~ Geology Geology and Major Structures of the Himalayas

The closing and subduction of the Tethyan Ocean, located between India and Asia during the Paleozoic, followed by collision of continents produced the structures and lithologies we see today in the Himalayas. Consequently, the mountains and surrounding regions are characterized by astounding complexity,represented by a variety of deformed and collision-produced lithologies and representing several phases of tectonic and deformational events. The Himalayas can be divided into six primary lithotectonic zones that occur in parallel belts. These zones consist of the Trans-Himalayan batholith, Indus-Tsangpo suture zone, Tethyan(Tibetan) Himalaya, Higher(Greater) Himalaya, Lesser(Lower) Himalaya, and Sub-Himalaya. Tectonic environments within these zones also vary. The emense collision of plates at 45 million years gave rise to an island-arc margin in the western Himalayas and an Andean-type margin in central-to-eastern Himalayan regions (Windley 1995).

Trans-Himalayan Batholith
The Trans-Himalaya zone is a linear plutonic complex. It is partly covered by forearc rocks and continental molasse sedimentary rocks. These assemblages are derived from uplift of magmatic rocks and their subsequent erosion. The igneous complex consists of I-type lithologies, including gabbros,diorites, and granites. Formation of the complex is thought to have occured in several phases, between 110 and 40 million years. Partial melting of a subducting NeoTethyan slab beneath the Asian plate is thought to have resulted in these magmas (Sorkhabi 1999). This zone varies in a west-east direction. To the west, plutons were emplaced into an area, called the Kohistan-Ladakh region, and represent an island arc environement. Contrastly, eastern igneous rocks represent an Andean-type environment(Windley 1995).
Western Trans-Himalaya
An island arc formed on the northern side of the NeoThethys and became trapped in between Asia and India. This allowed for two stages of deformation. The first consisted of collision of the arc with Eurasia, followed by collision with India, as the Tethys began to subduct under Asia (Windley 1995). The area is dominated by granodiorites and tonalite composed of a quartz+K-spar+biotite+hornblende+ sphene mineral assemblage. U-Pb in monazite and allunite dated the granitic rocks at 60.7 plus or minus 0.04 million years. Late stage bodies of pegmatite and leucogranitic dikes are also present. Pre-collision phases, consisting of felsic intrusions, have been dated by U-Pb in zircon techniques and record a date of 101 plus or minus 2 million years (Searle 1991).
Eastern Trans-Himalaya
The Kangdese sub-alkaline batholith extends along the north side of the Indus-Tsangpo Suture and represents the Andean style tectonic environment. Rock types in this zone include slates, phyllites,schists, gneisses, amphibolites, and migmatites of Ordovician-Cretaceous age (Windley 1995).
Indus-Tsangpo Suture Zone
The Indus-Tsangpo Suture Zone (ITZS) defines the areas of collision between the Indian plate and the Kohistan-Ladakh arc in the western Himalayas and the Tibetan Lhasa block in the east (Windley 1995). It also marks the zone along which the Tethys Ocean was consumed by subduction processes. The ITSZ can be traced for more than 2000km (Searle 1991) between these regions and host a variety of rock types that tell us quite a bit about the orogen. Complete successions of ophiolites occur, some containing diamonds as well, suggesting high pressures during subduction and rapid extrusion along the suture zone. Glaucophane schists also occur in narrow belts along the ITSZ in Pakistan. Olistoliths occur in northwestern India and consist of reef and and continental slope sediments in abyssal tubidite deposits. Mafic to felsic lavas as well as cherts, serpentinites, and dunites are also observed. Limestones and red sandstones are associated with Tethys Ocean sediments and found in the Ladakh region (Windley 1995). Such a wide variety of rock types along the ITSZ further indicates that the collision of plate boundaries was a complex one, effecting many terraines in many ways. Their one commonality is that this great structure separates Asian lithosphere from the Indian plate.
Tethyan(Tibetan) Himalaya
The Tethyan Himalayas are located to the south of the ITSZ. They consist of thick, 10-17km, marine sediments that were deposited on the continental shelf and slope of the Indian continent. This occured as India was drifting but still in the southern hemisphere (Verma 1997). Sediments are largely unmetamorphosed, which has made for excellent preservation of fossils and occur in synclinorium-type basins. Some however, have experienced greenschist facies deformation (Windley 1995). Fossils occur in this east-west zone within strata that are very clearly known. The large variety of size and distribution of fauna suggest that life was flourishing in this area before the orogen. Such success in biological diversity is accounted for by the relatively stationary position of the Tehthyan Zone between mid-Proterozoic and Eocene time. Episodic formation of land barriers enabled life to grow and diversify (Sorkhabi 1999).
