We determined the structure of the crust beneath the central and western Tien Shan by analyzing broadband (1--20 s) analog seismograms of converted P-SV phases generated by earthquakes at teleseismic distances and recorded by 14 seismograph stations. The one-dimensional structures that best explain the waveforms reveal pronounced differences in crustal velocities east and west of the Talasso-Fergana fault. Specifically, the transition zone between the crust and the mantle east of the fault is about 2 times broader than the west of the fault. Also, velocities at depth between 10 and 35 km are about 10% lower in the east, although the depth range of these lower velocities is not well resolved. The Talasso-Fergana fault is an important boundary for other observables; in addition to the structural discontinuities observed at the surface, the areas east of the Talasso-Fergana fault are associated with abnormally low mantle velocities, outcrops of basalt, low Q, and short-wavlength variations in anisotropy. Integrating these observations, we interpret the broad mantle gradient as being due to vertical intrusions of mantle material into the lower crust. Likewise, the low velocities in the midcrust could be thermally induced or due to the introduction of magmatically derived fluids. The gross contrast in structure east and west of the Talasso-Fergana fault could reflect a contrast in the dynamics of mountain building in these two regions. We postulate that while crustal shortening is the dominant mechanism controlling topography west of the Talasso-Fergana fault, vertical uplift caused by a mass deficiency in the upper mantle may contribute significantly to the generation and maintenance of the high elevations east of the fault. ¿American Geophysical Union 1993