A 450 km long north-south magnetotelluric profile spanning the Tien Shan from Kazakhstan to western China reveals lateral variations in the resistivity of the mantle lithosphere to depths of 140 km. Minimum changes of one order of magnitude in this depth range result from variations in temperature or composition, or both. Higher resistivities beneath a central portion of the range where the Moho is half as deep as elsewhere in the Tien Shan indicate a strong lithospheric block. We propose that this block protects this region of thin crust from appreciable deformation and instead transmits stresses due to India-Asia plate convergence from the southern to the northern parts of the range.