3-DIMENSIONAL ELASTIC-WAVE VELOCITY STRUCTURE OF THE WESTERN AND CENTRAL TIEN-SHAN

Arrival times of compressional (P) and shear (S) waves generated by ea rthquakes at local and teleseismic distances and recorded by seismogra phs located in the western and central Tien Shan are used to determine one- and three-dimensional elastic wave velocity structures of the cr ust and upper mantle beneath the mountain belt. The best fit one-dimen sional structures suggest that the average depth of the Mohorovicic di scontinuity in this area is 50 km. The three-dimensional structure of the upper crust reveals thick sediments within each of the major depre ssions in the region. A 7 km-thick wedge of sediment beneath the Chu D epression is outlined at depth by a south dipping plane of seismic act ivity, suggesting the presence of an active decollemont. These low vel ocities extend continuously to the southeast toward Issyk-Kul, suggest ing a structural relationship between the two. However, rather than be ing consumed, it appears that Issyk-Kul is overthrusting the surroundi ng ranges. The low-velocity sediments in the Fergana basin reach depth s of 10 km and are bounded on three sides by amorphous bands of seismi city. Velocities at midcrustal depths generally are lower beneath the central Tien Shan than beneath the western Tien Shan. This pattern bec omes more evident in the uppermost mantle, with P velocity contrasts o f as much as 10% across a boundary that corresponds roughly to the geo graphical position of the Talasso-Fergana fault. The low velocities be neath the central Tien Shan exceed 150 km depth but do not appear to b e deeper than 300 km depth. There is no evidence for a lithospheric ro ot beneath this part of the range; rather, the low velocities imply th e presence of a positive buoyancy force uplifting the mountains. Evide nce that this low-velocity region existed before the collision suggest s that the Tien Shan may not owe its rejuvenation simply to its locati on at the northern edge of a strong Tarim basin but rather to an anoma lous upper mantle that was easier to deform than the surrounding litho sphere.