Particle-size distributions of low-angle normal fault breccias: Implications for slip mechanisms on weak faults

Abstract

Slip on low-angle normal faults is not well understood because they slip at high angles to the maximum principal stress directions. These faults are considered weak and their motion cannot be explained using standard Byerlee friction and Andersonian fault mechanics. One proposed mechanism for weak fault slip is reduction of effective normal stress induced by high pore-fluid pressure. This mechanism is likely to allow dilation of the fault zone and, therefore, affect the particle-size distribution of fault breccia, which has been shown to differ for unconstrained versus constrained comminution. High pore-fluid pressure can cause dilation which leads to unconstrained comminution. We analyze samples from the footwalls of two low-angle normal faults in southern California (West Salton and Whipple detachment faults) to determine the fault-rock textures and grain-size distributions (GSDs). The GSDs are fractal with fractal dimensions ranging from ∼2.6 to 3.4. The lower end of this range is thought to reflect constrained comminution and only occurs in samples from the footwall of a small-offset “minidetachment” fault about 100 m below the Whipple detachment. The higher fractal dimensions are common in cataclasites related to the main faults and also reflect constrained comminution but are overprinted by shear localization. Our GSDs are similar to those from natural and laboratory-deformed fault rocks from strong faults. We conclude that if high pore-fluid pressure aided slip on these faults, it did not strongly affect mechanisms by which brecciation occurs, implying that fluid pressure generally was sublithostatic. Independent evidence exists for lithostatic fluid pressure that having dropped or cycled to hydrostatic levelsin the minidetachment, but our GSD results suggest that periods of high fluid pressure were too short or infrequent for unconstrained comminution to have been the dominant cataclastic mechanism. Fractal dimensions of ∼2.6 for these samples suggest that little subsequent abrasion occurred due to shear localization, consistent with minor offset on the minidetachment. Main detachment footwall samples with fractal dimensions ≥3 reflect constrained comminution followed by shear-related abrasion, and suggest that seismic cycling was important in formation of main detachment cataclasites.