Regional Geology & Structure of the Himalayas
The geological formations in the Himalayas take sharp turns forming hairpin bends at the eastern and western extremities of the arc and continue southwards into the Burmese and Baluchistan arcs respectively. These two points of acute inflexion constitute the major syntaxial bends in the Himalayas.
According to Gansser (1964) the Himalayas may be subdivided into five geographical divisions from west to east. In their longitudinal structure, the Himalayas are divided (from north to south) by a series of parallel tectonic zones (see map). The Sub- or Outer-Himalayas forming the foot-hill zone are delimited in the south by the large fans of Ganges alluvial deposits, whereas the northern edge is a clearly outlined tectonic feature -- the Main Boundary Fault, genetically linked with Miocene metamorphism in the Himalayas. Between the Main Boundary Fault in the south and the Main Central Thrust in the north lies the stretch of the Lesser Himalayas. Some workers, however, take a somewhat different stance on the northern limit of the Lesser Himalayas. The real boundary in their opinion is the plane that separates the Precambrian granite injected metamorphics of medium grade (Munsiari Formation) from the katazonal, very high grade metamorphics, making the bulk of the Higher Himalayas (Vaikrita Group). There is no thrust discernible between these two units, which together are considered by most workers to constitute the Central Crystalline Zone of the Higher Himalayas. An abrupt and dramatic change in the grade of metamorphism is evident, however, which hints at the presence of a thrust, which Valdiya (1978) has designated as the Vaikrita Thrust defining the base of the Vaikrita Group.
The Lesser Himalayas are composed of tectonically compressed blocks of Paleozoic and Mesozoic crystallines, metamorphics, and sedimentary rocks. The Main Central Thrust is a major tectonic feature of the Himalayas and has brought the crystalline rocks of the Higher Himalayas over the younger sedimentaries. The true amplitude of these movements cannot be determined. Many researchers consider these overthrust overlappings to be due to large scale movements.
The Higher Himalayas consist of a single range with an average height exceeding 6000 m. The width of this zone, mostly composed of granites and gneisses, is 24 km. The Central Crystallines occupy the core or the 'axis' of this range, and were considered to be Tertiary intrusives accompanying the compressional movements responsible for the uplift of the Himalayas by some earlier workers. The recent view, however, is that they are mostly Paleozoic or Precambrian in age and represent a geanticline between the unfossiliferous sediments of the Lesser Himalayas to the south and a highly fossiliferous sequence of the Tethys zone in the north. The stratigraphic order in the Tethys zone is well known because of well preserved fossils e.g. productus, ophiceras, etc., whereas in the Lesser Himalayas it is primarily based on structural-setting and lithological control.
There is evidence of continuous Precambrian continental basement extending north of India, through the Himalayas, into Tibet. It appears obvious that the sedimentation of the Himalayas took place in a sunken basement extending north of the Indian shield, which could have been in contact with the southern boundary of the Asian Plate, or the two might have been continuous. Buckling of this basement due to compression formed the barrier between the northern and southern basins, thus separating the two environments in the form of an upthrust wedge. The fact that the Central Crystallines are of Precambrian age supports this theory.
Important subordinate ranges on the northern side of the Higher Himalayas, corresponding to the Lesser Himalayas on the southern side, are the Zanskar, Kailash, and Nyenchentanglee. This zone, referred to as the Tibetan or Tethys Himalayas, contains a complete record of fossiliferous sediments from the Cambrian to Tertiary.
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Department of Geology
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