Paleostress directions near two low-angle normal faults: Testing mechanical models of weak faults and off-fault damage
Many large-slip faults, such as the San Andreas fault and low-angle normal faults (LANFs), appear to be weak relative to their surroundings or to laboratory friction measurements, and to be poorly oriented for slip in the regional stress field. Several models seek to explain the mechanics of slip and/or formation of such faults. Other models explain damage around faults as due to fault or earthquake rupture propagation or slip on nonplanar faults. Most of these models explicitly predict the near-fault stress field.

Exhumed footwalls of low-angle normal faults are advantageous natural laboratories for testing such models because they expose rocks that passed through the brittle-plastic transition and all or part of the seismogenic crust. We present reduced paleostress tensors derived from inversion of fracture and slip-line orientation data taken mainly from the fault cores and fractured damage zones in the upper footwalls of two LANFs, the Whipple and West Salton detachment faults of southern California. Frictionally weak materials probably were not significant along these faults except in the uppermost few kilometers of the crust, and pore-fluid pressure probably never approached lithostatic values.

Most results show that the faults were at a high angle to the near-fault maximum compressive stress (σ1) direction, in general accord with Andersonian extensional stress fields. Our results support a “strong-sandwich” mechanical model for slip in the upper crust, in which normal-friction LANFs are embedded in stronger surroundings and slip at high angles to σ1 and models of stress rotation across the thickness of the brittle crust, with moderately plunging σ1 near the brittle-plastic transition, provided that some mechanism allows the faults to propagate through the brittle crust at gentle dips as the footwalls are exhumed. Paleo-σ1 vectors oriented at moderate angles to the faults are sparse and may reflect early damage formed in the midcrust, while the angle between σ1 and the detachment was moderate or during alongstrike LANF or earthquake rupture propagation. Coulomb plasticity due to granular flow, which predicts faults at ∼45° to σ1, is not well supported because many paleo-σ1 vectors with moderate angles to the LANFs are from fractures below the cataclastic fault cores. Our results are inconsistent with “weak-sandwich” models that predict reorientation of σ1 to low angles (∼30°) to the fault within the damage zone and/or fault core due to local pore-fluid pressure or elasticity changes. Fracturing due to slip on non-planar faults is generally consistent with our paleostress results. However, the roughness of the LANFs studied is not known, but they may have very low roughness. The stress state used in this wavy-fault model to constrain the expected damage region is nearly identical to that inferred in the strong-sandwich model from field measurements. Fractures in the damage zone probably do not record up-dip fault or earthquake rupture propagation, which is expected especially for earthquake propagation, but along-strike propagation may have controlled fracturing at some sites. Some paleostress fields are probably related to folding of the detachments about slip-parallel axes.