The Gulf of California is believed to be an area of active spreading and movement along transform faults. Astride one of these major transform faults, the Guaymas lineament, lie two island chains, the Isla San Luis group in the north, and Isla San Lorenzo group in the south. The Isla San Luis Island group consists of five islands, all uplifted fault blocks of Miocene calcalkali volcanic rocks. These blocks are upthrown on the west and dip to the northeast. Young acidic volcanic rocks overlie these Miocene rocks on Isla San Luis. Fractures on these islands suggest active faulting related to the regional tectonic framework. The San Lorenzo group also consists of uplifted fault block islands exposing a Cretaceous tonalite basement complex intruded into a metasedimentary sequence. These rocks are overlain by Miocene calcalkali andesitic volcanic rocks. Unconformably over these rocks are fossiliferous lower Pliocene clastic and evaporite sedimentary rocks. Young volcanism is evidenced by Isla Raza which consists of olivine basalt. The chemistry of the volcanic rocks and the geology of the islands are compatible with current theories of the history of the Gulf of California.
