Seismotectonic range-front segmentation and mountain-belt growth in the Pamir-Alai region, Kyrgyzstan (India-Eurasia collision zone)

The Trans Alai Range along the northern perimeter of the Pamir region of Kyrgyzstan is at the northern edge of the India-Eurasia collision zone. The range defines a section of Main Pamir thrust, which is divided into eastern, central, and western segments that record differential absorption of plate convergence. The 50-km-long, east-west-striking, central fault segment of the Main Pamir thrust has dip-slip thrust fault offsets (with a minimum Holocene dip-slip rate of ∼6 mm/yr) and is linked to the other segments via northwest-striking dextral transfer. The western transfer faults are well characterized geomorphically, and the westernmost records the predominant transition to north-vergent thrusting along the western segment via north-vergent, low-angle thrust faults rooted in a steeply south-dipping dextral shear zone. In contrast, the eastern segment deformation is widely distributed and geomorphically less evident, but the transfer also takes place in a structurally complex zone. Asymmetric offset of a regionally correlated terrace, geomorphic zonation, and the distribution of historic earthquakes suggest that the central segment is mechanically linked with the western segment, but not with the eastern segment. The progressive closure of the Alai Valley by the northward advance of thrusting exemplifies the annihilation of an intramontane basin. If the high surface uplift rates implied by these geologic observations have been maintained for the last few million years, they attest to the high level of tectonic activity and the recency of construction of the Trans Alai Range.