COAST RANGE OPHIOLITE [c3, p64]

The Coast Range ophiolite represents oceanic crust on which much of the sedimentary rock of the Great Valley sequence was deposited. A complete ophiolite sequence consists of serpentinized harzburgite tectonite at the base, overlain by cumulate ultramafic and gabbroic rocks, passing upward into noncumulate gabbroic and related plutonic rocks, then into diabase dikes, and finally into pillow lavas. The Coast Range ophiolite, however, generally is highly sheared, dismembered, thinned, and locally missing, presumably as a result of faulting, at many places along the fault contact between Franciscan and Great Valley rocks. Only in a few places is a nearly complete lithologic sequence of Coast Range ophiolite preserved, and there the total stratigraphic thickness of the ophiolite is about 3 to 5 km (Hopson and others, 1981). Isotopic ages ranging from about 165 to 153 Ma (Hopson and others, 1981) indicate that the Coast Range ophiolite is Middle and Late Jurassic in age. Paleontologic and paleomagnetic evidence suggests that the Coast Range ophiolite formed in an equatorial setting and was transported great distances northward before being accreted to North America and overlain by the Great Valley sequence (Pessagno and others, 1984; Hopson and others, 1986; McLaughlin and others, 1988).