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Two-dimensional velocity structure along the synclinal axis of the Great Valley, California

U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MS 977, MENLO PARK, CALIFORNIA 94025

Abstract

In 1981 and 1982, the U.S Geological Survey conducted a seismic refraction study of the Great Valley, central California, in order to obtian a better understanding of the deep crustal structure and tectonic evolution of the region. We present the interpretation of a 147-km-long seismic refraction profile recorded along the synclinal axis of the Valley. The northern 98 km of the profile are reversed, whereas the remaining southern portion is unreversed. Observed travel times and amplitudes of primary and secondary arrivals from three shot points have been modeled using two-dimensional ray tracing techniques. The resultant P-wave velocity cross-section consists of a 27-km-thick crust whose isovelocity contours generally rise gently to the southeast. The crust is described in six layers, with average velocities (average layer thickness in parentheses) of 2.8 km/sec (3 km), 4.25 km/sec (3 km), 5.5 km/sec (2 km), 6.1 km/sec (6 km), 6.75 km/sec (6 km), and 7.2 km/sec (7 km). The second through fifth layers are separated by 1-km-thick high gradient transition zone; the lower crustal boundary (6.75/7.2 km/sec) and the Moho (7.2/8.1 km/sec) are modeled with first-order discontinuities. The first layer (Cenozoic sedimentary deposits) is characterized by a velocity gradient of 0.7 km/sec/km starting with a velocity of 1.7 km/sec at the surface. The second layer (4.25 km/sec) consists of the Cretaceous Great Valley Sequence and shows a very modest velocity gradient of 0.2 km/sec/km. The third layer (5.5 km/sec; "basement") may consist of metasedimentary rocks of the Franciscan assemblage, fractured crystalline rocks, or possibly serpentinized ophiolite. The fourth layer (6.1 km/sec) apparently has no velocity equivalent 30 km to the west in the Diablo Range; it and the layer beneath it (6.75 km/sec) may be correlated with the metamorphic and igneous rocks of the eastern foothills of the Sierra Nevada. The basal crustal layer (7.2 km/sec) is evidently mafic igneous rock. An independent interpretation of this profile using the reflectivity method produced a similar model. An interpretation of a parallel profile 10 to 15 km east of our line shows a structure similar to our model below 15 km, but a thinner sedimentary section and velocities as much as 0.9 km/sec faster 6 to 15 km deep. The crustal velocity structure of the central Great Valley is atypical of either continental or oceanic crust, but is consistent with the geologic hypotheses that the Valley is a thickened marginal basin, formed in either a forearc or backarc setting located west of the Mesozoic Sierra Nevadan arc.

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