Differential structural and geomorphic mountain-front evolution in an active continental collision zone: The northwest Pamir, southern Kyrgyzstan

Western, central, and eastern segments of the Trans Alai mountain front in the northern Pamir of Kyrgyzstan have accommodated varying degrees of approachment of the Pamir orogen with respect to the Tien Shan (Shan = Mountains) to the north. Ongoing collision between the northwestern corner of the Indian indenter and Eurasia has resulted in closure of the intramontane Alai Valley, which separates the Tien Shan and Trans Alai (Pamir) ranges. The different segments highlight the processes of shaping tectonically active mountain fronts in a semiarid environment. In this study, we have characterized this variation in processes with compilations of regional tectonic information, detailed geologic and geomorphic maps, topographic analyses, and interpretation of seismic reflection data. Along the sinuous western segment of the mountain front, dextrally oblique thrusting has created a wide (>500 m) zone of highly erodible fault gouge. This fault zone impinges on the southern Tien Shan, but complete basin closure is prevented by erosion due to the westward- flowing Kyzilsu River; the Kyzilsu valley forms the only outlet and is the vestige of a formerly contiguous sedimentary basin linking the Tarim Basin of China with the Tadjik Depression in the west. Numerous large landslides rooted in the fault zone have covered the active fault, which is partially undercut by the Kyzilsu River. Older, large landslides in this setting are associated with different levels of fluvial terraces of the former or present course of the Kyzilsu River, suggesting a causative relationship between lateral fluvial scouring, failure of mechanically weak mountain fronts, ongoing faulting, and mass transfer. Along the linear central segment, deformation is confined to a narrow single south-dipping thrust fault that juxtaposes Pliocene-Pleistocene and Holocene conglomerates. In this sector, the mountain front has numerous Holocene offsets. This prevailing structural style and the long-term deformation are underscored by multiple flights of gently sloping pediments and glaciogenic terrace surfaces that abruptly terminate at the steep mountain front, which also forms the boundary with the wide regraded piedmont. In contrast, closure between the Pamir and Tien Shan is complete along the eastern segment. The eroded and sinuous mountain front has been tectonically inactive during late Quaternary time. Small drainage-basin areas and low stream power apparently were not conducive to maintaining an eastern outlet to the Tarim Basin. Active deformation has stepped back into the orogen and now is concentrated along the Markansu Fault and within the Tien Shan to the north. The large drainage-basin area of the Kyzilsu River and the constant, glacially fed runoff guarantee that an effective interplay between tectonic uplift and erosion is maintained. Therefore, the geomorphically different mountain-front segments highlight the relationships between tectonic uplift and geomorphic processes, which in turn are controlled by lithology, topography, and the history of sediment routing throughout the landscape.