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

We examine the distribution, density, and orientation of outcrop-scale stru ctures 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 cu ts the Cretaceous Pelona Schist in the study area. The mesoscopic structure s examined include fractures, small faults, and veins; they were inventorie d using scan lines at closely spaced stations along three strike-perpendicu lar 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 ov er a distance of 50 m from the cores: fracture and fault densities appear t o increase 50-80 m from the fault cores, and vein densities are highly vari able 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 zon e. When coupled with previous geochemical and microstructural data, these d ata show that large-displacement faults of the San Andreas system, are up t o 200-250 m thick, and enclose zones of mineralogic and geochemical alterat ion 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 t rapped headwaves travel in the damaged zone, and that some aftershock event s produce slip on faults and Fractures, which often have orientations very different from the principal slip surfaces. (C) 2000 Elsevier Science Ltd. All rights reserved.