Reverse fault rupturing: competition between non-optimal and optimal fault orientations

Abstract

AbstractA dip histogram for intracontinental M>5.5 reverse-slip ruptures reveals a trimodal distribution with a dominant Andersonian peak (fault dip, δ=30±5°) flanked by subsidiary clusters at δ=10±5° and 50±5°, and no dips greater than 60°. For a simple compressional regime (σv=σ3), the dominant peak is in accord with the reshear of optimally oriented faults with a friction coefficient of μs=0.6±0.2, implying frictional lock-up at δ=60±10° consistent with the observed upper dip bound. The low-dip cluster (δ=10±5°) is dominated by thrusting in the frontal Himalaya and may incorporate staircase thrust systems in cover sequences with deflections along bedding anisotropy. The cluster of moderate-to-steep reverse fault ruptures (δ=50±5°) is likely dominated by compressional inversion of inherited normal faults. In both circumstances, however, there appears to be competition between Andersonian thrusts in various stages of development and non-optimal failure planes dipping at either high or low angles. A delicate balance between levels of differential stress and fluid-pressure determines whether or not a poorly oriented thrust or reverse fault reactivates in preference to the development of new, favourably oriented Andersonian thrusts.