EMERGENCE AND PETROLOGY OF THE MENDOCINO RIDGE

The Mendocino Fracture Zone, a 3,000-km-long transform fault, extends from the San Andreas Fault at Cape Mendocino, California due west into the central Pacific basin. The shallow crest of this fracture zone, k nown as the Mendocino Ridge, rises to within 1,100 m of the sea surfac e at 270 km west of the California Coast. Rounded basalt pebbles and c obbles, indicative of a beach environment, are the dominant lithology at two locations on the crest of Mendocino Ridge and a Ar-40/Ar-39 inc remental heating age of 11.0 +/- 1.0 million years was determined for one of the these cobbles. This basalt must have been erupted on the Go rda Ridge because the crust immediately to the south of the fracture z one is older than 27 Ma. This age also implies that the crest of Mendo cino Ridge was at sea level and would have blocked Pacific Ocean easte rn boundary currents and affected the climate of the North American co ntinent at some time since the late Miocene. Basalts from the Mendocin o Fracture Zone (MFZ) are FeTi basalts similar to those commonly found at intersections of mid-ocean ridges and fracture zones. These basalt s are chemically distinct from the nearby Gorda Ridge but they could h ave been derived from the same mantle source as the Gorda Ridge basalt s. The location of the 11 Ma basalt suggests that Mendocino Ridge was transferred from the Gorda Plate to the Pacific Plate and the southern end of Gorda Ridge was truncated by a northward jump in the transform fault of MFZ.