ACTIVE TECTONICS OF THE PAMIRS AND KARAKORAM

The source parameters of moderate- to large-sized earthquakes and the spatial distribution of earthquake hypocenters are used to investigate the active tectonics and three-dimensional configuration of the subducted lithosphere in the Pamir-Karakorum intracontinental convergence zone. The continental collision has resulted in intense deformation in the upper crust. The deformation is distributed over a broad area and is absorbed by thrust and strike-slip faulting. Along the northern margin of the Pamirs, where the deformation due to continental collision is most intense, the Northern Pamir Thrust and several major fault zones mark the present boundary. Most of the earthquakes that occurred in these fault zones are shallow crustal events with focal mechanism solutions correlated with the tectonic features of the region. Along the Pamir Front, thrust faulting dominates, while on the western and eastern edges of the Pamirs the style of deformation is characterized by oblique thrusting with a component of strike-slip motion. Thrust-type events beneath the Tadjik Depression indicate that both the sedimentary rocks and the basement are involved in shortening. Right-lateral strike-slip motion is observed on the eastern edge of the Pamirs where they border the Tarim Basin and in the Talas-Fergana-Kun Lun fault zone. A southward dipping seismic zone beneath the Pamirs and Karakorum indicates that the Asian lithosphere has been subducted along the Pamir Front to a depth of at least 200 km. This interpretation is consistent with published geological and geophysical data. Focal mechanism solutions of some intermediate-depth events beneath the Pamirs show strike-slip faulting with approximately N-S horizontally oriented P axes, indicating that nearly horizontal compression at the intermediate depth is the predominant mode of deformation. A 90-km-deep event beneath the Karakorum is interpreted as occurring in the subducted Asian lithosphere; the fact that the P axis of this event is oriented parallel to the descending plate suggests downdip compression within the Asian lithosphere. The overall tectonics of the region is interpreted as a consequence of the under-thrusting Asian lithosphere being impinged upon by the shallow northward underthrusting Indian lithosphere.