Geomorphology of anomalously high glaciated mountains at the northwestern end of Tibet: Muztag Ata and Kongur Shan
Muztag Ata and Kongur Shan massifs represent a significant area of anomalously high topography at the northwestern end of the Tibetan Plateau, rising to > 7500 m above sea-level (asl) from the plateau that has an average elevation of ~ 3500 m asl. These massifs provide an excellent opportunity to test geomorphic concepts, such as the glacial buzz-saw model. Using remote sensing, digital elevation modeling, field mapping and terrestrial cosmogenic nuclide (TCN) methods, the massifs were examined to determine the relative importance of tectonics and geomorphic processes in shaping the regional landscape and to provide a framework for testing geomorphic models. The gneiss domes that underlie the peaks are the result of exhumation along the Kongur detachment fault that has unroofed the massifs at a rate of between 4-6 km/Ma over the last few million years. This has resulted in rapid uplift and active seismicity, which is exemplified by the numerous fresh fault scarps throughout the region and large historic earthquakes. The geomorphic system is dominated by glaciation and the region contains extensive successions of moraines and paraglacial landforms, including fans, terraces and landslides. Glaciers have oscillated considerably throughout the latter part of the Quaternary, and three major glacier stages are recognized (Karasu [oldest], Olimde and Subaxh [youngest] glacial stage) that include at least 10 smaller glacial advance. The style of glaciation has changed over time from expanded ice caps to piedmont glaciers to valley and cirque glaciers. This possibly reflects a change in climate and/or topographic constraints as the massifs grew and became incised. The topography and glaciers in the region vary across the massifs divided by a broadly N-S trending high ridge and watershed. The western portion, situated upwind (the stoss slopes) of the mid-latitude westerlies, that bring moisture to the region, has gentle high topography and small valley glaciers. In contrast, on the eastern leeward slopes, gradients are higher and long debris-covered valley glaciers are present. The hypsometry of the region indicate two peaks in the distribution frequency of elevation (3600-4100 m and 4400-4800 m asl). These two elevation zones are consistent in space with the former equilibrium-line altitudes during the Olimde and Subaxh glacial stages and suggest that glacial erosion (most effective at the ELA) has helped control topography. This observation supports the glacial buzz-saw hypothesis, which argues that glaciers determine hypsometry by means of rapid surface erosion. Based on TCN methods, basin-wide rates of erosion range from ~ 0.1 to 1.4 km/Ma and are five to ten times lower than the unroofing rate of both massifs. The discrepancy over different time scales suggests that initial unroofing was produced by abrupt tectonic uplift and that the unroofing of the massifs has continued at a slower pace during the Late Quaternary.