Himalayas are one of the complex mountain systems of the world. They represent great variety of rock systems dating back from Cambrian to Eocene periods and from granites and gneisses to sandstones, limestone's, boulder conglomerates and shales. At several places rocks have been highly metamorphosed.
Intense folding has led to the formation of recumbent folds, overturned folds and even nappes. It is because of this complex situation that scholars have given divergent views regarding the origin of these mountains. These views may be grouped under three categories: (a) geosynclinals evolution, (b) plate tectonics, or (c) vertical movements (E. Ahmad, 1992, p. 21).
(a) Geosynclinals Evolution
Exponents of this opinion mainly base their deduction on the sedimentary nature of the Himalayan rocks majority of which have evidence of marine origin and contain fossils of marine organisms. Even the enormous thickness of the Himalayan rocks allows them to associate their deposition under the bed of sea whose floor underwent sinking with the increasing weight of the deposits.
The geosynclinals origin of the Himalayas has obtained maximum approval from the scholars. The theories of Suess, Argand, Kober etc. all belong to this category. According to geologists the disintegration of Pangaea led to the formation of a long mediterranean sea (called Tethys) between the two land masses of Angaral and (north) and Gondwanaland (south).
This sea was occupying the region of the Himalayas during the Mesozoic era (180 my.) During the end of the Palaeozoic era and beginning of the Mesozoic era the Tethys almost engirdled the whole earth running from Europe in the west to China in the east. Eroded material from the two land masses were deposited in the Tethys and could assume considerable thickness due to the sinking nature of the bed of the sea. During Cretaceous period the bed of the sea started rising which led to the folding of three successive ranges of the Himalayas.
The first upheaval, which led to the formation of the Greater Himalaya, took place during Eocene period. Similarly second upheaval during Miocene period folded the Lesser Himalayas and third starting in Pliocene period ended with the birth of the Siwalik Hills.
(b) Plate Tectonics
According to the plate tectonics the rise of the Himalayas is viewed as the outcome of the collision of the Indian plate with its Asian counterpart. This has resulted into the seduction of the northern margin of the Indian plate, crustal shortening, folding of the upper silica material, pilling up of nappes and isostatic recovery.
Following features of the Himalayas lend support to these views: (i) Flysch occurs along the Indus-Tsangpo and Shyok-Kailash zones, (ii) The low angle MCT (Main Central Thrust) separates the central crystallines from the meta- sedimentaries which occur below the thrust and are pre-Cambrian to lower Palaeozoic in age. Then Tethyan marine sediments occur over the central crystallines, (iii) the klippen and windows were also taken to support large-scale thrusting, (iv) The MBT (Main Boundary Thrust) separates the Pre- Cambrian Mesozoic metamorphics and sediments from the Tertiary deposits. Above facts suggest crustal shortening in the Himalayas which followed crustal consumption at the edges and the intercontinental collision along the Indus-Suture zone.
(c) Vertical Movements
Those who advocate vertical movements responsible for the upliftment of the Himalayas take support from the fact that the gravitational force, the main force among the various bodies in space, can act only radially inhibiting enormous horizontal translocation implicit in plate tectonics.
The facts in the Himalayas supporting this view include the high angle inclination of the MBT; absence of distinct root zones and possible occurrence of granite domes in various areas (E. Ahmad, 1992, pp. 21-22).
Following are some of the important views which throw light on the origin of the Himalayas:
Edward Suess-According to Suess the folding of the Himalayas has been caused by the com- pressional forces which have worked from the north and led to the folding of the detritus deposited in the bed of the Tethys. In this process the land mass of Angaraland lying north of the Tethys acted as backland whereas Gondawanaland along the southern margin of the Tethys behaved as foreland and remained stationary.
Due to the southward movement of Angaraland the Tethyan sediment was compressed against the Peninsular mass yielding place to three successive are like ranges from west to east owing to two extended horns of the Peninsula (the Aravallis and Delhi ridge in the west and Meghalaya plateau in the east).
The southward bend in the Himalayan ranges is cited as a strong argument in support of this theory which has almost lost its tenability in the light of recent studies on global plate tectonics.
Emile Argand-the Swiss geologist E. Argand expressed his views on the origin of the land mass of Asia in a paper entitled 'La Tectonique de 1 Asia' before the International Geological Congress at Brussels in 1922. According to this theory the northward movement of southern ancient rigid masses (consisting of Indian Peninsula and Arabian Peninsula) folded the Tethyn sediments against the northern rigid masses (consisting of Angaraland, Chinese Massif, Sardian Massif and Russian Platform) which gave birth to the Himalayas. The theory, which was earlier vehemently criticized by the scholars, has got new lease of life through plate tectonics.
Kober-Famous German geologist Kober has presented a detailed and systematic description of the surface features of the earth in his book 'Der Bau der Erde' in which he has tried to establish a relationship between ancient rigid masses and orogen (mobile zones or geosynclines).
