Microfracture fabric of the Punchbowl fault zone, San Andreas System, California

Abstract

The origin of fault zone structure is not completely understood. On the basis of mechanistic models of faulting, the characteristic internal structure of faults may largely be established early during growth of the fault, or it may develop throughout the faulting history as the result of wear during sliding along established fault surfaces. Microfractures are a common fabric element in fault zones and form in particular orientations that are related to the stress state at the time of formation. Thus, microfracture fabric may be used to investigate the origin of the internal structure of fault zones. I have conducted a study of the microfracture fabric within the Punchbowl fault zone, an exhumed, large-displacement, strike-slip fault of the San Andreas system. Open, healed, and sealed microfractures were analyzed with respect to density and orientation as a function of distance from the fault. Microfracture density decreases with distance from the fault core to background levels at approximately 100 meters from the fault surface. Microfractures display preferred orientations within about 30 meters of the fault surface, and fairly random fabric at greater distances. Within a very narrow zone (2.5 meters from the fault surface), microfractures occur in preferred orientations approximately parallel and perpendicular to the Punchbowl fault. Between approximately 2.5 and 15 meters from the fault surface, microfractures are preferentially oriented perpendicular to the fault surface and to the slip vector of the fault. The fabrics of the open and healed microfractures are similar. These data are most consistent with progressive microfracture development associated with wear during Punchbowl fault movement, and further suggest that the average stress state was maintained throughout much of the faulting history.