Tectonics with special reference to India

Abstract

The chief elements in the structure of the earth's crust are shield and stable areas (platforms), sedimentary basins; geosynclines, folded mountain ranges and the present mobile belts formed by compression; mid-ocean ridges, rifts and faults forming another group, due to tensional phenomena. Seismic and volcanic activities are associated with the folded and mobile belts as well as with the rifts. Both these types occur in a definite pattern. The compression phenomena are more or less confined to the Pacific margins and the Alpine-Himalayan belt of mountains on either side of Mediterranean. The tension rifts follow the middle of the Atlantic and Indian Oceans with branches going out into the South-Eastern Pacific and around Antarctica. In the case of the mobile belts, the continental margins which were the loci of sedimentation were compressed, folded and thrust over the ocean side, forming circular arcs of various dimensions. This is very clearly seen around the Pacific Ocean. Though arcuate mountain chains are also found along the Alpine-Himalayan mountain belt, the former Mediterranean Ocean has largely disappeared by the continents coming together. The mid-ocean ridges, particularly in the Atlantic and Indian Oceans, are broad mountain features which appear to have arisen from the stretching of the crust and extrusion of large masses of basic igneous material (Sima). Recent observations have shown that the middle of these ridges are marked by deep rifts with seismic and sometime volcanic activity. India consists of a stable peninsular shield with a mobile belt around the whole of its northern border. The structure of the peninsula is controlled by the Dharwarian, Eastern Ghats, Satpura and Aravalli structural trends, which are responsible for the triangular pattern. The Aravalli belt extends north-east from Gujarat through Delhi to Garhwal. To its east is the great Vindhyan basin which is moulded on the Bundelkhand granite massif and which presumably extends also into the Sub-Himalayas of U.P. and Nepal. It is not yet known whether there is a continental nucleus in the centre of the peninsula. The structural trends have determined more or less the shape of the coast and also the direction of faulting of the three major troughs of coal-bearing rocks, one along the Damodar and Sone valleys, the second along the Mahanadi valley and the third along the Godavari valley. There is also a well-marked rift along the valley of the Narmada which has been a marine belt from the Permo-Carboniferous to recent times. It is known that Madagascar was separated from East Africa in the Permian. The Mozambique Channel gradually widened, and from the Jurassic times marine strata were deposited, these being very closely related to the rocks of the same age found in Cutch, in the Baluchistan arc, and on the eastern side of Arabia. The eastern coast of India seems to have taken shape in the Upper Jurassic and there is a good marine succession from the Albian upwards. In Australia also there is evidence that its western coast was invaded by the incipient Indian Ocean in the Jurassic and more extensively in the Cretaceous. The great fissure-eruptions of basic igneous rocks which occurred in Africa, South America and India (Rajmahal) in Upper Triasic or Jurassic times are apparently the manifestations of tension which resulted ultimately in the breaking up of the southern continents which constituted Gondwanaland. That the final break-up occurred in the early Cretaceous times is proved by stratigraphic data from the different parts of Gondwanaland but, as stated already, the incipient Arabian Sea seems to have opened up from Permian times which is also the period during which the Narmada rift was formed. Another great fissure-eruption occurred in Western India, Somaliland and Ethiopea in the Upper Cretaceous to Eocene. The Alpine-Himalayan mountain ranges resulted from the compression and uplift of the great Equatorial Mediterranean geosyncline which existed during the Mesozoic era. The initial compression took place in the Upper Cretaceous when ultrabasic igneous rocks were intruded into the core of the mountain belt in Oman, in the Baluchistan arc and in the Himalayas as well as further east. The other major phases occurred in Upper Eocene, Lower or Middle Miocene and in the Plio-Pleistocene. These phases can be recognised over most of the Alpine-Himalayan belt though in varying degrees of intensity. The rocks of the southern part of Indian Peninsula have a close connection with those of Ceylon, Madagascar and East Africa with which they are closely related structurally. This indicates that they all probably formed parts of one unit, with Australia lying on the one side and Antarctica on another. India began to drift away from the rest of Gondwanaland early in the Cretaceous by translatory motion as well as by anti-clockwise rotation through something like 60 degrees of arc. It travelled far to the North and intensely crumpled up the Tethys basin and wedged itself into what is now Central Asia. The last is a region of numerous, more or less parallel, mountain ranges gathered up sheaf-like in the Pamir region. The ranges concerned are the Tien-Shan, Trans Alai, Kunlun-Lokzung, Altyntagh, Karakorum, Kailas, Zanskar and Himalaya from north to south. Their ages range from the Hercynian period in the north to the Alpine period in the south, the middle ranges having been affected by both of these revolutions, and perhaps also by the Cimmerian. All these ranges are characterised by high negative gravity anomalies which reach a maximum of a little over –560 milligals in the Kunlun and Lokzung ranges. This indicates that there is a very great thickness of Sial in Central Asia, which is still a very disturbed region as indicated by the occurrence of earthquakes along these mountain belts. This region will ultimately attain equilibrium by a redistribution of mass which is now occurring as a spreading of the Asiatic continent to the east and southeast as indicated by the active island arcs of Kurile, Japan, Riukiu, Philippines, Banda and Sunda arcs on the inside and the Marianas-Yap-Palau and the Nias-Mentawei-Andaman arcs on the outside. The movement of India to the north-east was accompanied by the movement of Arabia also in the same direction creating the tension rifts of East Africa as well as the Red Sea rift. The Persian Gulf is probably of the nature of afore-deep similar to the Gangetic plains, though it is continuous with the Mekran fault. Mountain building has presumably eased off around the northern borders of India, but as stated already it is still going on in South-Eastern and Eastern Asia. Perhaps even the spreading out of the Indian Ocean area is also still going on. The deep penetration of India into Central Asia is indicated by the enormous compression evident in the Himalayan region as well as by the two spectacular hair-pin bends near Nangaparbat on the west and Nancha Barwa in the east. The Kashmir and the Assam wedges have gathered up the mountain ranges in the north-west and north-east corners beyond the Indian shield, these regions being highly seismic. Most of these facts about the tectonic features of India and its neighbourhood are best explained by the hypothesis of continental drift associated with the names of Taylor, Wegener and Du Toit.