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

We introduce a new hypothesis to explain the unusual pattern of intermediate-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 crust that became neutrally buoyant and now hangs in the mantle underneath the center of the active mountain belt. We interpret an abrupt cutoff in seismicity shallower than 70 km under the Pamir as evidence that the slab has become decoupled from the surface deformation, and we suggest that the remnant slab has acted as a strain marker for flow in the upper mantle. The contortion of the slab can be explained by superimposed vertical and lateral simple shear related to the western edge of the Indian Plate. This model requires relatively low strain rates of the order of 10(-15) s(-1), consistent with strain rates estimated from earthquake moment release in Benioff zones. 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), which are much higher than inferred strain rates in subducting slabs. The results are important for understanding mantle dynamics, providing a unique way to measure strain in the upper mantle. The implied flow field also has significant implications for the kinematics of the entire collisional plate boundary in Tibet and the Tien Shan.