THE BAJA BASINS PROGRAM: MENTORED UNDERGRADUATE FIELD AND LAB RESEARCH INTEGRATING MULTIDISCIPLINARY GEOLOGIC DATA TO ADDRESS THE TECTONIC EVOLUTION OF THE GULF OF CALIFORNIA RIFT OF THE EAST-CENTRAL BAJA PENINSULA, MEXICO

The Baja Basins undergraduate research experience is unique because it features a high 1:1 to 1:2 mentor-to-student ratio that split fieldwork in Baja in January with laboratory analyses at UMKC in July and partnered with copper and gypsum mines, thus providing students with academic and industry workforce training. Organized into thematic research groups, the Baja Basins program focuses on modeling authentic research practices while supporting student growth in scientific inquiry and communication. As co-PI on the original REU, Cathy Busby was instrumental in launching the program. Since 2015, we have supported 64 U.S. and 15 Mexican undergrads. Major scientific advances of the Baja Basins program include new Ar/Ar ages and geochemistry of arc-to-rift transition volcanic rocks in the Santa Rosalia Basin (SRB) that show high-magnesian andesite lavas and intrusions on the western margin of the Guaymas oceanic rift basin that are as young as 6.06 Ma ±0.27 Ma (Busby et al. 2020) and contemporaneous with the Boleo Fm Cu mineralization. Mineral stability diagrams of the ore fluid indicate initial reducing conditions that become highly oxidized with little change in pH. Mn and probably other metals in the ores were leached from mafic minerals, mainly amphiboles, in the underlying volcanic rocks. Mineralogy and elemental analyses of clastic interbeds in the Boleo Fm gypsum clearly establish that the gypsum is coeval with ore deposition. Stratigraphic sections of the Boleo Fm across the SRB reveal a thin basal marine limestone overlain by a thick siliciclastic sequence composed of coarsening-up deltaic cycles and laterally equivalent distal gypsum. Basin architecture and structural relations suggest that subsidence occurred by growth of a large monocline that tilted the crust down to the ENE. New U-Pb dating of zircon from the Cinta Colorada tuff within the Boleo Fm yield an age of 5.86 Ma ±0.12 Ma. Pleistocene uplifted marine and fluvial terraces were studied with detailed new field mapping supported with RTK GPS elevation data, luminescence and cosmogenic geochronology, and soils chronosequence analysis. These data reveal a spatially complex uplift history at rates that have stayed approximately constant at 0.3 mm/ka since ~500 ka, and sedimentation linked to climatic cycles ranging between 21 to 100 kyr.