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Chapter 05
Geological Studies
however, does not include the shortening along the Indus suture line, where a considerable area (the exotic regions) must have disappeared into depth. Except for the northwards-directed thrust of the exotic Flysch masses towards the Trans-Himalayas southwards-directed tectonic elements dominate. This south vergency is at present underlined by the exceedingly elevated hinterland (the Tibetan Plateau) and the deeply depressed foreland- the Indo-Ganges and Brahmaputra plain. These features offer a striking contrast to the present configuration of the Alps. The picture was probably less accentuated during the major Himalayan orogeny. The final rise of the Tibetan plateau is coupled with the youngest, actually recent morphogenic rise of the Himalayas, and has as its counterpart the sinking of the Indian foreland.
In the eastern Himalayas, the foothills are steeper and the foreland basin has shrunk to only 30-50 km. It is not without interest to note that most of the larger Himalayan earthquakes are concentrated in the eastern Himalayas and their foreland. This fact is illustrated in Fig. 149 where some of the major earthquakes have been compiled. Practically all are of the shallow type, and their foci are believed to occur between 20 and 30 km in depth. This is somewhat less than the presumed depth of the Moho discontinuity, which, based on results are admittedly vague, varies from 35 km in the western Himalayas to over 40 km in the eastern part of the range. This amount is small if compared with the great crustal depression of the southern Alps, amounting to 70 km, which is probably so far the largest known crustal thickness. The fact that the Himalayas are not a geo-synclinal type mountain range except for the zone of the Indus suture, may be the reason for the relatively shallow crust below this high pile or mostly crystalline thrusts. Deeper depressions may be eventually found related to the Indus suture line.
E
W
Shib-chu
Fig.92: Transgression of the Jungbwa peridotiles directly on the Cretaccous Flysch without exotic blocks. Lower Shib-Chu Corge. S Tibet; after A. GANSSER (1939)
1. Upper Cretaccous Flysch
2. Ophicalcites
3. Junghwa peridotits
4. Pleistocene gravel terraces
In the whole Himalayas, upwards movements are still very active. The morphogenic phase is clearly reflected in the present history of the range. The main elevation of the Himalayas was
Central Himalayan Geological Observations
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