Uppermost mantle structure and its relation with seismic activity in the central Tien Shan

Using arrival data from GHENGIS and Kyrgyz Network (KNET) stations, we determined the uppermost mantle structure of the central Tien Shan from Pn tomography. Pn velocities in the north-central Tien Shan are slower than normal for stable lithosphere. They are very slow in the south-central Tien Shan, where a hot mantle previously has been postulated. The Pn velocity variation correlates well with seismicity. Most earthquakes are concentrated in the north-central Tien Shan, but almost no earthquakes in the south-central Tien Shan, even though both geologic and GPS observations show active convergence there. This correlation suggests a genetic relationship between seismicity and the state of lithospheric mantle. Specifically, we propose that the very slow Pn velocity beneath the south-central Tien Shan is caused by high temperatures produced by heating from mantle upwelling, and that this heat has conducted well up into the crust, inducing a ductile rheology that prevents significant seismicity.