COAST RANGE THRUST [c3, p65-66]

The upper plate of the Coast Range thrust, consisting of Great Valley sequence with Coast Range ophiolite at a the base, is cut by the San Andreas fault only along the west side of the Diablo Range in central California. There, the upper plate of the Coast Range thrust forms a broad blanket over the Franciscan rocks except in several places along the length of the Diablo Range where windows, or piercement structures, in the upper plate expose Franciscan rocks of the lower plate (Figure. 3.4: at Mount Diablo, Pacheco Pass, New Idria, and Park-field. This deformed antiformal structure (Bailey and others, 1964) is truncated on the west by the Hayward and Calaveras faults in its northern part and by the San Andreas fault in its southern part, and narrows to the southeast as the San Andreas fault converges with the south end of the Great Valley. The ophiolitic rocks are generally thinned and highly discontinuous along the faults that bound the windows except for the New Idria window, which is occupied mostly by serpentinite. South of the Pacheco Pass window, the structure is complicated by a series of west-northwest-trending synforms that cross the axis of the Diablo Range antiform at a low angle, and by multiple strands of the San Andreas fault. In the Parkfield area, the western part of the antiform has been virtually destroyed by its proximity to the San Andreas fault. The Franciscan rocks of the Parkfield window, and the associated serpentinite and Great Valley sequence of the highly dissected upper plate, now are mostly elongate fault slices that form part of the San Andreas fault zone (see Dibblee, 1980).

Most of the serpentinite of the Coast Ranges of California is related to the ultramafic parts of the Coast Range ophiolite. Serpentinite is strikingly absent along the San Andreas fault north of San Francisco, a situation that reflects the absence of the upper plate of the Coast Range thrust in that area. Although many elongate fault-bounded bodies of serpentinized ultramafic rocks in the Coast Ranges traditionally have been mapped and described as part of the Franciscan, most are probably dismembered and dislocated parts of the Coast Range ophiolite or equivalent. Recognition that the ultramafic rocks in most places belong to the Coast Range ophiolite rather than to the Franciscan assemblage is important to a tectonic analysis of the region. The common presence and possible seismotectonic significance of serpentinite along creeping segments of the San Andreas and related faults were described by Allen (1968) and Irwin and Barnes (1975).