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Upper crustal structure, Livermore Valley and vicinity, California coast ranges

DEPARTMENT OF GEOLOGY AND GEOPHYSICS RICE UNIVERSITY, P.O. BOX 1892, HOUSTON, TEXAS 77251
U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MS 977, MENLO PARK, CALIFORNIA 94025

Abstract

We have interpreted two reversed seismic refraction profiles in the Livermore Valley area of California using travel times and amplitudes calculated with asymptotic ray theory. The upper crustal structure to 5 km depth is different on either side of the Greenville fault zone. The fault does not appear to extend below 5 km. The fault zone disrupts arrivals of rays which turn in the fault zone, but does not appear to affect rays that turn below 5 km. East of the Greenville fault zone, the boundary between the Cretaceous Great Valley sequence and Franciscan assemblage rocks occurs along a north-dipping interface below 4.5 km depth. West of the Greenville fault, the boundary between valley fill sediments and Franciscan rocks was not observed. Miocene sediment having a high compressional velocity, 5.0 km/sec, is present beneath Livermore Valley to at least 5 km depth. In contrast, the maximum velocity associated with adjacent Cretaceous sediments is 4.2 km/sec. The high velocity of the Livermore basin sediments seen in the refraction data is also observed on sonic logs made in exploratory wells in the basin which bottom in Miocene sediments. This last finding is important, since velocities of 4.8 to 5.0 km/sec have previously been associated with shallow Franciscan rocks. This observation calls into question the use of seismic velocities to distinguish between different Coast Range-Great Valley lithologies without other corroborating data.

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