Seismotectonic segmentation of the India-Eurasia collision zone in the northern Pamir Mountains, Kyrgyzstan

ARROWSMITH, J R., Department of Geology, Arizona State University, Tempe, AZ, 85287,; STRECKER, M., Institut fuer Geowissenschaften, Universitaet Potsdam, Postfach 60 15 53, D-14415 Potsdam, Germany,

The Trans Alai Range along the northern perimeter of the Pamirs of Kyrgyzstan is at the leading edge of the India-Eurasia collision zone where continental crust is subducted southward. The range defines a section of the plate margin which is divided into eastern (E of 72 degrees, 55 E; 39 degrees, 30 N), central, and western (W of 72 degrees, 15 E;39 degrees, 28 N) segments bounding seismotectonic blocks that record differential absorption of plate convergence. The 40-km-long east-west striking central fault segment has dip-slip thrust fault offsets and is linked to the other segments via NW-striking transfer faults with dextral strike-slip kinematic indicators. Along the central segment, the active mountain front is straight due to repeated offset of regionally correlative fluvioglacial terrace gravels which grade into moraine deposits upstream. The vertical offset of the correlated terrace surface is 0 m at the western transfer zone; it rapidly increases to 13 m 4 km E; and reaches a maximum of 18 m about 8 km E of the transfer, but diminishes again to 0 m at the eastern transfer. At the Syrinanjar River, intercalated organic material from faulted gravels of the terrace deposit yield a dendrocorrected radiocarbon age of 4535 to 4350 years BC (a maximum age for the terrace surface). The vertical offset of the terrace there is 18 m and with a dip of the fault zone of 27 degrees we obtain a total slip of 40 m, resulting in a minimum Holocene dip-slip rate on the principal thrust fault of 6.2 mm/yr. The western transfer faults are characterized by striated fault surfaces, pressure ridges at stepovers, aligned springs along straight mountain fronts, and beheaded stream channels. The faults terminate in dextrally oblique thrust faults which bound more sinuous mountain fronts and uplifted terrace and pediment surfaces. The westernmost transfer fault records the structural transition to predominantly N vergent thrusting that persists for 10s of km to the W via N vergent low-angle thrusts rooted in a steeply S-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 of transfer faults.