Gravity anomalies over the Ferghana Valley, a deep sedimentary basin in the western Tien Shan, differ from those expected for local Airy isostatic equilibrium by 120 mGal and imply a deficit of mass beneath the basin equivalent to a Moho deeper by 10 km than prevails where isostasy holds. Although the Ferghana Valley is underlain by thick Mesozoic and Cenozoic sediment, measurements of density show that this material cannot account for the deficit of mass. Gravity anomalies over both the surrounding mountain belts and the Kazakh Platform farther north imply that local Airy isostatic equilibrium is approached; flexure of a relatively thin, effectively elastic, plate (thickness ~10--15 km) loaded by the present topography can account for the range of Bouguer anomalies of -50 to -450 mGal over these regions. We infer that approximately north-south shortening of the relatively thin lithosphere has created mountains north and south of the basin, has warped the basement of the immediate surroundings of the basin up by folding the mantle lithosphere, and has forced the basin floor down beneath the Ferghana Valley. It appears that the preexisting thermal structure and variations in crustal thickness have dictated the styles and distribution of deformation in this region. ¿ 1998 American Geophysical Union