An intermontane basin formed by damming and drainage reorganization: The Kathmandu Basin of Nepal

In the central Himalaya, there are dramatic contrasts in topography and drainage configurations along the strike of the range. The Kathmandu Basin of central Nepal is a relatively high (∼1250 m from mean sea level msl), small alluviated intermontane basin drained by the Bagmati River. In contrast, the neighboring trans-Himalayan Koshi and Gandaki rivers are steeply incised bedrock rivers (∼550 m from msl). It has been hypothesized that the Bagmati River of the Kathmandu Basin represents the remnants of a Pliocene trans-Himalayan river (Proto-Bagmati River) that was comparable to the modern Koshi and Gandaki rivers, but that has since been captured by these neighboring catchments. In this study, we employ multiple sedimentological techniques to analyze Pliocene fluvial deposits preserved at the base of the Kathmandu Basin, which lie unconformably on Precambrian−Paleozoic basement rocks. We reconstruct hydrology, current directions, and sediment transport distances of the Pliocene fluvial deposits to approximate the scale of the Proto-Bagmati River relative to the modern. We also examine the provenance of granitic clasts in the Pliocene conglomerates using U-Pb zircon ages. Our results indicate that the discharge and transport distance of the Proto-Bagmati River was greater in Pliocene times. The provenance investigation also supports that the network extended northward into the High Himalaya. When combined with structural evidence, we propose a tectono-stratigraphic model whereby localized accelerated rock uplift in the Himalayan thrust wedge resulted in damming and alluviation of the paleo-Bagmati River. The contrast of channel gradients between the alluviated paleo-Bagmati River and the neighboring Koshi and Gandaki rivers is interpreted to have resulted in drainage reorganization of the paleo-Bagmati river network by its steeper neighboring catchments, and consequent alluviation and formation of the present-day Kathmandu Basin.