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Crustal structure in central California is grossly two dimensional, as can be readily inferred from the crustal-thickness map (Figure. 8.3A). There are five blocks or provinces with subparallel fault boundaries: an accretionary prism, which is partly off shore; the Salinian block, which underlies the western Coast Ranges, including the Santa Cruz Mountains and Gabilan Range; a complex block between the San Andreas and Calaveras faults, underlying the Santa Clara Valley; the Diablo block, beneath the Diablo Range; and the Great Valley/Sierran block (Figure. 8.2). To illustrate the crustal structure of central California, we have modified and reinterpreted the part of Centennial Continent-Ocean Transect C2 (Saleeby, 1986) that extends from offshore California at Monterey Bay to the Sierran foothills near Modesto (Figures. 8.2, 8.4). Seismic control, which is exceptionally good along this transect, has been augmented since Saleeby's (1986) study primarily by analysis of seismic-refraction profiles in the Great Valley (Figure. 8.4A). The reader is referred to Hill (1978) for an earlier treatment of deep structure along approximately this same transect.