EARLY TO MIDDLE MIOCENE SYN-EXTENSIONAL MAGMATISM IN THE SOUTHERN GULF OF CALIFORNIA

The Gulf of California (GoC) constitutes an example of crustal stretching leading to the birth of new oceanic basin at the site of a former convergent margin. In the southern Gulf the rifting process broke apart a Late Cretaceous to Paleocene batholitic belt, now exposed in southern Baja California and mainland Mexico, which is extensively covered by mid Eocene to Lower Miocene ignimbrites of the Sierra Madre Occidental silicic large igneous province (SMO). The intrusive part of SMO has been scarcely studied in the southern Gulf due to its limited exposure. Previous reports were limited to a few bodies in Sinaloa (mainland Mexico) and Bahia Concepcion (Baja California).

Here, we present the result of an extensive study of the Tertiary intrusive rocks of the southern Gulf sampled along its margins and in the submerged rifted blocks. Emplacement and cooling ages were determined on 66 samples by U-Pb and 40Ar/39Ar methods, and define two main magmatic pulses in Early and Middle Miocene. The Early Miocene magmatic pulse (22 to ~18 Ma) is observed across the entire southern GoC, from southern Baja California to Nayarit and Sinaloa, with a rapid cooling at ~18 Ma suggesting that emplacement was concurrent with crustal stretching and normal faulting forming the Gulf Extensional Province. The mid-Miocene magmatic pulse (16 to 13 Ma) tends to focus in the central part of the Gulf, around the Pescadero Basin. Granitoids emplaced in this area also show a rapid cooling with ⁴⁰Ar/³⁹Ar ages in the 13-12 Ma range. As a whole the age pattern suggest a transition from wide to a narrow rift mode during the Miocene.

It’s widely known that ignimbrites of the last flare-up of the SMO (24 to 18 Ma) reached southern Baja California. Present results indicate that the early to middle Miocene magmatism (source of this silicic pulse), was not only located in northern Nayarit and Sinaloa but also in the area of the future Gulf and that this region was already under extension in the Early Miocene. Therefore, crustal stretching preceding the formation of oceanic crust in the GoC started significantly before the end of subduction and was likely the main factor controlling the genesis of magmatism.