SHEAR-WAVE SPLITTING AND SMALL-SCALE CONVECTION IN THE CONTINENTAL UPPER MANTLE

THE deformation of the upper mantle beneath a collisional belt is key to the dynamics of the belt, but this deformation is difficult to observe. Seismic azimuthal anisotropy manifested by shear-wave splitting provides the best geophysical evidence of deformation in the upper mantle. Here we use observations of shear-wave splitting to investigate deformation in the upper mantle beneath the Tien Shan in Central Asia. We find that underneath most of the Tien Shan the fast direction of azimuthal anisotropy is roughly parallel to the axis of the belt, but it deviates by nearly 90-degrees where the upper mantle is anomalously hot. The former relationship, previously observed in other collisional belts, seems to be usual for this environment. The anomalous directions, however, provide evidence of a rising plume beneath the range, suggesting more generally that the along-strike flow of the mantle beneath collisional belts is a reflection of thermally driven convection.