CRUSTAL VELOCITY STRUCTURE NORTH OF THE MENDOCINO TRIPLE JUNCTION

A 140-km-long refraction/wide-angle reflection profile recently record ed by Stanford University and the U. S. Geological Survey imaged the s ubducting Gorda slab beneath northern California. The profile, which i s subparallel to the coast from Cape Mendocino northward, indicates th at the North American plate is 13- to 14-km-thick along the coast nort h of Cape Mendocino. The crust is characterized by relatively uniform, low velocities of less-than-or-equal-to6 km/s interpreted as Francisc an rocks. Two strong reflections define the upper and lower boundaries of the subducting Gorda crust. Our data indicate that the subducting Gorda crust thickens northward from Cape Mendocino from 7-km-thick jus t north of Cape Mendocino to 10-km-thick 120 km to the north. This cha nge in thickness is coincident with a change in velocity from 6.7 km/s south to 6.2 km/s north. Mantle velocities of 7.7 km/s are observed f or offsets greater than c. 80 km. We interpret our model to indicate t hat the Gorda slab is not imbricated on a crustal scale beneath our pr ofile, that sediments and/or a tectonically thickened oceanic layer 2 are present to the north but not in the vicinity of Cape Mendocino, an d that proximity to the Mendocino triple junction affects the way sedi ments are subducted.