Pseudotachylites in the Eastern Peninsular Ranges of California

A continuous zone containing pseudotachylites in the Eastern Peninsular Ran ges of California extends over more than 15 km from Deep Canyon in the nort h to at least Toro Canyon in the south. Pseudotachylites are found in rocks of tonalitic to dioritic composition. While the overall compositions of ho st rock and pseudotachylite veins are similar, veins are characteristically enriched in Fe and Ti and depleted in Si. Many veins are cataclastic, and all contain fragments, but a large number have a groundmass with skeletal a nd spherulitic microlites of calcic plagioclase (An 40-50), biotite and ilm enite, indicative of partial melting. The pseudotachylites formed mainly during a late brittle event (56-62 Ma) a nd postdate the high-strain, ductile mylonitic deformation in the Santa Ros a mylonite zone (65-87 Ma) as ascertained by Ar-40/Ar-39 ages. The size of veins ranges from less than 1 mm to more than 10 cm in thickness and aspect ratios display a broad log-normal distribution. Three-quarters of the vein s are more or less parallel to the foliation. The orientation of the veins is consistent with a top to the west displacement, similar to that for the ductile mylonites. From the sizes, energies required for melting are estima ted to range between 10(5) J for smaller veins and 10(10) J for larger vein s. Interestingly, the energy distribution and geometry of these pseudotachy lites correspond closely with energy distributions of current microseismic events along the San Andreas fault at Parkfield, suggesting that basic mech anisms may be similar and due to intrinsic mechanical properties of rocks. In particular these indicate that local stress concentrations, rather than average crustal stresses, are relevant for earthquakes. (C) 2000 Elsevier S cience B.V. All rights reserved.