LAUSR

Oblique plate convergence is common, but it is not clear how the obliquity is achieved by continental fold-and-thrust belts. We address this problem in the Qilian Shan, northeastern Tibetan Plateau, using fieldwork observations, geomorphic analysis and elastic dislocation modeling of published geodetic data. A thrust dips SSW from the northern range front, and underlies steeper thrusts in the interior. Cenozoic thrust-related shortening across the Qilian Shan is ~155-175 km, based on two transects. Elastic dislocation modeling indicates that horizontal strain in the interseismic period is consistent with oblique slip on a single low angle detachment thrust below ~26 km depth, dipping SSW at ~17o. We suggest this detachment is located above North China Block crust, originally underthrust during Paleozoic orogeny. Horizontal shear strain is localized directly above the up-dip limit of creep on the detachment, and is coincident with the left-lateral Haiyuan Fault. This configuration implies oblique slip on the detachment below seismogenic depths is partitioned in the shallow crust onto separate strike-slip and thrust faults. This is consistent with strain partitioning in oceanic subduction zones, but has not previously been found by dislocation models of continental interiors. The marginal, strike-slip, Altyn Tagh Fault influences thrusting within the Qilian Shan for 100-200 km from the fault, but does not control the regional structure, where Paleozoic basement faults have been reactivated. The Qilian Shan resembles the main Tibetan Plateau in nascent form: active thrusts are marginal to an interior that is developing plateau characteristics, involving low relief, and low seismicity.