evidenced within this unit. An almost complete stratigraphic record ranging from the Upper Proterozoic to the Eocene is preserved within the sediments of the TH. The stratigraphic analyses of these sediments yields important indications on the geological history of the northern margin of the Indian continent from its Gondwanian evolution to its continental collision with Eurasia. The transition between the generally low-grade sediments of the Tethys Himalaya and the underlying low- to high-grade rocks of the High Himalayan Crystalline Sequence is usually progressive. Yet, in many places along the Himalayan belt, this transition zone is marked by a major extensional structure, the Central Himalayan Detachment System (also known as South Tibetan Detachment System or North Himalayan Normal Fault).

The Nyimaling–Tso Morari Metamorphic Dome, NTMD: In the Ladakh region, the Tethys Himalaya synclinorium passes gradually to the north in a large dome of greenschist to eclogitic metamorphic rocks. As with the HHCS, these metamorphic rocks represent the metamorphic equivalent of the sediments forming the base of the Tethys Himalaya. The Precambrian Phe Formation is also here intruded by several Ordovician (~480 Ma; Girard and Bussy, 1998) granites.

The Lamayuru and Markha Units, LMU are formed by flyschs and olistholiths deposited in a turbiditic environment, on the northern part of the Indian continental slope and in the adjoining Neotethys basin. The age of these sediments ranges from Upper Permian to Eocene.

4) the Indus Suture Zone, ISZ (or Indus-Yarlung-Tsangpo Suture Zone) defines the zone of collision between the Indian Plate and the Ladakh Batholith (also Transhimalaya or Karakoram-Lhasa Block) to the north. This suture zone is formed by:

the Ophiolite Melanges: which are composed of an intercalation of flyschs and ophiolites from the Neotethys oceanic crust;

the Dras Volcanics: which are relicts of an Upper Cretaceous to Upper Jurassic volcanic island arc and consist of basalts, dacites, volcanoclastites, pillow lavas and minor radiolarian cherts;

the Indus Molasse: which is a continental clastic sequence (with rare interbeds of marine saltwater sediments) comprising alluvial fan, braided stream and fluvio-lacustrine sediments derived mainly from the Ladakh batholith but also from the suture zone itself and the Tethyan Himalaya. These molasses are post-collisional and thus Eocene to post-Eocene.

The Indus Suture Zone represents the northern limit of the Himalaya. Further to the North is the so-called Transhimalaya, or more locally Ladakh Batholith, which corresponds essentially to an active margin of Andean type. Widespread volcanism in this arc was caused by the melting of the mantle at the base of the Tibetan bloc, triggered by the dehydration of the subducting Indian oceanic crust.