ULTRACATACLASITE STRUCTURE AND FRICTION PROCESSES OF THE PUNCHBOWL FAULT, SAN-ANDREAS SYSTEM, CALIFORNIA

The Punchbowl fault is an exhumed, 40+ km displacement fault of the Sa n Andreas system. In the Devil's Punchbowl, the fault contains a conti nuous ultracataclasite layer along which the Punchbowl Formation sands tone and an igneous and metamorphic basement complex are juxtaposed. T he fabric of the ultracataclasite layer and surrounding rock indicate that nearly all of the fault displacement occurred in the layer. By an alogy with nearby active faults, we assume that the Punchbowl fault wa s seismogenic and that the ultracataclasite structure records the pass age of numerous earthquake ruptures. We have mapped the ultracataclasi te layer at 1 : 1 and 1 : 10 to determine the mode of failure and to c onstrain the processes of seismic slip. On the basis of color, cohesio n, fracture and vein fabric, and porphyroclast lithology, two main typ es of ultracataclasite are distinguished in the layer: an olive-black ultracataclasite in contact with the basement, and a dark yellowish br own ultracataclasite in contact with the sandstone. The two are juxtap osed along a continuous contact that is often coincident with a single , continuous, nearly planar, prominent fracture surface (pfs) that ext ends the length of the ultracataclasite layer in all exposures. No sig nificant mixing of the brown and black ultracataclasites occurred by o ffset on anastomosing shear surfaces that cut the contact or by mobili zation and injection of one ultracataclasite into the other. The ultra cataclasites are cohesive throughout except for thin accumulations of less cohesive, reworked ultracataclasite along the pfs. Structural rel ations suggest that: (1) the black and brown ultracataclasite were der ived from the basement and sandstone, respectively; (2) the black and brown ultracataclasites were juxtaposed along the pfs; (3) the subsequ ent, final several kilometers of slip on the Punchbowl fault occurred along the pfs; and (4) earthquake ruptures followed the pfs without si gnificant branching or jumping to other locations in the ultracataclas ite. By comparison with rock friction experiments, the slip localizati on along the pfs in the ultracataclasite implies rate weakening behavi or with a critical slip distance similar to laboratory values, and thu s relatively small nucleation and breakdown dimensions for earthquake ruptures. Of the various mechanisms proposed to explain the low streng th of the San Andreas and to produce dynamic weakening of faults, thos e that require or assume extreme localization of slip are most compati ble with our observations. (C) 1998 Elsevier Science B.V. All rights r eserved.