Isostatic Impact of the Tibetan Inland

A glacio-isostatic drop of the plateau of about 600-700 m must have occurred (Kuhle, M. & Kuhle S. 1997 p.124-129). (Density difference of ice versus material of the earth's mantle (g=1: 2.4 at an ice thickness of 1000 to 2700 m=417 to 1125 m glacio-isostatic drop). Since deglaciation occurred about 9000 years ago, remnants of glacio-isostatic uplift with downward tendency should still be observable, as in Central Scandinavia (Morner 1978) or the Pleistocene glaciation of North America (Andrews 1970)

The latest tectonic map to be published in China (Seismographic Map of China, 1987) shows rates of uplift of more than 10 mm/y for Central Tibet (Chen 1991; Fang 1991). This value is two to three times higher than what had been established for the much younger High

Himalayas, which were actually uplifted more rapidly than Central Tibet since the Late Tertiary as the evidence of antecedent valleys demonstrates (Kuhle 1982a).

Schneider (1957, pp.468, 475) puts the uplift of the NW-Karakorum at 12,000 m since the end of the Late Tertiary. This corresponds to 3-4 mm/year for Tibet and the Himalaya since the Late Tertiary. Gansser (1983, p. 19) gives an integral value of 10-15 mm/year for Tibet and the Himalaya while for the Himalaya alone a value of 4-8 mm/year is given (personal communication, 1982). The Swedish triangulation by Norin (1932, 1982) showed that since the Survey of India in 1861 the profile by way of Leh and into SW Tibet, had undergone an uplift of 37 m. This implies a rate of uplift of 521 mm/yr. In the Scandinavian centre of uplift such extreme rates of uplift only took place around 10-8 ka (see Morner 1978 Fig.1 and for North America: Andrews 1970). They decreased to the present rates of 10 mm/yr by about 4 ka.

One possible conclusion is that in the region of Na-K'ot Ts'o (Fig.1, No.59; Fig.3a NW of Kamet), which is characterised based on that Swedish triangulation profile by very strong uplift, the Tibetan ice persisted several thousand years longer than the Scandinavian inland ice.

On the other hand young subduction processes in the southern Himalayas might contribute might to strong uplift as well. Thus a melting history that is comparable to other areas of the world, is also possible. New drillsites on the Tibet Plateau characterising individual ice advances and retreats are thus needed.

As Fig.3b shows, the ice burden decreased in the direction of the outlet glaciers on the rim of the Tibetan highland as a function of its steep gradient curves. On the one hand this serves to reduce the uplift, owing to the reduction in pressure. On the other hand, mountains south of the inland ice, like the Himalayas, were affected merely by ice filling of the valleys. This leads to a comparatively linear ice burden, whereas the Central Plateau carried an extensive ice sheet.

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