PORE PRESSURE, STRESS INCREASE, AND FAULT WEAKENING IN LOW-ANGLE NORMAL FAULTING

Low-angle (dip < 30-degrees) normal faults accommodate much extension of the continental crust. They apparently move under low resolved shear stress and are anomalously weak, characteristics that they share with the San Andreas fault. Structural, textural, and geochemical arguments suggest that low-angle normal faults are weak in both the ductile and brittle regimes, partly or totally due to elevated pore fluid pressure. High pore pressure in detachment zones may be contained by upper-plate strata, mineral precipitation in their hanging walls, formation of low-permeability microbreccia layers, threshold pressure gradients, and low-permeability mylonites below chlorite breccia. Mechanical analysis shows that fault weakening may preclude equality of the regional and fault-zone stress tensors, and predicts reorientation and increase of principal stresses in weak fault zones. These changes suppress hydraulic fracturing in the brittle detachment zone and allow slip under frictional sliding conditions typical of upper crustal rocks. Fault weakening focuses extension in the upper crust onto low-angle normal ductile-brittle shear zones in the midcrust, promoting propagation of low-angle brittle normal faults into the upper crust,