Chapter 9: Regional crustal structure and tectonics of the Pacific Coastal States; California, Oregon, and Washington

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Abstract
The Pacific Coastal States form a complex geologic environment in which the crust and lithosphere have been continuously reworked. We divide the region tectonically into the southern transform regime of the San Andreas fault and the northern subduction regime, and summarize the geophysical framework with contour maps of crustal thickness, lithospheric and seismicity cross sections, and results from site-specific geophysical studies. The uniformity of crustal thickness (30 ± 2 km) in southern California is remarkable, and appears to be primarily the result of crustal extension in the Mojave Desert and ductile shear of the lower crust along the plate transform boundary. Southern California seismicity defines a broad zone of deformation that extends from the Borderland to the Mojave Desert (about 300 km). The geophysical framework of central and northern California records magmatism and accretion associated with the Mesozoic and Cenozoic subduction, late Cenozoic transform faulting, and in the Basin and Range to the east, extension. The crust thickens from about 20 km at the coast to as much as 55 km in the Sierra Nevada, and thins to about 30 km in the Basin and Range. Cross sections of the crust show that seismic velocities and densities vary significantly over short distances perpendicular to the coast, reflecting processes that include the accretion of oceanic sediments and igneous crust, and significant lateral motion of crustal blocks. Maximum hypocentral depths in central California become deeper as the crust thickens to the west, but seismicity is low beneath the Great Valley and Sierra Nevada, which together appear to form a relatively undeforming block. The lower crust of the Pacific Coastal States has a high average seismic velocity (6.7 km/sec or greater), which probably is the product of tectonic underplating of oceanic crust and/or magmatic underplating by a basaltic melt. The geophysical framework of the subduction regime is dominated by the subduction of the Gorda and Juan de Fuca plates, arc magmatism in the Cascade Range, and plateau volcanism and rifting in the back arc. As defined by earthquake hypocenters, the Juan de Fuca plate dips at a shallow angle (3°) within 50 km of the trench, increases to 10° beneath the continental shelf and coastal province, and plunges more steeply (25° dip) a short distance west of the Cascade Range. Whereas a true continental Moho exists from the Cascade Range to the east, the Moho is that of the subducting oceanic lithosphere west of the range. Crustal thickness increases from about 18 km at the coast to about 42 km beneath the Cascades Range, a distance of about 200 km. The crustal velocity structure and crustal thickness of the Cascades Range is relatively uniform along its axis. The velocity structure shows high velocities (greater than 6.5 km/sec) at all depths greater than 10 km, indicating rocks of an intermediate-to-mafic composition, and a relatively low upper-mantle velocity of 7.7 ± 0.1 km/sec, indicating high temperatures. Seismological studies at the volcanic centers of the Cascades indicate that the dimensions of subsurface magmatic systems are small, on the order of a few kilometers. Some 1 to 6 km of Miocene and younger basaltic extrusives cover much of the back arc, thereby obscuring most of the pre-Miocene geology. However, geophysical data demonstrate the importance of Mesozoic compression and Cenozoic (particularly Eocene) extension, accompanied by magmatic underplating of the crust.

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