The Pamir-Hindu Kush seismic zone as a strain marker for flow in the uppermantle

We introduce a new hypothesis to explain the unusual pattern of intermediat e-depth earthquakes in the Pamir-Hindu Kush region of central Asia. In our model the seismicity of the zone is defined by a remnant piece of oceanic c rust that became neutrally buoyant and now hangs in the mantle underneath t he center of the active mountain belt. We interpret an abrupt cutoff in sei smicity shallower than 70 km under the Pamir as evidence that the slab has become decoupled from the surface deformation, and we suggest that the remn ant slab has acted as a strain marker for flow in the upper mantle. The con tortion of the slab can be explained by superimposed vertical and lateral s imple shear related to the western edge of the Indian Plate. This model req uires relatively low strain rates of the order of 10(-15) s(-1), consistent with strain rates estimated from earthquake moment release in Benioff zone s. In contrast, the double-facing subduction zone model applied by a number of previous authors requires strain rates of the order of 10(-13) s(-1), w hich are much higher than inferred strain rates in subducting slabs. The re sults are important for understanding mantle dynamics, providing a unique w ay to measure strain in the upper mantle. The implied flow field also has s ignificant implications for the kinematics of the entire collisional plate boundary in Tibet and the Tien Shan.