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Abstract

Fluvial drainage patterns in orogenic belts reflect interactions among tectonics, climate, and lithology. The central South Tian Shan displays a complex fluvial drainage pattern that shifts from longitudinal (flowing parallel to mountain ranges) in the west to transverse (flowing across ranges) in the east. Whether such drainage patterns reflect underlying patterns of tectonic deformation, lithology, climate, or the influence of surface processes within a drainage basin is often unclear. We focus here on the large Saryjaz catchment of SE Kyrgyzstan, which marks the transition between longitudinal and transverse drainages. We analyse topographic and fluvial metrics including slope, river steepness (ksn), and the integral proxy (χ) along the river profile, and we map the spatial distribution and characteristics of knickpoints to discern the possible controls on the observed drainage pattern. We discriminate between knickpoints of different origin, namely tectonic, lithologic, glacial, and those linked to transient waves of incision. We find a series of transient knickpoints in tributaries downstream of a sharp 180° bend in the main stem of the Saryjaz river, which also marks a striking increase in channel steepness. Both observations indicate accelerated incision along this lower reach of the catchment. Knickpoint elevations and ksn below the knickpoints decrease downstream, whereas χt values of knickpoints (χ measured from the tributary junctions) remain near-constant among tributaries. These results, together with a comparison of other rivers in the vicinity that show no evidence of transient knickpoints, suggest that transient incision in the Saryjaz catchment is driven “top-down” by a large-magnitude river capture event rather than “bottom-up” by accelerated uplift or base-level fall. We estimate an erodibility parameter from 10Be-derived catchment-average denudation rates and use this to estimate the celerity of knickpoints. We find that the knickpoints started retreating at a similar time between ca. 1.5 and 4.4 Myr ago. Considering the river patterns and the timing constraints, we suggest that this capture event was likely driven by the overfill of Neogene intermontane basins, potentially affected by both tectonic and climate factors.