Seismotectonics of the Tajik Basin and Surrounding Mountain Ranges

Abstract

Seismologic and geologic fault-slip data characterize the active deformation of the intramontane Tajik basin and its margins, the Tian Shan, Pamir, and Hindu Kush at the northwestern tip of the India-Asia collision zone. Within this complexly deforming region, the Tajik basin lithosphere forms the backstop for the north-dipping Indian-slab subduction beneath the Hindu Kush but itself delaminates and retreats west and northward beneath the Pamir. Herein, we link crustal deformation to these lithosphere-scale processes, using data from recently deployed seismic networks and geologic observations. Transpressive strike-slip deformation dominates the bounding fault zones along the basin's northern and eastern margins. Seismicity is most intense in the Garm region/Peter I. range of the northeastern basin, where these bounding faults converge and gain a dominant thrust component. Within the basin, seismically and geologically derived P axes align with the ~W-trending GPS velocity vectors. Seismicity is concentrated in and at the base of a southward deepening, ∼9–15-km-thick wedge. Seismic deformation at the basin's southern margin occurs beneath the Afghan platform, where deep crustal earthquakes likely trace the western end of the Hindu Kush subduction zone. Roughly NNE-striking sinistral strike-slip events outline the Hindu Kush-Pamir transfer system, a zone of distributed shear in the crust overlying the transition of the two oppositely dipping slabs at subcrustal depths. Our observations suggest that crustal deformation in the Pamir and Hindu Kush links with lithosphere-scale processes, whereas deformation in the basin is controlled by the westward gravitational collapse of the Pamir and the resultant basin inversion.