PATTERNS OF NEOGENE BASIN SUBSIDENCE IN THE GULF OF CALIFORNIA-SALTON TROUGH

Neogene sediments exposed in the Salton Trough, California, along the coast of Baja California, Mexico, on gulf islands, and along the Nayarit coast of mainland, Mexico provide insights into the subsidence of basins intimately involved in the evolution of the Gulf of California. These sequences, along with data from DSDP sites in the gulf mouth area, allow analysis of the subsidence and deformation of basins formed in a variety of crustal and depositional settings. Geochronologic, lithologic, and paleobathymetric data from six stratigraphic sections arrayed from the northern reach of the gulf (Fish Creek-Split Mountain, California) to the mouth of the modern gulf (Maria Madre Island, Mexico) allow application of backstripping techniques to derive rates and patterns of basin subsidence. The resulting subsidence curves reflect discrete phases in the subsidence, uplift and deformation of gulf basins. Estimated rates of middle Miocene subsidence during non-marine deposition in the western Salton Trough are typical of thermal decay. Earliest marine flooding of the southern gulf occurred during Miocene continental extension (ca 10 Ma) after spreading and subduction of microplates stopped off southern Baja California about 13 Ma. Initial rapid mechanical subsidence to bathyal water depths occurred in the gulf mouth region between 10-6 Ma. Subsidence to bathyal water depths also occurred in the northernmost gulf at least as early as 7-6 Ma. Widespread rapid subsidence to lower bathyal water depths occurred from 5-3 Ma accompanying Pliocene transform rifting and spreading. Seismic reflection profiles and data from onshore and DSDP sites illustrate that deformation of Miocene "proto-gulf" sediments commenced along the eastern margin of the gulf as spreading and subsidence occurred along the evolving transform-rift boundary to the west. Based upon analysis of benthic foraminifera at DSDP Site 475, abyssal water depths were attained in the western gulf mouth region by 3.5-3 Ma.