crust of much of California was formed at an Andean-type continental margin
during the Mesosoic and early Cenozoic, and was modified by large strike-slip
offsets along the San Andreas fault system during the late Cenosoic. Decoupling
within the crust, as implied by present upper-crustal tectonic wedging in central
California, and decoupling between the crust and mantle, as implied by "subduction"
of lithospheric mantle in southern California, indicates that the San Andreas
fault system must change with depth in its location and (or) style of deformation.
Studies of the crustal and upper-mantle structure of California along the San Andreas fault system have been underway for more than half a century, beginning with the early studies by Byerly and Wilson (1935) and Byerly (1946) in northern California and by Gutenberg 1943) in southern California. Crustal profiling along and near the San Andreas fault was first accomplished in the early 1960's by Eaton (1963), Healy (1963), and Roller and Healy (1963). Research accelerated after the 1966 M=6.0 Parkfield, Calif., earthquake to include both detailed crustal profiling and installation of dense seismic networks for the study of earthquakes (see chap. 5; Eaton and others, 1970). Since 1970, a wide variety of seismologic methods have been used to investigate crustal and upper-mantle structure in the vicinity of the San Andreas fault system. In this chapter, we summarize the main features of this structure and relate the structure to broad-scale tectonic processes.
Seismologic studies of crustal and upper-mantle structure in California make use of three primary data sources: (1) traveltimes of local earthquakes as measured by permanent and temporary seismic arrays, (2) seismic-refraction and reflection profiles, and (3) teleseismic delay times measured by seismic arrays. Traveltimes of local earthquakes, in addition to containing the information needed to locate earthquakes, contain a wealth of information regarding the seismic-velocity structure of the crust and upper mantle. Velocity structure can be determined from these traveltimes by iteratively adjusting an initial velocity model and associated hypocentral parameters, using inversion methods (for example, Crosson, 1976; Eberhart-Phillips and Oppenheimer, 1984). The resolution of velocity structure from local earthquake data is a function of the interstation spacing of the network and the abundance and distribution of seismicity.
Seismic refraction and reflection profiles together form a complementary set of seismic measurements. Seismic-refraction profiles provide the highest resolution of seismic P-wave velocities in the crust and upper mantle. The seismic-refraction method, however, generally does not provide the sharpest picture of lithologic interfaces, from which geologic structure is inferred; such a picture is better provided by seismic-reflection profiling.
Teleseismic delay-time studies offer the most effective means of determining the structure of the subcrustal lithosphere. The method is based on interpreting relative arrival times of compressional waves throughout a seismic array in terms of velocity variations at depth beneath the array. The Earth structure in the volume beneath the array generally is described by a series of blocks, and velocity deviations are derived for each block from the observed delay times (Aki and others, 1977; Thurber and Aki, 1987). The California seismic array is ideally suited for such investigations because of its large areal extent and the length of time it has been in operation (see chap. 5).
The primary product of the seismologic methods described above is a model of the seismic P-wave-velocity distribution in the crust and upper mantle. However, the interpretation of seismic P-wave velocities in terms of rock type is highly nonunique because laboratory velocity data indicate that numerous rock types can have similar velocities (for example, Birch, 1960). This interpretation is further complicated by the fact that, in rocks at pressures of less than 2 kbars (depths above 8 km), seismic velocities are strongly affected by the presence of cracks (on all scales) and porosity. In addition, rock velocities are affected by temperature and the presence of water. Thus, the interpretation of P-wave velocities in terms of rock types must involve other data sets, including laboratory velocity measurements on rocks at different pressures, temperatures, and water saturations, surface geologic data, well data, and other geophysical data, including gravity and magnetic data. Fortunately, abundant laboratory velocity data (for example, Stewart and Peselnick, 1977; Lin and Wang, 1980), geologic data (see chaps. 1, 3), and geophysical data (see chap. 9) are available for California, making the lithosphere of this region one of the best studied in the world.
In this chapter, we summarize the lithospheric structure and tectonics along the San Andreas fault system of California (Figure. 8.2) with maps of crustal thickness and upper-mantle seismic-velocity anomalies, and with crustal cross sections for central and southern California. Structure changes more rapidly parallel to the San Andreas fault in southern California than in central California, and so we supplement the cross section for southern California with a map showing crustal-block motions and a diagram illustrating the different motion of the lithospheric mantle below. Seismic and other data currently are still not dense enough to construct a cross section along the San Andreas fault system itself.
Construction of the crustal cross section for central California led us to a new interpretation of upper-crustal tectonic wedging, the mechanism whereby the Franciscan assemblage was emplaced in the Coast Ranges during the late Mesozoic(?) and Cenozoic. This interpretation extends that of Wentworth and others (1984) to include a two-part history whereby the observed structures atop the wedge, which include both extensional and compressional faults, were created. We further speculate that similar tectonic wedging occurred in southern California from the Mojave Desert to the Chocolate Mountains to emplace the Rand schist and the Pelona-Orocopia schist of Haxel and Dillon (1978) into rocks east of the San Andreas fault.