Spatial distribution and focal mechanisms of mantle earthquakes in the Hindu Kush–Pamir region: A contorted Benioff zone

Abstract

Seismological evidence based on the spatial distribution and focal mechanism solutions of mantle earthquakes in the Hindu Kush–Pamir region supports a model in which suboceanic lithosphere that existed between the converging Eurasian and Indian continents is subducted into the upper mantle. Two lines of evidence support this model. First, a careful selection of well-defined hypocenters in the region from 1950 through 1973 defines a thin (about 25 km thick) contorted Benioff zone within the upper mantle. The zone dips steeply (about 70°) toward the north in the western part of the region and has a shallower dip (about 50°) toward the southeast in the eastern part of the region. Second, focal mechanism solutions of intermediate-depth earthquakes have extensional (T) axes oriented parallel to the dip of the Benioff zone. The thin slab-like distribution of hypocenters and the downdip extensional stress are similar to features found in many other more typical regions of active lithospheric subduction. In addition, comparison of the focal mechanism solutions and the spatial distribution of earthquakes demonstrates the presence of a fault plane at about 200 km depth within the western part of the subducted lithosphere. Similar fault planes have also been observed in other areas of subduction. Comparison of the seismicity of the Hindu Kush–Pamir region with the apparent lack of mantle earthquakes along the main Himalayan arc suggests that the region is the site of the final stage of subduction of suboceanic lithosphere along the collision boundary between the Indian and Eurasian plates.