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NORTHERN AND CENTRAL CALIFORNIA

FRANCISCAN ROCKS [c3, p63-64]

Franciscan rocks form the east wall of the San Andreas fault for virtually its entire course through the Coast Ranges of central and northern California, although the Franciscan is concealed along some reaches of the fault by overlying rocks. The Franciscan is a heterogeneous assemblage that consists largely of dismembered sequences of graywacke, shale, and lesser amounts of mafic volcanic rocks, thin-bedded chert, and rare limestone. These rocks also occur with serpentinite and tectonic pods of blueschist in melange zones that are the locus of much shearing within the Franciscan and that generally separate blocks of the more coherent sequences. The sedimentary and volcanic Franciscan rocks were formed in a marine environment, as attested by the abundance of foraminifers in the limestone and by radiolarians in the chert. Most of these rocks are probably Late Jurassic and Cretaceous in age (Bailey and others, 1964), but some of the chert and associated volcanic rocks are as old as Early Jurassic (Pliensbachian) (Irwin and others, 1977; Blome and Irwin, 1983). In the northern Coast Ranges, some of the rocks assigned to the coastal belt of the Franciscan assemblage are as young as late Tertiary and are thought to have accreted to North America during post-middle Miocene time (McLaughlin and others, 1982). The age and origin of Franciscan melange is problematic. Mid-Cretaceous limestone in melange near Laytonville in the northern Coast Ranges, 225 km northwest of San Francisco, has a paleomagnetic inclination that indicates an origin several thousand kilometers to the south (Alvarez and others, 1980). Similarly, Franciscan pillow basalt about 45 km northwest of San Francisco is thought to have moved northward 19 of latitude (approx 2,000 km) from its site of origin (Gromme, 1984). These and other features indicate that some, possibly much, of the Franciscan has been transported great distances northward along the Pacific margin relative to a stable North America.

The Franciscan rocks are locally overlain structurally by the Coast Range ophiolite and the Great Valley sequence, and are separated from them by the Coast Range thrust (Bailey and others, 1970). The original extent of the Coast Range thrust is not clearly known because most of the ophiolite and Great Valley sequence that formed the upper plate of the thrust has been removed from the top of the Franciscan except in the general area of the Diablo antiform, which is marked by a line of windows from Mount Diablo to Parkfield, and along the west edge of the Great Valley (Figure 3.3 and Figure. 3.4). A few small outliers of upper-plate rocks are present elsewhere east of the San Andreas fault as far north as Pillsbury Lake, 35 km east of Willits, and the Camp Meeker area, 17 km northeast of Bodega Head, and at several localities west of the Salinian block as far south as the Santa Ynez fault in the Transverse Ranges (see maps at front of book). Much of the serpentinite in Franciscan melange may well be sheared-in fragments of dismembered Coast Range ophiolite.