The pre-existing Farallon Trench from 10 to 30 Ma.
Current SE Farallon Island
San Andreas Fault Sequential Diagrams Atwater
The Farallon Trench was an ancient oceanic trench on the west coast of North America during the Late Cretaceous Epoch. The trench started disappearing when the Farallon Plate began subducting under the North American Plate. The Subduction of oceanic lithosphere occurs through trench migration. So that after the Farallon Plate completely subducted, there are only Juan de Fuca Plate remaining. The Farallon Trench then turned into the San Andreas Fault. Since then, it has spread out to the north, trench above Juan De Fuca Plate, and south, trench above Central America. The time duration of subduction began from 165 Ma, Farallon Trench replacing the Mezczlera promontory, till 5 Ma, San Andreas fault straightening.