Vertical extent of the newborn San Andreas fault at the Mendocino triple junction

Three-dimensional seismic refraction data reveal the structure of the plate boundaries at the Mendocino triple junction. First arrival traveltimes wer e inverted to obtain a three-dimensional seismic velocity model. The tomogr aphy results indicate a strong lateral velocity contrast in the lower crust . This velocity contrast is beneath the surface trace of the San Andreas fa ult and tracks along the fault to the triple junction and then along the Me ndocino fracture zone. Velocity values appropriate for mafic rocks are more than 5 km shallower on the Pacific plate than east of the San Andreas faul t, and 1-3 km shallower than under the Cascadia accretionary prism. The mod el suggests that the San Andreas fault, which is growing as the triple junc tion migrates northward, cuts steeply through the entire crust. Pacific oce anic crust does not extend beneath North America at this location. The slab less window south of the triple junction and east of the San Andreas fault must be filled with upwelling asthenosphere. The sharp corner of the Pacifi c plate migrates northward, displacing the Cascadia accretionary prism and forcing the prism material to move north or east.