Seismotectonics of the Tajik basin and north-eastern Afghanistan

The Pamir-Hindu Kush at the north-western tip of the India-Asia collision zone stands out due to its worldwide unique zone of intense intermediate depth seismicity, which has been attributed to ongoing continental subduction. The lithosphere of the adjacent intra-montane Tajik basin was proposed to form the backstop for this north-dipping Indian-slab subduction beneath the Hindu Kush but itself delaminates and retreats west and northward beneath the Pamir. In the south, the Tajik basin is bounded by the poorly studied Afghan Platform and Hindu Kush of NE Afghanistan. Frequent Mw 7+ earthquakes occur both at crustal and sub-crustal depths within this remote region. This poses the question to which extent crustal deformation is coupled to sub-crustal processes.

Herein, we present seismological data allowing to characterize the active deformation within the Pamir-Hindu Kush mountains and the Tajik-Afghan basin. Our seismotectonic analysis is based on the recordings of the TIPTIMON seismic network, deployed in the Tajik basin, Hindu Kush, and western Pamir between 2012 and 2014. This data is extended by a 15-sites covering seismic network in NE Afghanistan which was installed in autumn 2017. The network is situated on top of the nest of intermediate depth seismicity and further west in the Afghan Platform.

Results on seismicity distribution show that seismic deformation in the Tajik basin is focused on the bounding fault zones along the basin's northern and eastern rims. Hypocenters in the Tajik basin gradually deepen southward from 9 to 15 km, possibly outlining the Jurassic evaporite décollement or a deeper detachment. The stress orientations derived from seismic moment tensors coincide with the trends of westward oriented GPS-velocity vectors. Thus, deformation in the basin seems to be mainly influenced by the gravitationally driven westward extrusion of Pamir's crust into the depression occupied by the Tajik basin. Preliminary results from our most recent network suggest that crustal seismicity in NE Afghanistan is dominated by compression at the western edge of the Hindu Kush continental subduction zone. Further, there seems to be a spatial correlation between the sub-crustal termination of the subducting slab and the region of most pronounced crustal seismicity.