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U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MS 77, MENLO PARK, CALIFORNIA 94025
Abstract
The compressional-wave velocity structure of the crust of the Coast Ranges of central California was modeled from seismic refraction data reported previously. For both the Diablo Range (east of the San Andreas fault) and the Gabilan Range (west of the fault), two alternative velocity models were derived for each range by iterative two-dimensional ray tracing. Each pair of models shows a sedimentary layer and an underlying crust composed of three or four layers. For the Diablo Range, the velocities in the sediments range from 2.9 to 4.8 km/sec, and the depth to basement varies from 0.2 km within the central Diablo Range to a maximum of 3.4 km north of the Livermore Valley. In the Gabilan Range, the velocities in the sediments overlying the granitic basement are higher north of the Gabilan Range (3.6 to 4.6 km/sec) than to the south (2.2 to 3.8 km/sec), and the depth to basement both north and south of the range is less than 2 km.
Below the sediments and fractured near-surface material, a resolvable difference in the crustal velocity structure on opposite sides of the San Andreas fault indicates compositional differences at depth. The upper crust has an average velocity of 5.7 km/sec at depths between 3 and 16 ± 3 km in the Diablo Range and an average velocity of 6.1 km/sec at depths between 2 and 9 ± 1 km in the Gabilan Range. The lower crust has an average velocity of 6.9 km/sec at depths between 16 ± 3 and 30 km in the Diablo Range and an average velocity of 6.5 km/sec at depths between 10 and 24 km in the Gabilan Range. The models also show that the depth to Moho differs by several kilometers between the two ranges. In the Diablo Range models, the crust-mantle boundary becomes shallower from south to north, rising from a depth of 30 to 26 km. In the Gabilan Range models, the Moho is at a depth of 24 to 26 km, depending on the velocity assumed at the base of the crust.
Laboratory measurements of rock velocities and the mapped surface geology allow us to interpret the velocity models in terms of crustal composition. We conclude that the Diablo Range probably consists largely of metagraywacke to a depth of 16 ± 3 km, and gabbroic material below this depth. The crust of the Gabilan Range probably consists of granitic material to a depth of 9 ± 1 km and gneissic material below this depth. Franciscan rocks are not regionally present in the crust of the Gabilan Range. If the gneissic lower crust consists of the same rocks as are found at the surface in the Gabilan Range, then the presumed large-scale lateral motion of the Salinian block has taken place at or below the crust-mantle boundary, rather than along a mid-crustal slip plane.
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