Late Pleistocene to Holocene landscape controls on former higher lake levels in Tibet and its interactions with surface processes

Hon Chim Chiu

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Abstract

Very rapid uplift has had a major influence on surface processes, including erosion rates and the associating sedimentological responses in the Qinghai-Tibetan Plateau. Many of the river courses in Tibet display characteristics that strongly suggest tectonic and glacial controls on their formation and evolution. It is generally agreed that the river systems appear antecedent to the uplift of the plateau (Duff and Holmes, 1993) and that they have incised the newly formed Himalayan ranges, creating a series of deep valleys.

Outburst floods from palaeolakes can be one of the mechanisms to create such incision. For instance, sedimentological and geomorphological evidence of palaeolakes in southeastern Tibet suggests the existence of large palaeolakes north of the Himalayan Ranges, with lake levels high enough that extensive coalescence of adjacent lakes in the Tibetan plateau in the LGM may have occurred, consistent with the long held Pan-Lake Hypothesis (Zheng, 1996; Zhu et al. 2003).

Only patchy evidence of surface breaching of the palaeolakes, however, could be observed. While glacial dams, tectonic displacements, and other local effects on geomorphology and sedimentology of the lowering of the lake levels could have occurred, there is a lack of regional synthesis on the resultant effect of losing a significant proportion of water on the surface in the erosional rates over a relatively short period of time.

Through the study of Palaeolake Tingri, a large lake system possibly existing in south-eastern Tibet during Late Pleistocene, the concept of 'focused incision' through an outlet towards Yö Ri Gorge was examined. Topographic and sedmentological evidence suggested that Phung Chu, the present river system running through the Gorge, may have been both a cause and the consequence of ‘focused rock uplift’ (Brookfield, 1998; Clark et al., 2004; Montgomery and Stolar, 2006) that focused incision power across the Ama Drime High Himalayan Crystallines to form the current path of the river.

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