Pseudotachylites in the Eastern Peninsular Ranges of California

Abstract

A continuous zone containing pseudotachylites in the Eastern Peninsular Ranges of California extends over more than 15 km from Deep Canyon in the north to at least Toro Canyon in the south. Pseudotachylites are found in rocks of tonalitic to dioritic composition. While the overall compositions of host 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 and spherulitic microlites of calcic plagioclase (An 40–50), biotite and ilmenite, indicative of partial melting. The pseudotachylites formed mainly during a late brittle event (56–62 Ma) and postdate the high-strain, ductile mylonitic deformation in the Santa Rosa mylonite zone (65–87 Ma) as ascertained by 40Ar/ 39Ar 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 veins 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 estimated to range between 10 5 J for smaller veins and 10 10 J for larger veins. Interestingly, the energy distribution and geometry of these pseudotachylites correspond closely with energy distributions of current microseismic events along the San Andreas fault at Parkfield, suggesting that basic mechanisms 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.