When did the Indus River of South-Central Asia take on its “modern” drainage configuration?

Abstract

For sedimentary archives to be used as a record of hinterland evolution, the factors affecting the archive must be known. In addition to tectonics, a number of factors, such as changes in climate and paleodrainage, as well as the degree of diagenesis, influence basin sediments. The Indus River delta-fan system of South-Central Asia records a history of Himalayan evolution, and both the onshore and offshore sedimentary repositories have been studied extensively to research orogenesis. However, a number of unknowns remain regarding this system. This paper seeks to elucidate the paleodrainage of the Indus River, in particular when it took on its modern drainage configuration with respect to conjoinment of the main Himalayan (Punjabi) tributary system with the Indus trunk river. We leverage the fact that the Punjabi tributary system has a significantly different provenance signature than the main trunk Indus River, draining mainly the Indian plate. Therefore, after the Punjabi tributary system joined the Indus River, the proportion of Indian plate material in the repositories downstream of the confluence should have been higher than in the upstream repository. We compared bulk Sr-Nd data and detrital zircon U-Pb data from the Cenozoic upstream peripheral foreland basin and downstream Indus delta and Indus Fan repositories. We determined that throughout Neogene times, repositories below the confluence had a higher proportion of material from the Indian plate than those above the confluence. Therefore, we conclude that the Indus River took on its current configuration, with the Punjabi tributary system draining into the Indus trunk river in the Paleogene, early in the history of the orogen. The exact time when the tributary system joined the Indus should correlate with a shift to more Indian plate input in the downstream repositories only. While the upstream repository records no change in Indian plate input from Eocene to Neogene times, a shift to increased material from the Indian plate occurs at the Eocene−Oligocene boundary in the delta, but sometime between 50 Ma and 40 Ma in the fan. Though further work is required to understand the discrepancy between the two downstream repositories, we can conclude that the tributary system joined the Indus trunk river at or before the start of the Oligocene.