Tectonic Evolution of the Himalayas
The magnificent heights to which the Himalayas have been uplifted must have been a result of several orogenic movements. The tectonic history of the Himalayas is a much debated subject. Some workers have recognized five phases of deformation on the basis of small scale shortening structures. Others have deciphered four major tectonic movements, the earliest dating back to the Precambrian or Lower Paleozoic, while the other three postdating the Permian. A few workers however, recognise only three episodes of orogenic activity, post-dating the Cretaceous.
The first phase of Himalayasn orogeny - the Daling event (Precambrian or Lower Paleozoic) is marked by medium grade metamorphism but intense deformation accompanied by both acid and basic igneous intrusives. The second phase of orogeny known as the Ladakh phase involved the beginning of earth movements which were responsible for the major uplift of the Himalayas. It is during this phase that the Indian plate is believed to have collided with the Asian plate. This caused a shallowing of seas, emplacement of ophiolites along the Indus Suture zone, granitic intrusions, palingenesis, and granitization. Patches of Eocene rocks over the Tal (Jurassic), Krols (Permo-Trias), Simla Slates (Paleozoic?), and Deoban rocks (Paleozoic?) indicate folding and erosion of these sedimentary basins before transgression of the sea in Eocene period, and subsequent deposition of Eocene sediments. This phase of the Himalayasn orogeny was active from Late Cretaceous to early Eocene times.
The third and most vigorous of all orogenic movements is termed the Dharamsala phase which culminated in the Middle Miocene. The consequent compression due to under-thrusting of the Indian plate beneath the Tibetan (=Asian) plate caused large scale isoclinal and recumbent folding of the Precambrian crystalline rocks. This resulted in the thrusting of huge blocks of these rocks that moved southwards, and came to rest upon the younger sedimentaries. Most of the thrust sheets of the Lesser Himalayas and widespread regional metamorphism and acid igneous activity are associated with this phase. Due to the great uplifting, a foredeep was created in front of the rising Himalayas into which the Siwalik molasse sediments were deposited.
The fourth phase of Himalayasn orogeny is the Siwalik phase. This phase was very active from the Late Pliocene to Middle Pliestocene period. Most of the present structures of the Himalayas developed during this phase. The Siwalik sediments were also uplifted. Tectonic creep along some of the thrust faults of the Himalayas, translation of upper Siwalik rocks over the subrecent gravel beds, and seismicity in the region strongly suggest that this phase has not yet ceased.
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