Mantle transition zone discontinuities beneath the Tien Shan

To better understand geodynamic processes of intracontinental mountain building, we conduct a systematic investigation of the mantle transition zone (MTZ) beneath the Tien Shan and its surrounding areas using a receiver function method under non-plane wave front assumption. The resulting apparent depths of the 410 km (d410) and 660 km (d660) discontinuities and the MTZ thickness display significant lateral variations. Both the central Tien Shan and the Pamir Plateau are characterized by a thick MTZ, which can be well explained by the existence of lithospheric segments resulted from possible break-off of the subducted slab or lithosphere delamination. A thin MTZ and an obviously depressed d410, which may be induced by asthenosphere upwelling associated with the dropping lithospheric segment, are revealed beneath the Kazakh Shield. Seismic evidence is obtained for the potential existence of lower mantle upwelling beneath the Tarim Basin based on the observed thin MTZ and relatively significant uplift of d660. The subduction of the Kazakh Shield and Tarim lithosphere driven by the India–Eurasia collision possibly plays an essential role in the formation and evolution of the Tien Shan orogenic belt, and the lower mantle upwelling revealed beneath the Tarim Basin may promote the uplift of the Tien Shan by softening the upper mantle.