3.3 MA SEA WATER INCURSION OF SAN NICOLÁS AND EL MANGLE BASINS DURING OPENING OF THE PROTO-GULF OF CALIFORNIA, MEXICO

Within the Baja California central domain of the Proto-Gulf extension, several depocenters were formed, including the El Boleo basin, followed by the Bahía Concepción basin to the south, the San Nicolás basin to the southeast, and the southernmost El Mangle basin. The San Nicolás basin is associated with the transfer zone coupled to the Bahía Concepción accommodation zone. The episode of extension was responsible for tilting of volcanic blocks of Miocene age. The San Nicolás Basin, a half-graben structure, was formed during a major episode of crustal extension related to the opening of the Proto-Gulf. It is one of the multiple synthetic basins associated with the transfer zone coupled to the Bahía Concepción accommodation zone. The extensional process resulted in uplift and exhumation of Cretaceous (99 ±2 Ma) granodiorite basement. Tobas San Antonio member is the oldest San Nicolás basin unit (3.3 ±0.5 Ma) demonstrating a younger basin age than Bahia Concepción (ca. 5.1 Ma). The member consists of lithic tuffs, deposited subaerially, and draped, with high dip angles around volcanic and plutonic blocks. The El Mangle basin, located east of the Cerro Mencenares Volcanic complex, 30 km north of Loreto, Baja California, Sur, contains oldest units of the same age as those in San Nicolás basin (3.3 ±0.5 Ma). Basal tuffs in both basins lie unconformably on the local basement, and are clearly post-extensional. The oldest El Mangle units are lithic tuffs, deposited in marine or brackish water. Preliminary data indicate that younger El Mangle tuffs were deposited in shallower water than were older tuffs. Equally preliminary data indicate El Mangle tuffs vary in composition from andesitic basalt (to the south) to andesite (to the north), consistent with post-subduction magmatic compositions reported by Bigioggero and others. Both San Nicolás and El Mangle basins were invaded by sea water at much younger ages than the extensional basins to the north, demonstrating a clear difference in the timing of marine incursion into these two sets of Gulf of California basins.