NEW INSIGHTS INTO THE NORTH OF BAJA CALIFORNIA MICROPLATE RECONSTRUCTION ATCA. 12 MA: PETROGRAPHY, GEOCHEMISTRY, AND PALEOMAGNETISM OF THE IGNIMBRITEOF HERMOSILLO – TUFF OF SAN FELIPE IN SAN CARLOS, SONORA, MÉXICO

Los Algodones mountain range is located north of the town of San Carlos in Guaymas, Sonora, Mexico. Los Algodones is characterized by an igneous-metamorphic basement of Triassic-late Cretaceous age and two tilted volcanic sequences and dikes of Miocene age. The first volcanic sequence overlays the basement and has lavas of early-middle Miocene with intermediate to mafic composition. Late Miocene lavas represent the second sequence with intermediate to felsic composition, including two ignimbrite deposits. Stratigraphic position, and petrographic, geochemical and paleomagnetic studies of Los Algodones Ignimbrite (LAI), one of the ignimbrite deposits, allow establishing that this unit is part of the ca. 12 Ma Ignimbrite of Hermosillo - Tuff of San Felipe (IGH - TSF), a widespread stratigraphic marker in NW Mexico. Based on LAI petrologic characterization, this deposit can be correlated with a lithological unit reported at Cataviña, Baja California, Mexico, 430 km away from the study area. This fact is tectonically important, because it allows us to propose a new reconstruction of the Baja California microplate to its original position on the North American plate. Paleomagnetic studies confirm a high tilted and a significant vertical axis rotation in the IGH – TSF, which are related to transtension during the evolution of the Proto-Gulf of California oblique rift.