Late Quaternary morphotectonic evolution of Upper Indus valley profile: A cosmogenic radionuclide study of river polished surfaces

摘要

We have studied cosmogenic exposure ages in a set of five samples from the river polished surfaces (1-46 m above present river level) of the Indus river (Ladakh), at maximum discharge yield during August. These ages not only provide an opportunity to unravel the complex morphotectonic history of the Upper Indus valley, but also yield estimates of river incision rates during down-cutting of bedrocks. The three lower surfaces, 21 m, 6 m and 1 m above the river level have monotonically decreasing cosmic ray exposure ages of 22, 15 and 3 ka BP, suggesting that the river has been cutting the bedrocks initially (22-15 ka) at high rate of 2 mm yr(-1) which reduced to 0.3-0.4 mm yr(-1) since 15 ka BP, The latter is at par with the reported values of exhumation rate for the Ladakh region. The present data together with similar data from the Indus valley west of Skardu, reported earlier, give a better understanding of the so far poorly understood interplay of regional exhumation, incision rate and the role of climate in the morphotectonic evolution of the Upper Indus valley. [References: 23]