Thus he has tried to explain the origin of mountains on the basis of his geosynclinal theory. According to this theory Tethys geosyneline occupied the present-day place of the Himalayas and was bordered by Angaraland in the north and Gondawanaland in the south both of which acted as foreland.
During the Eocene period both these rigid masses (kratogens) started converging as a result of which folds were formed along the northern and southern borders of the Tethys sediments giving birth to the Kunlun mountains in the north and Himalayas in the south.
Tibetan plateau as median mass between these two mountains remained unaffected by the folding, although it was slightly raised due to the intense nature of the compressional forces.
During the upheaval of the Himalayas a fore deep was formed whose infilling led to the formation of the Ganga Plain. Although Kober's views seem to be more logical but majority of scholars believe in one sided movement.
S.G. Burrard-Burrard put forth his contraction theory regarding the origin of the Himalayas in 1912. According to this theory below the earth's surface there is layer which is cooling at a slower rate. After cooling this layer contracts and is broken apart.
This promotes contraction in the surface layer also and folding of the Himalayas. The drifting of the broken parts produces vacuum which is filled up by the fresh arrival of deposits by the rivers.
The folding of one such deposit is responsible for the upheaval of the Siwalik. Because this theory creates many difficulties in respect of isostasy it has not received adequate approval from the scholars.
Fox and Weddel-According to Fox and Weddle the rise of the Himalayas is the outcome of two different processes. In one such process the compression from the hinterland area of the Tibetan plateau produces wrinkles along its southern border which denotes the position of present Himalayas.
Under the second process agents of erosion start eroding these folds and removal of huge quantity of eroded material produces deep valleys and gorges. Thus in order to maintain is static balance erstwhile folds are further uplifted giving present gigantic height to the peaks of the Himalayas.
So according to this theory the main cause of towering height of the Himalayan peaks are the deep valleys and gorges carved out by the Himalayan Rivers. Fox and Weddel have cited the terraced valley of the Zakar Chu River which is a tributary of the Arun River.
Heim-According to Heim gradual rise in the Himalayan region is causing subsidence in the neighbouring Ganga Plain. This is the main cause of occurrence of earthquakes in these areas.
E.H. Pascoe-Pascoe raises the question in relation to the Himalayas that it is not clear whether folding caused uplift or uplift caused folding. There is an apparent difficulty brought to light by gravity surveys in India according to which maximum density is found along the belt extending through Orissa, Jabalpur and Sindh called 'Hidden Range' which is not the lowest topographic belt in the country.
In 1932 Glennie tried to explain this apparent anomaly by crustal upward. "The gradual rise of the Hidden Range would have brought die tachylite nearer and nearer to the surface, until die rigidity of die overlying granitic layer broke down the result being die outpouring of die Deccan Trap. While die Hidden Range was being formed, the Himalayan region was occupied by the broad, shallow depression of the Tethys geosynclines.
The continued and excessive sinking of these geosynclines is presumed to have caused crustal weakness and a rapid depending and narrowing of dies trough. The sediments are imagined to have been folded as result of this narrowing, and the surplus sedimentary matter to have folded up above the trough to produce dies Himalayan Chain." (E.H. Pascoe, 1964, pp. 2107-2112.
Wager's suggestion that die uplift to the Himalayas may be regarded as consequence of the erosion of the southern margin of the Tibetan highland falls under is stays. This appears to be proved by greater uplift in the east where the valleys and gorges are cut deeper dean in die west.
In a recent study of is stays in the Himalayas Chugh and Bhattacharji have found that "is ostasy is generally in existence in the Himalayas." They noted the order of undercompensation in die Outer Himalayas as 10% and that of overcompensation in the Ganga Plain as 40% which proves that erosion in die mountains and deposition in the plains" tend to maintain is static equilibrium in the region as a whole."
A. Holmes-the upheaval of the Himalayas may also be explained by Holmes' Convection Current Theory. According to this theory the final stage in sub-crustal thermal convection currents is a period of waning currents by gradual uplift until is static balance is achieved in the zone. This zone is marked by sedimentation-subsidence in die initial stage.
This is followed by a relatively short duration of orogenic compression and root formation. Uplift in this second stage is prevented by the down-dragging power of subsiding currents. Folding associated in later phase's wittier metamorphism causes uplift and die formation of mountain belts. "One of die most astonishing facts is that, while the original cover is being removed by erosion or gravity sliding, die granitic heart or core of a mountain range may continue to rise until it is exposed in the flanking valleys, and eventually at the summit itself.
Everest and some of the Himalayan neighbours are celebrated examples of this culminating feature of mountain building". Also the uplift precedes napes and giant napes with granite and magmatic cores are related to gravitational exogenesis (A. Holmes, 1965).