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

Abstract

Magnetotelluric measurements in the foreland of the Daxue Shan have imaged low‐angle (<20°) 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 offsets of 7 km on 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 convergence rates of 1.3 mm/yr and 2.5 mm/yr on the Chang Ma and North Hills thrusts, respectively. These slip rates correspond to ∼ 3 mm/yr of sinistral slip on the Altyn Tagh Fault between 96°E and 97°E. This is consistent with the 4 ± 2 mm/yr of slip estimated by geological studies. Assuming comparable slip rates and similar fault geometry in the neighboring 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 geologic studies that favor a relatively high rate of slip on the Altyn Tagh fault. This, in turn, implies that lithospheric extrusion contributes significantly in accommodating the ongoing convergence between India and Asia. Farther west, the Altyn Tagh Fault is imaged on four magnetotelluric profiles as a vertical resistivity contrast extending to a depth of at least 8 km. Two strands of the North Altyn Tagh Fault are imaged east of the asperity near Subei (39°30′N, 95°E).