Quaternary channel-focused rapid incision in the Phung Chu-Arun River in Central Himalaya: Implications for a Quaternary capture event

The Phung Chu-Arun River, one of the few major trans-Himalaya rivers, consists of a large drainage area captured in the northern slope of the Himalaya, and provides a key to understand tectonic, surface processes and their interplay in topographic development of the Himalaya. In this study, we carried out thermochronologic and topographic analysis along the Phung Chu-Arun River in Central Himalaya to reveal its incision development. Apatites from a downstream section along its major tributary yield very young fission track and (U-Th)/He ages of 0.8-1.5 Ma. These cooling ages, combined with previously reported data for higher elevations, reveal an abrupt change of the slope in the age-elevation plot suggesting an accelerated channel incision initiated at between ∼1.5 and 2.5 Ma. The age-elevation relationship yields a high apparent exhumation rate (AER) of ∼2.6-3.8 mm/a during Quaternary, which is spatially focused along the mainstream of the Kharta-Chentang reaches and the associated nearby tributaries. Topographic analysis suggests that this channel-focused incision acceleration was probably facilitated by a Quaternary capture event in upper drainage of the palaeo Arun River. This capture event greatly enhanced the effective discharge draining southward, causing an accelerated incision wave along the mainstream and tributaries of the palaeo Arun River. Furthermore, thrusting at physiographic transition zone is also required to sustain high channel gradient and the long-term rapid incision. Channel morphology presenting common knick points in major tributaries suggests that the drainage basin in Ama Drime Range area likely remain in pre-steady state adjustment of this capture event.