Geotectonic Features of the Himalayas
Separating the Siwalik Formations of the Sub-Himalayas from the older rocks lying to their north, the Main Boundary Fault is a major structural plane traceable throughout the length of the Himalayas (see map). The irregularity and sinuosity of the fault trace is evidence of a gently inclined plane. The older rocks of the Lesser Himalayas are thrust over the Siwaliks along a series of more or less parallel thrust planes. The Main Boundary Fault is a reverse fault with large scale movements and is still very active. Measurements made by some workers demonstrate that the present day movement is of the order of 0.92 cm/yr.
The Main Central Thrust, marking the boundary between the Lesser and Higher Himalayas, is a zone of more or less parallel thrust planes along which the rocks of the Central Crystallines have moved southwards against, and over the younger sedimentary and metasedimentary (Paleozoic ?) rocks. It is a major tectonic feature, well defined, and easily recognisable in general. In some places, however, the thrust is not clearly discernible because of a lack of metamorphic or tectonic discontinuity and presence of rocks of similar lithology on either side.
In the Ladakh region of the Kashmir Himalay, all along the upper course of the Indus valley, a significant linear tectonic zone is evident and is well known as the Indus Suture (see map). The rocks of the Indus Suture zone comprise ophiolites, orbitulina limestone, flysch wedges, and molasses sediments. Gansser (1964) believes that this belt is thrust southwards along the Indus Suture Zone. There is a lot of controversy regarding the nature and origin of the Indus Suture and the associated ophiolites and exotic blocks. One point of view considers this as a line of junction of the Indian and Asian plates, where the Indian plate is subducting beneath the Asian plate, and that the ophiolites are parts of the oceanic crust abducted during the collision. This concept has been strongly opposed by other workers who consider the northern boundary of the Indian plate along the very high seismic activity belt of Tien-Shan mountains, and the Indus Suture as being a deep fracture reaching up to the mantle along which, Cretaceous onward, flysch type sediments were deposited; volcanics and pyroclasts poured out; and blocks of Alpine type ultramafics were emplaced - almost in solid state.
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