| Abstract |
The San Andreas transform system joins the Gulf of California divergent plate boundary in the Salton Trough region of Southeastern California. Here several strands of the system end on the southeast at active spreading centers beneath about 6 km of upper Cenozoic sediments, both marine and nonmarine. Neotectonic deformation, earthquakes, geothermal activity, and volcanicity document these spreading centers. Before this pattern of tectonics and sedimentation developed about 4.5 m.y. ago and the present Gulf of California began to open, the San Andreas fault was already active. It apparently originated during oblique plate convergence 8 or 10 m.y. ago along the margins of a proto-gulf. Conglomerates of this age have now been offset laterally about 320 km from their source areas. Before this, beginning abut 22 m.y. ago, basin-range tectonics prevailed. These basins and their surrounding terranes pre-date the San Andreas system in Southeastern California. In early tertiary and late Cretaceous time, plate convergence prevailed and the region lay near the northeastern margin of a forearc basin. |
