Microfracture analysis of fault growth and wear processes, Punchbowl Fault, San Andreas system, California

Fabric and timing relations of mode I microfractures are used to test current hypotheses for the origin of damage along large-displacement faults by the processes of fault growth and wear. Oriented samples 0.075 m to 1 km from the Punchbowl fault surface (i.e. ultracataclasite layer) document an increase in development of preferred orientation and increase in density of microfractures towards the ultracataclasite layer, defining a zone of fault-related microfracture damage about 100 m thick. A distinct microfracture set that is perpendicular to the slip direction of the fault is present throughout the damage zone. This implies that the average orientation of the maximum principal compressive stress within the damage zone was nearly normal to the fault surface. Two additional microfracture sets are present, one oriented at low angles to the fault within meters of the ultracataclasite layer, and another low-angle set that occurs in the outermost damage zone. The preferred orientations and timing relations are most consistent with local damage accumulation from stress cycling associated with slip on a geometrically irregular, relatively weak fault surface. Low-angle microfractures nearest the ultracataclasite layer also may record wear associated with the passage of earthquake ruptures, and those in the outermost damage zone may be consistent with Andersonian fault formation and subsequent fault weakening.