GEOLOGIC MAPPING AND STRUCTURAL ANALYSIS OF SAN MARCOS ISLAND, BAJA CALIFORNIA SUR, MÉXICO

The Baja Basins IRES Program is a U.S.-México collaborative research project funded by NSF that aims to provide field research experiences to undergraduate students. Field investigations are focused on the Santa Rosalia Basin (SRB) in Baja California Sur, where a rare and accessible Neogene stratigraphic record sheds light on the tectonic and magmatic evolution of the active oblique rift margin of the Gulf of California. San Marcos Island (SMI) hosts an active gypsum mining operation and shares a similar stratigraphy to the SRB, yet the island has never been mapped geologically, and its potential correlation with the SRB is uncertain. We conducted detailed geologic mapping and structural analyses on the south half of SMI to better understand the structure and tectonic evolution of the island, and to evaluate its relationship with the SRB. The oldest volcanic rocks on SMI are ca. 9–7 Ma, dip moderately and uniformly to the W or NW. These are unconformably overlain by shallow marine siliciclastic strata and gypsum of the latest Miocene San Marcos Fm, which are in turn unconformably overlain by the fossiliferous marine clastic and carbonate rocks of the Pliocene Carmen Fm. Fault kinematic analysis indicates a predominance of normal faulting on N- to NE-striking faults and a mean E-W extension vector oriented at 276°. Extension commenced at ca. 8–6 Ma (prior to deposition of the San Marcos Fm), and continued until after 3.9 Ma during deposition of the marine Carmen Fm. The similar fault pattern, extension direction (276° on SMI vs. 267° in the SRB), and timing of faulting in the SRB support a shared deformation history and suggest that SMI has likely not experienced significant vertical-axis rotation relative to the Baja peninsula. Additionally, the composition and ages of the volcanic rocks on SMI are very similar to the rift-transition volcanic sequence in the SRB (Busby et al., 2020; Sommer et al., 2022), and the gypsum and clastic facies correlate well with similar strata of the Boleo Fm in the northern SRB. These findings support the hypothesis that SMI has been offset from the northern SRB to its current location by up to ~40 km of SE-directed right-lateral slip on undocumented offshore transform faults.