Structure of the Altyn Tagh Fault and Daxue Shan from magnetotelluric surveys: Implications for faulting associated with the rise of the Tibetan Plateau

Magnetotelluric measurements in the foreland of the Daxue Shan have imaged low-angle (< 20 degrees) thrust faults that extend to a depth of 3 km. The surface projections of the faults at depth coincide with the mapped traces of the Chang Ma (M=7.6, 1932) and North Hills thrusts. Minimum cumulative o ffsets of 7 km oil the Chang Ma thrust and 14 km on the North Hills thrust are estimated from the horizontal extent of underthrust sediment. Assuming that regional thrusting began at 5-6 Ma, this corresponds to minimum conver gence rates of 1.3 mm/yr and 2.5 mm/yr oil the Chang Ma and North Hills thr usts, respectively. These slip rates correspond to similar to3 mm/yr of sin istral slip oil the Altyu Tagh Fault between 96 degreesE and 97 degreesE. T his is consistent with the 4 +/- 2 mm/yr of slip estimated by geological st udies. Assuming comparable slip rates and similar fault geometry in the nei ghboring ranges, this requires a minimum of 11 mm/yr of shortening parallel to the Altyn Tagh fault between the Dang He Nan Shan and Qilian Shan. Both the style of thrusting and rate of shortening are in agreement, with geolo gic studies that favor a relatively high rate of slip on the Altyn Tagh fau lt. This, in turn, implies that lithospheric extrusion contributes signific antly in accommodating the ongoing convergence between India and Asia. Fart her west, the Altyn Tagh Fault is imaged oil four magnetotelluric profiles as a vertical resistivity contrast extending to a depth of at least 8 kin, Two strands of the North Altyn Tagh Fault are imaged cast of the asperity n ear Subei (39 degrees 30 'N, 95 degreesE).