Mesoscopic structure of the Punchbowl Fault, Southern California and the geologic and geophysical structure of active strike-slip faults

Abstract

We examine the distribution, density, and orientation of outcrop-scale structures related to the Punchbowl Fault, an exhumed ancient trace of the San Andreas Fault, southern California, in order to determine the structure of the fault zone. The Punchbowl Fault has 44 km of right-lateral slip, and cuts the Cretaceous Pelona Schist in the study area. The mesoscopic structures examined include fractures, small faults, and veins; they were inventoried using scan lines at closely spaced stations along three strike-perpendicular traverses 200–250 m long across the fault. The fault zone thickness is a function of the type of structure measured. Slip along narrow (<2 m wide) ultracataclasite cores of the faults results in foliation reorientation over a distance of 50 m from the cores: fracture and fault densities appear to increase 50–80 m from the fault cores, and vein densities are highly variable across the fault zone. Fractures and faults in the damaged zone have a variety of orientations, but most are at high angles to the main fault zone. When coupled with previous geochemical and microstructural data, these data show that large-displacement faults of the San Andreas system, are up to 200–250 m thick, and enclose zones of mineralogic and geochemical alteration that are 20–30 m thick. Extreme slip localization occurs over zones 1–5 m thick. When reconciled with geophysical imaging, our data suggest that trapped headwaves travel in the damaged zone, and that some aftershock events produce slip on faults and fractures, which often have orientations very different from the principal slip surfaces.