Recent drainage diversions under thrusting conditions in the Suusamyr Valley, the Tien Shan Range, Kyrgyzstan

The Suusamyr Valley in the Tien Shan Mountain Range is an intramontane depression bounded by convergent thrusts. This study focuses on the morphology of drainage diversions as a response to the progressive regional compression and the active deformation. Topographic cross sections, aerial photo analyses, satellite images, trenching, and luminescence dating of sediments were used in this study, as well as information from previous studies. The spectacular wind gaps and the buildup of shutter ridges indicate two morpho-tectonic stages related to the process of drainage diversions. Initial tectonic warping is evidenced by a linear hump topography or by a mild convex-upward longitudinal profile of streams. The trenches revealed steep (>15°) upstream-tilted layering and ponding of lenticular floodplain deposits. There is no evidence for heavy ponding followed by spillover and overriding of tectonic barriers. Morphologically, the fault activated by the 1992 earthquake (M_s = 7.3) continues the long-term northward migration of the Suusamyr River, thereby undermining the already heavily landslide-stricken slopes of the valley fill. The drainage system adapted to minor changes in relief and proved to be the finest sensor for recording slight initial deformations. The luminescense ages indicate a mid-Holocene to historic period of active deformation.