Arrival times of compressional (P) and shear (S) waves generated by earthquakes at local and teleseismic distances and recorded by seismographs located in the western and central Tien Shan are used to determine one- and three-dimensional elastic wave velocity structures of the crust and upper mantle beneath the mountain belt. The best fit one-dimensional structures suggest that the average depth of the Mohorovicic discontinuity in this area is 50 km. The three-dimensional structure of the upper crust reveals thick sediments within each of the major depressions in the region. A 7 km-thick wedge of sediment beneath the Chu Depression is outlined at depth by a south dipping plane of seismic activity, suggesting the presence of an active decollemont. These low velocities extend continuously to the southeast toward Issyk-Kul, suggesting a structural relationship between the two. However, rather than being consumed, it appears that Issyk-Kul is overthrusting the surrounding ranges. The low-velocity sediments in the Fergana basin reach depths of 10 km and are bounded on three sides by amorphous bands of seismicity. Velocities at midcrustal depths generally are lower beneath the central Tien Shan than beneath the western Tien Shan. This pattern becomes more evident in the uppermost mantle, with P velocity contrasts of as much as 10% across a boundary that corresponds roughly to the geographical position of the Talasso-Fergana fault. The low velocities beneath the central Tien Shan exceed 150 km depth but do not appear to be deeper than 300 km depth. There is no evidence for a lithospheric root beneath this part of the range; rather, the low velocities imply the presence of a positive buoyancy force uplifting the mountains. Evidence that this low-velocity region existed before the collision suggests that the Tien Shan may not owe its rejuvenation simply to its location at the northern edge of a strong Tarim basin but rather to an anomalous upper mantle that was easier to deform than the surrounding lithosphere. ¿ American Geophysical Union 1993