3D geometry of range front blind ramp and its effects on structural segmentation of the southern Longmen Shan front, eastern Tibet

Abstract

A 3D fault morphology model has been constructed from seismic profiles covering the piedmont of the southern Longmen Shan (LMS), eastern margin of the Tibetan Plateau. It reveals a spatial correlation between an ∼3.5 km-high geometrical bump on the range front blind ramp (RFBR) and the gap between the 2008 Mw 7.9 Wenchuan and 2013 Mw 6.6 Lushan earthquakes. Sandbox modeling is then used to probe the effects of the geometrical irregularity of the RFBR on fault behaviors and the LMS range front deformation. Sand wedges are designed to thrust over artificial blind ramps, analogous to the thrusting of the LMS orogenic wedge toward the Sichuan Basin through the RFBR. The experiments reproduce the first-order deformation features presented in the southern LMS range front, confirming that along-strike geometrical variations of the RFBR have induced the segmentation of both fault motion and the range front deformation. Their results also suggest that the gap between the Wenchuan and Lushan earthquakes is located directly within the structural transition zone originating from the geometrical bump on the RFBR. Additionally, there is a duplex structure occurring in the hanging wall of the geometrical bump, which localizes significant shear strain with progressive shortening and represents a fracture zone corresponding to the observed geophysical anomaly (low seismic velocity) underneath the seismic gap. It is likely that this fractured body protected the gap from the ruptures of the Wenchuan and Lushan earthquakes.