TEARING APART OF AN EARLY MIOCENE SILICIC IGNEOUS PROVINCE IN THE SOUTHERN GULF OF CALIFORNIA

We have studied a large set of samples from the rifted continental crust submerged in the southern Gulf of California (SGC) on both sides of the Alarcon, Pescadero and Farallon oceanic basins that were recovered by a remote operated vehicle and by dredging during two cruises in 2008 and 2009. Once integrated with the subaerial geology of the conjugated margins in southern Baja California, Nayarit, and Sinaloa, these data allow a better understanding of the pre-rift magmatic history. Previous works considered intrusive rocks exposed in southern Baja California (Los Cabos block and in the Gulf islands of Espiritu Santo, San Jose, San Diego, Santa Cruz, and Santa Catalina) as part of the basement and correlatable with the Late Cretaceous Peninsular Range batholith. However, here we document the occurrence of some early Miocene intrusive, and their volcanic equivalents, on both margins of the southern Gulf as well as in the submerged rifted blocks.

For intrusive rocks from submerged continental blocks of the SGC we obtained an U-Pb age of 19.9 Ma and four Ar-Ar ages of 17.4-18.5 Ma for samples recovered from the southern Pescadero transform scarp, from submarine extensional faults to the SE of Alarcón basin, and from the Cerralvo bank. In southern Baja California, we obtained Ar-Ar ages of 19.9 to 18.1 Ma for intrusive rocks forming the southern part of Santa Catalina island and 18.6 Ma for a granite at Bahía Concepción. These rocks are very similar to plutonic bodies exposed along the Santiago and San Pedro river in Nayarit for which we obtained Ar-Ar ages ~ 19 to 20 Ma. The volcanic counterpart of these plutonic rocks is also present in the SGC. Ignimbrites dredged at sites offshore Espiritu Santo island yielded U-Pb ages of 21.3 to 20.5, and ignimbrites and andesites from Montserrat, Santa Catalina and Danzante islands, Ar-Ar ages of 18.9 to 17.7. These ages are similar to those reported for the lower part of the Comondú group in southern Baja California (e.g., Hausback, 1984; Drake, 2005) and for the last ignimbrite pulse in the southern Sierra Madre Occidental (Ferrari et al., 2002, and our unpublished data). The distribution of silicic plutons on both sides of the Gulf and in the rifted blocks indicate that the early Miocene silicic province extended along the entire area now occupied by the Gulf and was later dismembered by the rifting process.