GEODETIC MEASUREMENT OF CRUSTAL MOTION IN SOUTHWEST CHINA

Global Positioning System measurements performed over a 2-4 yr period confirm the Xianshuihe-Xiaojiang fault system as one of the primary ac tive structures in southwest China. Stations southwest of these faults show southerly motions of 5-15 mm/yr relative to the western Sichuan basin, and stations in a 200 km geodetic network located northwest of the basin move at only 0-5 mm/yr. These results are consistent with cl ockwise rotation of southwestern Sichuan and western Yunnan about the eastern Tibetan syntaxis, accommodated by left-lateral slip of 12 +/- 4 mm/yr on the Xianshuihe-Xiaojiang fault system, The results imply th at if eastward extrusion of crustal material from the plateau occurs a t present, it is not accommodated by east-west shortening along the ma rgin of the plateau and must involve wholesale eastward motion of low- lying regions to the east.