Cenozoic right-slip faulting along the eastern margin of the Pamir salient, northwestern China

Abstract

The Pamir salient defines the western end of the Himalayan-Tibetan orogen and has overthrust the Tarim-Tajik basin to the north by ∼300 km along a late Cenozoic, south-dipping intracontinental subduction zone. Field mapping, structural measurements, and analysis of mesoscale structures along a 32-km-long reach of the Yarkand River document the tectonic evolution of the east flank of this salient, between the North Pamir to the west and the Western Kunlun Shan to the east. The study area is cut by a set of four, north-northwest–striking, steeply dipping brittle faults. Microstructures and asymmetric outcrop- to map-scale folds indicate right slip along these faults. Between these structures, fault-bounded panels of Phanerozoic strata are deformed by en echelon folds with axes that trend more westerly than the adjacent faults, consistent with dextral transpression. The fault system described here extends for ∼350 km along the eastern flank of the Pamir salient. Transpressional right slip along this set of faults, here called the Kashgar-Yecheng transfer system, appears to have accommodated late Cenozoic separation of the North Pamir from the Western Kunlun Shan during south-directed intracontinental subduction beneath the leading edge of the Pamir salient. Correlation of major faults suggests total slip along the Kashgar-Yecheng transfer system is likely on the order of ∼280 km. This offset estimate implies long-term slip rates of 7–15 mm/a along the Kashgar-Yecheng transfer system when combined with previous sedimentologic, stratigraphic, and thermochronologic data that indicate deformation along the east flank of the Pamir started between the late Eocene and early Miocene. These results imply that the first-order structures on the western and eastern flanks of the Pamir are asymmetric: previous work has shown that deformation in the west was accommodated by anticlockwise vertical axis rotation of the Pamir over the eastern margin of the Tajik basin. This rotation is generally interpreted to reflect northwest-directed radial thrusting, in contrast to the transpressional right-slip transfer faulting on the east side reported here.