As to the causes which led to the building of this largest mountain of the world one could argue in favor of any one of the more important theories of mountain building like Jeffrey’s Thermal Contraction hypothesis or Wegener’s Continental Drift, or Daly’s hypothesis of Sliding Continents or Holmes’s Thermal Convection Current or Plate Tectonics but with regard to all of these there is some danger of “begging the question” to various degrees, i.e., the cause of the cause is not known or it is inadequately known or it is insufficient to account for orogeny and its various aspects coherently. Then the sequence or orogenic events is also not known. Thus, Pascoe is not sure whether in the Himalaya folding was the cause of uplift or uplift caused folding.
One cause of the origin of the Himalayas was given by Glennie in 1932. While the Hidden Range (a sub-surface range indicated by gravity anomalies) extending from Jodhpur to Orissa, was being formed, the Himalayan region was occupied by the Tethys geosyncline (This, as we have seen earlier, is doubted by many). The excessive shrinking of the geosyncline, caused folding and the surplus sediments were raised as the Himalayan mountains.
Wager thought that the uplift of the Himalaya is due to erosion and isostatic uplift. Obviously, it relates only to a phase of the mountain. First, there must be a mountain then there will be erosion. We have examined this elsewhere. Isostatic uplift in response to great erosion appears proved in eastern Himalaya where the erosion and heights are greater. Here rainfall is higher and valleys and gorges have been deeper. In the Great Himalayan belt, adjoining Tibet enormous erosion in the gorges has caused the uplift of the highest peaks and ridges.
Continental drift, which to a certain extent is similar to the modern Plate Tectonics, was also invoked by Pascoe as a cause of orogeny on the northern (Himalaya) and other margins of Gondwanaland.
The Thermal Convection Current Hypothesis of Holmes is one of the latest theories of mountain building and the immediate predecessor of Plate Tectonics.
The probable movement of Angaraland and Indian Peninsula towards each other caused the formation of the Himalayas. One of the astonishing phenomena related to the Himalayas and other orogenic mountains is the continued rise of the granitic core till it is exposed in the valleys and gorges and finally in the peaks like Mt. Everest and its other towering associates.
It is probably the thermal current, which is the source of “a rising heat front” after compression, has died down. This rising thermal front is responsible for the latest metamorphic progress “and the initiation of the morphogenic phase (the finale of the Himalayan orogeny)”.
While these inferences are, valid it is surprising that such late phase metamorphism resulting from batholythic expansion and granitization affected only Pre-Cambrian rocks, leaving Cambrian and later sedimentaries. First, the Pre-Cambrians were thrust by orogenic movements. Then they underwent metamorphism which obscured61 thrust contacts creating serious problems in unraveling the geology and structure of the Himalayas.
While Thermal Convection Current can explain all such phenomena as compression, uplift, batholythic expansion, gravity sliding, granitisation, morphogenic phase, thrusting and metamorphism in the Himalaya there is no sufficient explanation for the thermal current itself. Thus, arises the problem of ‘begging the question’. There is wide gap between the temperature that can be generated by radioactive heating in the crust and the temperature, which is essential for melting the rocks of these regions. It is believed that the gap is bridged by supply of hot matter from the mantle. This conjecture needs proving. This is why Holmes says that the convection hypothesis is almost raised to the dignity of a theory.
Plate Tectonics may also be invoked in the origin and formation of the Himalaya. According to Davies, the Himalaya is a region of convergence between Eurasian and Chinese plates in the north and Indo-Australian plate on the south.
According to Sutton, the Himalayas are an example of collision between two continental plates.
Vine regards the Himalayas located on the junction of the Indian and Eurasian plates where the schematic direction of movement is from south-southwest in the Indian plate towards the Himalayas, there being no indication of corresponding opposite movement in the Eurasian plate.
According to Tarling also the Himalayas are situated in the zone of interaction between the Indo-Australian plate in the south and the Eurasian plate (its sub-plate called China plate) in the north, the direction and pattern of movement being the same as indicated by Vine.
Crawford wrote a sizable article on the Himalaya. He believed that there was sea-floor spreading in the Tethys sea region. This caused the formation of the rift called Indus Suture Line, which was later plugged by volcanic matter. The Himalayas themselves represented a pile of fractured slices in the middle of the Indo-Tibetan plate. According to him there was no continent-to-continent collision. The Himalayas were formed within the northern part of the same plate, i.e., Gondwana plate which extended as far north as Tarim Basin.
These views of Crawford are opposite to those of Sutton, Vine, Gansser, etc. who believed in continent-to-continent collision. Crawford’s thesis is unconvincing because collision and compression is the view commonly accepted of the G.S.I, and the occasional foreign students of Himalayan geology regarding the Himalaya.
According to Gansser the major part of the Himalaya is not geosynclinal. There was “deepseated tectonic disturbance” in the Tibetan part of the Himalaya causing the outflow of basic lavas seen along the Indus suture or rift. He believed that there was thrusting of the Indian or Gondwana shield (or plate) against and under the Tibetan region. This caused the formation of the Northern Thrust, the elevation of Tibetan plateau, compression of the Flysch belt along the Indus rift and the formation of the Himalayan mass itself.
According to a recent publication of the G.S.I, “the northern and southern (plates) had collided and subduction of the Indian plate below the Tibetan plate had started.” Continued plate interaction might have uplifted wedges of oceanic crust—The northward movement of the Indian plate as a result of sea-floor spreading on the Indian Ocean ridge caused the closing of the Tethys which was “about 5,000 km wide”, in late Mesozoic.
It is believed by some that the Indian plate terminates on the Indus suture in the west and the Arakan-Andaman arc in the northeast and east.
What is known as Counter Thrust is northward inclined thrust in the Indus Suture zone.
On the basis of seismological evidence Gupta found that the “Indian and Asian plates have underthrust into the upper mantle forming a V-shaped pocket of intermediate focus earthquakes”.
According to analysis of data related to seismicity and faulting near India-Tibet-Nepal boarder Srivastava found that thrust faulting orthogonally to the trend of the Himalaya has been the common feature. Cymatogeny, i.e., “upwelling of molten material from the upper mantle” might also he a cause of Himalayan orogeny.
According to Qureshy vertical movement appears to have played the main role in the elevation of the Himalaya. Schwiderski (1967) showed from satellite data that there has been upward movement of matter from the mantle under the Central Asian mountains and the Himalayas.
According to N.M. Savage, the Himalayas have been caused by plate collision. The forces involved for plate tectonics are similar to those for thermal convection hypothesis. The question of the origin and cause of the Himalaya, therefore, remains open and unresolved and uncertain in the present state of knowledge.
We note a few broad points regarding the tectogenesis of the Himalaya. One does not find a regular coincidence between tectonic or geologic zones and relief regions. Original geosynclinal sediments occur along linear zones like the Indus, Shyok and Tsangpo valleys. Dislocation along the Main Boundary Fault began before Pleistocene and might be extant. The Northern Thrust is inclined northwards while the Indus Thrust is inclined both southwards and northwards (ambivergent). This might suggest the rise of the mountain in between.
There are several evidences of continued rise of the Himalaya. These include:
(ii) River terraces, e.g., the gravel terraces of the Upper Karnali, the Upper Sutlej valley and Kargil basin,
(iii) The tilting of the uppermost Siwalik Boulder conglomerates,
(iv) Remains of low altitude animals like rhinoceros found in upper Siwalik beds 1500 meters above the sea-level,
(v) Growing desiccation of Tibetan lakes, interpreted as due to growing check on the entry of the monsoon by the rising Himalaya.