GUAYMAS BASIN AND SURROUNDINGS: STRUCTURE, TECTONICS, AND MAGMATISM

The Gulf of California (GOC) marks a young transtensional plate boundary in western Mexico between the North American and Pacific plates. In the Guaymas Basin segment, in the central Gulf of California, the active tectonic and volcanic features include the Guaymas and Carmen transform faults, the Guaymas spreading center dominated by formation of sill-sediment complexes, young volcanoes on the Baja California peninsula, the Isla Tortuga volcano, and submarine volcanic ridges and knolls away from the basin axis. This presentation reviews the state of knowledge of the Quaternary to recent tectonic and volcanic history of this region, as an example of the complexities that develop as seafloor spreading centers are in the process of localizing at transtensional plate boundaries at continental rifted margins. The Quaternary volcanoes on the Baja California peninsula include the La Reforma Caldera and resurgent dome, the El Aguajito caldera, and the youngest volcanoes including El Azufre and La Virgen. The rocks derived from eruptions at the three vents comprising the Tres Vírgenes-El Aguajito-La Reforma caldera system include airfalls, subaerial pyroclastic density current deposits, andesite and dacite lavas, and submarine rhyolite domes (now uplifted, so mappable near the coast). These range in age from Holocene back to more than 1 Ma and have been studied by various authors. After the ignimbrite-forming eruptions at La Reforma Caldera and El Aguajito Caldera (1.1 Ma), eruptions have been documented at 300 ka, 173 ka, 128 ka, 112 ka, and 22 ka. Other volcanic sources inferred to have erupted below sea level form likely volcanic knolls visible in bathymetric surveys at various points in the Guaymas Basin. Studies of regional seismicity delineating active faulting onshore and in the marine basin come from Mexican national seismic networks and local seismic networks in the Tres Virgenes geothermal field. Active seismic studies have imaged the marine strata and the sills in the near surface and provided a cross section of the entire spending system down to the Moho. Basin sediments were drilled in Deep Sea Drilling Project (DSDP) Leg 64 in 1978-1979 and International Ocean Discovery Program (IODP) Expedition 385 in 2019. Details of these sedimentary strata are discussed in other presentations in this topical session.