Regional Pliocene Exhumation of the Lesser Himalaya in the Indus Drainage

Abstract. New bulk sediment Sr and Nd isotope data, coupled with U-Pb dating of detrital zircon grains from sediment cored by International Ocean Discovery Program in the Arabian Sea, allow reconstruction of erosion in the Indus catchment since ~ 17 Ma. Increasing εNd values from 17 to 9.5 Ma imply relatively more erosion from the Karakoram/Kohistan, likely linked to slip on the Karakoram Fault and compression in the Southern and Eastern Karakoram. After a period of relative stability from 9.5 to 5.7 Ma there is a long-term decrease in εNd values that correlates with increasing relative abundance of > 300 Ma zircon grains that are most common in Himalayan bedrocks, precluding large-scale drainage capture as the cause of decreasing εNd values in the submarine fan. Although the initial increase in Lesser Himalaya-derived 1500–2300 Ma zircons after 8.3 Ma is consistent with earlier records from the foreland basin the much greater rise after 1.9 Ma, has not previously been recognized and suggests that widespread unroofing of the Crystalline Lesser Himalaya and to a lesser extent Nanga Parbat did not occur until after 1.9 Ma. No simple links can be made between erosion and the development of the South Asian Monsoon, implying a largely tectonic control to Lesser Himalayan unroofing.