GRAVITY-ANOMALIES OVER THE FERGHANA VALLEY (CENTRAL-ASIA) AND INTRACONTINENTAL DEFORMATION

Gravity anomalies over the Ferghana Valley, a deep sedimentary basin i n the western Tien Shan, differ from those expected for local Airy iso static equilibrium by 120 meal and imply a deficit of mass beneath the basin equivalent to a Moho deeper by 10 km than prevails where isosta sy holds. Although the Ferghana Valley is underlain by thick Mesozoic and Cenozoic sediment, measurements of density shaw that this material cannot account for the deficit of mass. Gravity anomalies over both t he surrounding mountain belts and the Kazakh Platform farther north im ply that local Airy isostatic equilibrium is approached; flexure of a relatively thin, effectively elastic, plate (thickness similar to 10-1 5 km) loaded by the present topography can account for the range of Bo uguer anomalies of -50 to -450 mGal over these regions. We infer that approximately north-south shortening of the relatively thin lithospher e has created mountains north and south of the basin, has warped the b asement of the immediate surroundings of the basin up by folding the m antle lithosphere, and has forced the basin floor down beneath the Fer ghana Valley. It appears that the preexisting thermal structure and va riations in crustal thickness have dictated the styles and distributio n of deformation in this region.