The local appearance of voluminous, monomineralic, foliate serpentinite breccias in the Wilbur Springs section of the Great Valley Sequence record a direct sedimentological response to a late Neocomian accretion event in the Coast Ranges. Field and petrologic studies establish the following events: 1) Early Cretaceous emplacement of lower plate Franciscan rocks (Indian Valley terrane) beneath the proto-Coast Range thrust, 2) deformation of both upper and lower plate rocks into the southeast plunging Wilbur Springs anticline, and 3) contemporaneous protrusion of serpentinite breccias.
Recently discovered modern examples of diapiric serpentinite protrusions of similar magnitude from the Mariana arc-trench system suggest many parallel analogies between the tectonics of active forearcs and the late Mesozoic of the California margin.
Two modes of occurrence of serpentinite have been recognized in the Wilbur Springs area. The first type, serpentinite derived from the alteration of ultramafic tectonite, is widely distributed throughout the Coast Ranges (Bailey et al., 1964). Once considered to be altered ultramafic intrusions and included as part of the Franciscan Complex, these serpentinite masses occur as regionally extensive, allochthonous fault slices or fault-bounded mèlange belts, and tend to be preferentially concentrated along or near contacts between the Franciscan complex and the Great Valley Sequence (Fig. 61). Based on the characteristic spatial association of the serpentinite with layered mafic plutonic rocks, submarine basaltic lavas, and pelagic sedimentary sequences, these serpentinized ultramafic rocks are now generally accepted to represent the basal, mantle tectonite of an ophiolite sequence, the Coast Range Ophiolite, upon which the sediments of the Great Valley Sequence were deposited (Bezore, 1969; Bailey et al., 1970; McLaughlin, 1974; Evarts, 1977; Hopson and Frano, 1977; Hopson et al., 1981).