Small-scale convection in the upper mantle beneath the Chinese Tian Shan Mountains

Small-scale convection of the upper mantle beneath the Chinese Tian Shan (Tien Shan) is investigated in terms of numerical modeling. The finite element method combined with the marker-in-cell technique is used to describe the flow of the heterogeneous upper mantle. The density model is derived from the P-wave velocity structure of the crust and upper mantle along the Kuche-Kuitun profile across the Chinese Tian Shan, which is obtained using the seismic travel time tomography technique. Our computational results reveal the southward-counterclockwise and northward-clockwise upper mantle convective cells underneath the Junggar-north Tian Shan and Tarim-south Tian Shan, respectively. Our results also show the convective scale reaches to ∼500 km and the convective speed at the top of the upper mantle should not be less than 20 mm/year for a normal viscosity model. The northward extrusion of the Tarim block plays a key role in the Tian Shan mountain building since the Cenozoic period, but it nearly does not influence the upper mantle convection. The present-day tectonic deformation in the Chinese Tian Shan is related to the small-scale convection of the upper mantle.