During the Plio-Quaternary, numerous volcanic centres of contrasted compositions were emplaced in the Baja California Peninsula and adjacent Gulf of California. The 43 km2 Isla San Esteban (28o 42 N and 112o 34' W), located in the central part of the gulf near the Pliocene extinct spreading centre of Lower Tiburón basin, is mostly made of andesitic, dacitic and rhyolitic subaerial lava flows, domes, and pyroclastic deposits. A new geological sketch map of the island is presented, as well as new K-Ar ages ranging from 4.5 to 2.5 Ma. Isla San Esteban lavas belong to two nearly contemporaneous magmatic series emplaced within an extensional tectonic framework related to the continental breakoff followed by the progressive oceanization along the Gulf of California. The first one is medium-K calc-alkaline, and its magmas evolved from basaltic andesites to rhyolites by fractionation of plagioclase, pyroxenes, titanomagnetite and apatite. The second one includes genuine adakites, and dacites derived from them through separation of plagioclase plus amphibole. We propose that these adakites originated from the partial melting of oceanic crust slivers, incorporated to the subcontinental lithospheric mantle during the Late Miocene opening of a slab window followed by the detachment of the deep part of the Farallon slab. The high thermal flux linked to the opening of the Gulf would have triggered their melting, as well as that of the subduction-modified mantle, which represents the likely source of the associated calc-alkaline lavas.