Inversion of slant stacks of deep crustal refraction data from the Great Valley, California

Abstract

Analysis in the τ( p ) domain of a refraction profile recorded in the Great Valley of California provides a check on models derived by raytracing. Coherency filtering based on a method developed for marine data has proved effective in suppressing aliasing in slant stacks of source gathers with receiver spacings greater than 800 m. Depth bounds derived by extremal inversion of τ( p ) data for two reversed shots range from 2 to 8 km in width and are consistent with two-dimensional models derived by trial-and-error using T ( X ) data. Both methods produce models with features such as a prominent discontinuity at the base of the sediments and a steep velocity gradient in the mid-crust, demonstrating that the major phases used to derive the two-dimensional models have also been imaged successfully in the τ( p ) domain. Differences between τ( p ) arrivals picked for the reversed shots are also consistent with differences predicted by the earlier models. Events interpreted as precritical reflections from the Moho have proved useful in constraining total crustal thickness (25 to 29 km). Average crustal velocities for slowness-depth models corresponding to extremal depths range from 5.2 to 5.5 km/sec (if sediments with velocities less than 5.0 km/sec are not included: 6.5 to 6.7 km/sec). Velocities at the base of the crust fall between 7.1 and 7.4 km/sec, while predicted critical distances for the Moho reflection range from 63 to 71 km. Predicted normal two-way travel times to the Moho lie between 9.8 and 10.2 sec, in agreement with Moho reflections identified on nearby reflection profiles. In the lower half of the crust, layers with velocities greater than 6.7 km/sec are interpreted as mafic rock. We suggest that a roughly 4-km-thick layer with velocities around 6.1 km/sec consists of metamorphosed graywacke rather than granite, as had been suggested by previous workers. This would be more consistent with models that interpret the crust beneath the Coast Ranges and western half of the Great Valley to be a complex of marine clastics and basement obducted onto the North American continent during the late Mesozoic.