[1] During subduction at the Franciscan trench beginning at 170-160 Ma and continuing to the present, marine sedimentary and lesser volcanic rocks have been underthrust, accreted, and metamorphosed to form the Franciscan accretionary wedge. The South Fork Mountain Schist (SFMS) forms the eastern margin and structural top of the wedge and so was apparently the first unit of substantial size to accrete into the Franciscan.

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Plagiogranites are a minor but widespread component of the Samail ophiolite plutonic member. They crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the Tethyan spreading ocean ridge, and their ages are thought to mark the time of ocean crust formation. Isotopic U‐Pb ages of zircons from 13 plagiogranites collected along a 270‐km segment of the Samail ophiolite subparallel to the regional trend of the sheeted dike complex (the former spreading ridge axis direction) define a narrow time interval of 93.5–97.9 m.y., with a pronounced clustering about 95 m.y. The zircon ages of the plagiogranites agree remarkably well with the early Cenomanian to early Turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them (Tippit et al., 1981). The agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite U‐Pb ages closely date the time span of ocean crust formation. No step changes in age patterns are observed along the ridge axis (sheeted dike) direction, suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse. One possible exception occurs at the southeastern end of the sampled interval (Ibra area), where a 3 m.y. discontinuity might be caused by an unmapped fault. Assuming that the regional trend of the sheeted dikes (N10°–25°W) marks the direction of the former spreading ridge axis, the present array of sample localities spans a distance of 130 to 195 km normal to that axis (i.e., in the spreading direction). The data as a whole do not define a clear‐cut age trend normal to the spreading axis, but by eliminating samples that may be aberrant due to faulting, the data array suggests a pattern of increasing ages from east to west. This would indicate that the Samail ophiolite is derived from the western (southwestern) side of the Tethyan spreading system—a conclusion reached independently from geological evidence. Also, the gentle age gradient in the spreading direction (i.e., between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m.y.) suggests a very fast spreading rate. This conclusion also is supported by independent geologic evidence.