EVIDENCE FOR A HIGH-VELOCITY SLAB ASSOCIATED WITH THE HINDU-KUSH SEISMIC ZONE

We used teleseismic travel time residuals to determine lateral velocit y variations of the crust and upper mantle in the Pamir-Hindu Kush reg ion in Tadjikistan and Afghanistan. Data from 29 analog seismic statio ns in Tadjikistan and northern Afghanistan were used to determine trav el time residuals for 210 teleseismic events ranging in distance from 28 degrees to 87 degrees and covering a broad range of azimuths. We in verted for velocity perturbations over a rectangular grid with a block size of 99 x 99 km. The model extended to a depth of 350 km with a 50 -km-thick first layer and two 150-km-thick deeper layers. The results show a strong and well-resolved zone of high velocities in the upper m antle at depths greater than 200 km, coincident with the location of t he Hindu Kush seismic zone. No clear velocity perturbations are associ ated with the Pamir seismic zone. Above 200 km little correlation is o bserved with the seismic zone, but indications of thicker crust under the Pamir and thinner crust under the Tadjik Depression are seen. The high velocities are most likely caused by the presence of oceanic lith osphere at depth.