RAPID EXHUMATION OF EARLY TO MIDDLE MIOCENE INTRUSIVE ROCKS IN THE SOUTHERN GULF OF CALIFORNIA: THE EARLY STAGES OF CONTINENTAL BREAKUP

Arc magmatism is a prominent feature of western Mexico, represented by the Cretaceous Peninsular range and Sonora-Sinaloa batholiths as well as the Eocene-Miocene voluminous silicic volcanism of the Sierra Madre Occidental (SMO), the world´s largest Cenozoic ignimbrite province. Nevertheless the plutonic magmatism associated to this latter volcanic activity is still poorly known. Plutonic bodies associated to the last silicic pulse of the SMO in the Early Miocene are particularly interesting, as they may have been emplaced during an already extensional setting (Ferrari et al., this meeting). Previous reports of Early Miocene intrusives are limited to a few bodies in Sinaloa and Nayarit. Our geologic and geochronological study recognized many other locations on both sides of the southern Gulf as well as in the submerged rifted blocks. Distinctive, medium to fine grained granitoids exposed in the SMO foothills in northern Nayarit (San Miguel, San Juan Bautista, Aguamilpa, El Cajon, Plan de Barrancas) and southern Sinaloa (El Rosario) yielded intrusion ages of 21.5 to 18.7 Ma. Similar intrusives were also recognized in the Santa Catalina Island offshore Baja California (22 Ma). Inside the Gulf of California, sampling using ROV Jason during the 2008 ROCA cruise recovered intrusive rocks with U-Pb ages ranging from 14 to 19 Ma on both sides of the Pescadero basin and on the eastern side of the Alarcon basin. A thermochronologic study (U-Pb, Ar-Ar, U-Th/He) of the Early to Middle Miocene intrusives is in progress. Preliminary results suggest that these plutons were emplaced at shallow depths and were rapidly exhumed immediately afterward. Our study shows that the sources of the Early Miocene volcanism are not restricted to the southeastern SMO but that this magmatic episode was also rooted in the area now occupied by the southeastern part of the Gulf. Intrusives were probably emplaced during ongoing extensional deformation and contributed to focus the deformation, which eventually resulted in the opening of the Gulf of California.