Sandbox modeling of evolving thrust wedges with different preexisting topographic relief: Implications for the Longmen Shan thrust belt, eastern Tibet

Abstract

Abstract To understand the effects of substantial topographic relief on deformation localization in the seismically active mountains, like the Longmen Shan thrust belt in the eastern Tibet, sandbox experiments were performed based on the framework of the critical taper theory. First, a reference experiment revealed that the critical taper angle was 12° for our experimental materials. Subsequently, different proto wedges (subcritical (6° in taper angle), critical (12°), and supercritical (20°)) were introduced to cover the range of natural topographic relief, and we used two setups: setup A considered only across‐strike topographic relief, whereas setup B investigated along‐strike segmentation of topography, consist of two adjacent proto wedges. In all experiments, thrust wedges grew by in‐sequence accretion of thrust sheets. Setup A revealed an alternating mode of slip partitioning on the accreted thrusts, with large‐displacement thrust and small‐displacement thrust developing in turn. And contrasting wedge evolutions occurred according to whether the proto wedge was subcritical or critical‐supercritical. In setup B, the differential deformation along the strike produced transverse structures such as tear fault and lateral ramp during frontal accretion. The observed tear fault and its associated thrust system resemble the seismogenic fault system of the 2008 M w 7.9 Wenchuan earthquake. Our experimental results could also explain first‐order deformation features observed in the Longmen Shan. Consequently, we conclude that topographic features, including topographic relief across the range and along‐strike segmentation of topography, contribute significantly to the kinematics and deformation localization in such active mountains.