LATE QUATERNARY MORPHOTECTONIC EVOLUTION OF UPPER INDUS VALLEY PROFILE - A COSMOGENIC RADIONUCLIDE STUDY OF RIVER POLISHED SURFACES

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 morphotec tonic history of the Upper Indus valley, but also yield estimates of r iver incision rates during down-cutting of bedrocks. The three lower s urfaces, 21 m, 6 m and 1 m above the river level have monotonically de creasing cosmic ray exposure ages of 22, 15 and 3 ka BP, suggesting th at the river has been cutting the bedrocks initially (22-15 ka) at hig h rate of 2 mm yr(-1) which reduced to 0.3-0.4 mm yr(-1) since 15 ka B P, The latter is at par with the reported values of exhumation rate fo r the Ladakh region. The present data together with similar data from the Indus valley west of Skardu, reported earlier, give a better under standing of the so far poorly understood interplay of regional exhumat ion, incision rate and the role of climate in the morphotectonic evolu tion of the Upper Indus valley.