Higher(Greater) Himalayas
The Higher Himalayas are also known as the Central Crystalline zone, comprised of ductily deformed metamorphic rocks and mark the axis of orogenic uplift. Mica schist, quartzite, paragneiss, migmatite, and leucogranite bodies characterize this uppermost Himalayan zone. They represent a multiphase deformation event, the first being Barrovian type, or normal geothermal gradient conditions. There was then a shift to Buchan-type metamorphism, low pressure and high temperature conditions, with temperatures greatly exceeding normal gradient temperatures(Sorkhabi 1999). Local retrograde events have also been noted. Analyses show that peak orogenic temperatures and pressures were 475-825 degrees Celsius and 500-800 megapascals. Corresponding minerals assemblages are dominated by biotite to sillmanite, representing greenschist to amphibolite facies deformation. Deformation seems to have occured in a north to south direction and is associated with the Main Central Thrust Fault (MCT), which brings the higher Himalayas on top of the lower Himalayas (Sorkhabi 1999). Initially, it was thought that approximately 350km of shortening had occured in the Greater Himalayan sequence of rocks. However, through studies by DeCelles etal. (1998), a major thrust fault within the zone was discovered. As a result, it is now estimated that between 600 and 650km of shortening occured here. There was also a question of provenance for Great Himalayan rocks. Previous work suggested that lower Indian crust comprised this area. New interpretations of rocks there indicate that the higher Himalayas are actually made of supercrustal rock. This idea states that upper crustal material of India accreted northward onto the Asian continent and that crustal material was origanlly an appendage of India that was, itself, accreted to India during Paleozoic time. This study implies that India probably had significantly more continental crust than previously thought, much more crust to be shortened in the formation of the Greater Himalayas.
Lesser(Lower) Himalayas
The Lesser Himalayan zone is bounded the Main Central Thrust(MCT) in the north and Main Boundary Thrust(MBT) to the south. Unlike the higher Himalayas, the lessers only experienced up to greenschist facies metamorphism. The rock types present here are also different. They are primarily sedimentary rocks from the Indian platform. Rock units here also show a series of anticlines and synclines that are in many cases quite sheared. Fossils have been documented in this zone, but they do not occur at the same frequency as Tehtyan zone fossils.
Main Central Fault (MCT)
This thrust fault was first described by Heim and Gansser (1939) when they noted a contact between terrigenous carbonate rocks and thick overlying metamorphic rocks, mica schists and gneiss (Sinha 1987). The Main Central fault marks the boundary between the higher and lesser Himalayan mountains. It is a longitudinal thrust fault, and in many places is marked by a several kilometer thick zone of deformed rocks with varying degrees of shearing and imbrication (Sorkhabi 1999). Mylonitization and retrograde metamorphic assemblages also occur here. The MCT is the actual suture between Gondwanaland (India) and the Proto-Tehtys microcontinent to the north (Spikantia 1987). Movement along the fault has brought crystalline rock from the Higher Himalayan zone on top of Lesser Paleozoic sediments in the form of klippen in synclines (Windley 1995). These units are called the Outer Crystallines, as noted above on the map. Outer crystalline rocks, garnet and kyanite-bearing, were exposed by slip along the MCT followed by uplift and erosion of 10km of overlying rock (Molnar 1986).
Sub-Himalaya
This foreland zone consists of clastic sediments that were produced by the uplift and subsequent erosion of the Himalayas and deposited by rivers. These rocks have been folded and faulted to produce the Siwalik Hills that are at the foot of the great mountains. Sub-Himalayan rocks have been overthrust by the Lesser Himalayas along the Main Boundary Thrust Fault. This steep thrust flattens with depth, dveloped during the Pliocene time and has been shown as active through the Pliestocene (Ni 1984). In turn, the Sub-Himalayas are bounded by a thrust fault to the south and are forced over sediments on the Indian plate. This fault system is called the Himalayan Frontal thrust (Sorkhabi 1999).

Another view of Himalyan zones is the following schematic cross section modeled after DeCelles (1998). It not only shows geological divisions within the mountain belt, but also structures and geologic relationships between rock types and structures. It is effective in showing relative motion along faults.

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