EVOLUTION OF THE GORDA ESCARPMENT, SAN-ANDREAS FAULT AND MENDOCINO TRIPLE JUNCTION FROM MULTICHANNEL SEISMIC DATA COLLECTED ACROSS THE NORTHERN VIZCAINO BLOCK, OFFSHORE NORTHERN CALIFORNIA

The Gorda Escarpment is a north facing scarp immediately south of the Mendocino transform fault (the Gorda/Juan de Fuca-Pacific plate bounda ry) between 126 degrees W and the Mendocino triple junction. It elevat es the seafloor at the northern edge of the Vizcaino block, part of th e Pacific plate, similar to 1.5 km above the seafloor of the Gorda/Jua n de Fuca plate to the north. Stratigraphy interpreted from multichann el seismic data across and close to the Gorda Escarpment suggests that the escarpment is a relatively recent pop-up feature caused by north- south compression across the plate boundary. Close to 126 degrees W, t he Vizcaino block acoustic basement shallows and is overlain by sedime nts that thin north toward the Gorda Escarpment. These sediments are t ilted south and truncated at the seafloor. By contrast, in a localized region at the eastern end of the Gorda Escarpment, close to the Mendo cino triple junction, the top of acoustic basement dips north and is o verlain by a 2-km-thick wedge of pre-11 Ma sedimentary rocks that thic kens north, toward the Gorda Escarpment. This wedge of sediments is re stricted to the northeast corner of the Vizcaino block. Unless the wed ge of sediments was a preexisting feature on the Vizcaino block before it was transferred from the North American to the Pacific plate, the strong spatial correlation between the sedimentary wedge and the tripl e junction suggests the entire Vizcaino block, with the San Andreas at its eastern boundary, has been part of the Pacific plate since signif icantly before 11 Ma.