SALINIAN BLOCK [c3, p66]

The west wall of the San Andreas fault consists mainly of rocks of the Salinian block from the Transverse Ranges northward to Bodega Head. At the latitude of the San Francisco peninsula, the Salinian block is separated from the San Andreas fault by a narrow fault slice of Franciscan rocks (Figure. 3.3). Northward, from just beyond Bodega Head to Point Arena, the rocks that crop out along the fault are chiefly Upper Cretaceous and Tertiary sedimentary strata, but because of the presence of a relatively small exposure of spilitic volcanic rocks that may be Franciscan, it is questionable whether these sedimentary rocks overlie granitic and metamorphic rocks of the Salinian block or whether they are separated from the Salinian block by another fault just off shore (Wentworth, 1968). The north end of the Salinian block off shore is thought to be at about the latitude of Point Arena (see McCulloch, 1987). At Point Delgada (see maps at front of book), which many workers consider to be the location of the northernmost onland trace of the San Andreas fault, the rocks on both sides of the fault are Franciscan.

The Salinian block is composite, consisting of central, western, and northern belts, and commonly is considered to include the Barrett Ridge slice (Figure. 3.3; Ross, 1984). The basement rocks of the Barrett Ridge slice, though poorly exposed, are thought likely to be a northward continuation of the rocks of the San Gabriel Mountain area because they include similar-appearing metamorphic and granitic rocks, as well as schist similar to the Pelona Schist, along a possible exposure of the Vincent(?) thrust (Ross, 1984). The principal formations of these central, western, and northern belts are granitic and metamorphic rocks, locally overlain by Upper Cretaceous and younger strata. The metamorphic rocks, which commonly are moderate- to high-grade gneiss, granofels, impure quartzite, and minor schist and marble, probably represent a metamorphosed thin-bedded sequence of siltstone and sandstone, with lesser amounts of shaly, marly, and calcareous strata (Ross, 1978). The metamorphic rocks of the western belt are higher in metamorphic grade than those of the central and northern belts. The stratigraphic age of the protoliths of the metamorphic rocks is not known. The plutonic rocks are mostly granite and tonalite, but they range in composition to gabbro. U-Pb-isotopic measurements on zircon in the plutonic rocks indicate that plutonic activity began about 120-105 Ma in the northwestern part of the Salinian block and migrated southeastward over a period of 40 m.y., with the youngest plutons intruding the Barrett Ridge slice about 80-75 Ma (Mattinson and James, 1985).

The basement rocks of much of the Salinian block do not clearly differ from those of the Sierra Nevada, and so many workers have speculated that the Salinian block may be a displaced part of the Sierra Nevada (for example King, 1959; Page, 1981). According to Ross (1984), who compared the two terranes in considerable detail, the similarities are so great that strong data would be required to support an alternative origin. Paleomagnetic data, however, indicate that the Salinian block may have been displaced 2,500 km northward since Cretaceous time (Champion and others, 1984) and that it may have originated near the latitude of Central America or Mexico in the axial part of the Cordilleran Cretaceous plutonic arc (Page, 1982).