CRUSTAL AND UPPER-MANTLE STRUCTURE UNDER THE TIEN-SHAN FROM SURFACE-WAVE DISPERSION

Long-period seismograms of nuclear explosions and natural earthquakes in central Asia, recorded at three digital stations of the Laboratoire de Detection Geophysique (LDG) network in France, attest to a signifi cant propagation effect in the Tien Shan area. This allows for an inve stigation of the deep structures under the range, which has been the s ite of particularly intense intracontinental deformation since about t he middle Miocene. Group velocity curves have been obtained by regiona lization for pure paths across either the central or eastern Tien Shan . They indicate a crust of approximately 50-55 km in thickness, and pa rticularly low shear-wave velocities under the central Tien Shan. Our study suggests lateral variation of the deep structures under the rang e, which could be related to a westward increase along the range of cu mulative amount and rate of shortening.