Drainage development in an intra-continental mountain belt: A case study from the south-Central Tian Shan

Abstract

As the highest intra-continental orogen in Central Asia, with summits above 7000 m, the Tian Shan orogenic belt has experienced multiple phases of orogeny, and has been reactivated since the early Cenozoic in response to the India-Asia collision. In the south-Central Tian Shan, sedimentary and thermochronology records suggest that Cenozoic deformation initiated from the late Oligocene to the early Miocene, leading to the building of widely-spaced mountain ranges. The Kyrgyz south-Central Tian Shan is characterized by a significant contrast between a longitudinal (i.e., strike-parallel) drainage pattern in the west and a transverse (i.e., strike-perpendicular) drainage in the east. However, it is not clear how the drainage pattern, a key topographic feature in orogenic belts, responded to Cenozoic structural reactivation and uplift of individual ranges.We focus here on the transition area between the regions of longitudinal and transverse drainage: the anomalously large Saryjaz catchment, which drains the highest part of the south-Central Tian Shan and shows a complex and peculiar drainage pattern. Through geomorphic observations and existing geological (i.e., structural and lithological) data, we analyze drainage characteristics, including longitudinal profiles in χ-space, knickpoints, and normalized channel steepness to understand the possible controls on the observed drainage pattern. We discriminate between knickpoints of different origin: tectonic (potentially linked to active faults), lithologic, glacial and linked to transient waves of incision.We find a series of transient knickpoints in tributaries downstream of a sharp U-shaped bend along the main stem of the Saryjaz catchment, which also shows a striking increase in channel steepness. Both observations indicate recent incision along this reach. The incision depth and the elevation of knickpoints both show a decreasing trend downstream. These results suggest that incision is driven “top-down” by a large-magnitude capture event rather than “bottom-up” by a base-level drop. We link this capture to ongoing replacement of the longitudinal drainage system to the west by the transverse one to the east, consistent with inferred patterns of drainage development in other intra-continental mountain belts and suggesting a more mature stage of drainage development in the east.