Ultracataclasite structure and friction processes of the Punchbowl fault, San Andreas system, California
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The Punchbowl fault is an exhumed, 40+ km displacement fault of the San Andreas system. In the Devil's Punchbowl, the fault contains a continuous ultracataclasite layer along which the Punchbowl Formation sandstone and an igneous and metamorphic basement complex are juxtaposed. The fabric of the ultracataclasite layer and surrounding rock indicate that nearly all of the fault displacement occurred in the layer. By analogy with nearby active faults, we assume that the Punchbowl fault was seismogenic and that the ultracataclasite structure records the passage of numerous earthquake ruptures. We have mapped the ultracataclasite layer at 1 : 1 and 1 : 10 to determine the mode of failure and to constrain the processes of seismic slip. On the basis of color, cohesion, fracture and vein fabric, and porphyroclast lithology, two main types of ultracataclasite are distinguished in the layer: an olive-black ultracataclasite in contact with the basement, and a dark yellowish brown ultracataclasite in contact with the sandstone. The two are juxtaposed along a continuous contact that is often coincident with a single, continuous, nearly planar, prominent fracture surface (pfs) that extends the length of the ultracataclasite layer in all exposures. No significant mixing of the brown and black ultracataclasites occurred by offset on anastomosing shear surfaces that cut the contact or by mobilization and injection of one ultracataclasite into the other. The ultracataclasites are cohesive throughout except for thin accumulations of less cohesive, reworked ultracataclasite along the pfs. Structural relations suggest that: (1) the black and brown ultracataclasite were derived from the basement and sandstone, respectively; (2) the black and brown ultracataclasites were juxtaposed along the pfs; (3) the subsequent, final several kilometers of slip on the Punchbowl fault occurred along the pfs; and (4) earthquake ruptures followed the pfs without significant branching or jumping to other locations in the ultracataclasite. By comparison with rock friction experiments, the slip localization along the pfs in the ultracataclasite implies rate weakening behavior with a critical slip distance similar to laboratory values, and thus relatively small nucleation and breakdown dimensions for earthquake ruptures. Of the various mechanisms proposed to explain the low strength of the San Andreas and to produce dynamic weakening of faults, those that require or assume extreme localization of slip are most compatible with our observations.
