Southwestern Isla Tiburon (SWIT), Gulf of California, Mexico, is an area approximately 30 square kilometers, containing sedimentary and volcanic strata which are predominantly Early and Middle Miocene in age. A fossiliferous marine conglomerate on SWIT is believed the oldest marine deposit in the Gulf of California region, and supports the conclusion that a marine protogulf existed 15 to 8 Ma. K-Ar ages from this and previous studies provide a time constraint on the deposition of the marine conglomerate and the overall volcanic history of SWIT. Dips of beds suggest listric faulting occurred throughout much of the Miocene: Redbeds dip 40-50°N, basalt flows 23ºN, conglomerates 20ºN, and an ignimbrite 5ºNW. A fault along Arroyo II displays right-lateral, strike slip and may be related to the La Cruz Fault which trends NW-SE along the southwest end of the island. In the late Cretaceous-early Tertiary, SWIT was part of the arc system along the western margin of the North American Plate. At this time, SWIT was adjacent to northeastern Baja California. In the Early Miocene Basin and Range extension led to the formation of small trough-shaped basins, and fluvio-lacustrine deposition resulted. Volcanic and roof pendant debris was shed into small basins on SWIT from the north. At 21-19 Ma andesitic volcanism occurred and a volcanic collapse structure formed which was later filled with andesitic lahar. The andesitic magma apparently formed out of a primary melt of the lower crust. From 19-16 Ma basaltic volcanism was dominant on SWIT and both tabular flows and ring dikes developed. The basalt of SWIT is alkalic olivine-augite, is high in incompatible elements and is similar to Early and Middle Miocene basalts of the Imperial Valley-northeastern Baja California region. Dacitic pyroclastic volcanism followed basaltic volcanism on SWIT, and between 14 and 11 Ma a protogulf developed which was filled with pyroclastic and conglomerate debris. At 11 Ma volcanism on SWIT became predominantly rhyolitic. First, rhyolite ignimbrite extruded, and, in the late Miocene, rhyolite pods, coulees, and crystal dikes developed. In the early Pliocene a dacitic ignimbrite with a tholeiitic character extruded reflecting conditions associated with modern Gulf of California rifting and seafloor spreading.