Stable Drainage Pattern and Variable Exhumation in the Western Himalaya since the Middle Miocene

Sedimentary records in peripheral basins of mountain belts record changes in erosion dynamics and drainage-network reorganization, but it is often difficult to discriminate between these different controls. Geochemical provenance data on paleo-Indus deposits from the western Himalayan foreland provide constraints on the possible variation of the position of the drainage divide between the Indus and Ganges river systems. Here we present geochemical (trace element and Hf-Nd isotopic) and thermochronological (detrital zircon fission-track [DZFT]) analyses of modern Indus and Miocene Siwalik sediments from northern Pakistan and compare these with published data on the Indus Fan. Available bedrock isotopic data are used to define three end-member sediment sources (Himalaya, Karakorum, and the Kohistan-Ladakh arc) and to calculate the contribution of each of these sources to the foreland basin and Indus Fan. Our results indicate that since the Miocene the contribution of the Himalayan rivers reaching the Indus in the foreland remained constant, whereas the contributions of sediment sources of the upper Indus catchment changed: those of the Kohistan-Ladakh arc diminished strongly in favor of Karakorum and Himalayan sources. Analysis of the DZFT data from the Miocene foreland basin and sediments of the modern upper Indus reach suggests that the exhumation pattern changed due to an increase in exhumation rate of the Karakorum and Himalayan units of the syntaxis since Miocene times, whereas that in the Kohistan-Ladakh arc remained relatively stable. These results imply that the Indus sediments record changing relative erosion rates in the different source regions rather than widespread drainage rearrangement, as suggested previously.