HORIZONTAL TRANSLATION OF A LONGITUDINAL RIVER VALLEY DURING OROGENESIS: DENUDATION OF THE INDUS VALLEY, NORTHERN INDIA AS RECORDED BY LOW TEMPERATURE THERMOCHRONOMETRY AND LANDSCAPE ANALYSIS

The horizontal translation of longitudinal rivers in response to thrust shortening challenges the widely held view that major rivers are largely antecedent in mountain belts. Recent developments in the application of (U-Th)/He as a low temperature thermochronometer allows for direct measurement of denudation in an area where there is strong geomorphologic evidence for interaction between tectonics and erosion. The Indus River valley, north India, represents the major longitudinal river system of the western Himalaya. The Ladakh batholith (NW Himalayas, India) was emplaced prior to collision of India and Eurasia between 52 and 50 Ma. The batholith trends WNW is 600 km long and between 30 and 50 km wide. The batholith is relatively undeformed since Cretaceous times and so preserves long term geomorphic features. It is bounded to the north by the Shyok river and to the south by the Indus river, both of which form major longitudinal river valleys and into which numerous tributaries flow. The batholith is divided asymmetrically by the main catchment divide identified on digital elevation models (DEM). Either side of the catchment divide clear variations are evident in the river drainage patterns and as a consequence in the development of geomorphic features across the Ladakh batholith. Two vertical transects across the Ladakh batholith have been sampled for analyses by (U-Th)/He to determine the horizontal component of valley incision. The measurements of the shape of drainage basins have been used to test the hypothesis that local drainage form is a function of the horizontal motion of major longitudinal valleys in response to tectonic activity and to quantify the relationship between erosion history and catchment development. These and future results from the thermochronometric and geomorphic analyses will be used in assessing landscape development in the region and will question the traditional interpretation of river systems.