STRUCTURE AND KINEMATIC EVOLUTION OF THE SANTA ROSALÍA BASIN, CENTRAL BAJA CALIFORNIA, MÉXICO – BAJA BASINS 2020 IRES

The Santa Rosalía Basin (SRB) in central Baja California, México formed during the late Miocene transition from subduction to oblique rifting associated with the birth of the Gulf of California. The structural architecture of the SRB and the timing and style of faulting are not well understood, yet they are critical for understanding the structural evolution of the Gulf of California and for evaluating potential links between faulting and extensive stratiform Cu-Co-Zn mineralization in the Boleo Fm. This study explores the spatial and temporal patterns of faulting in the SRB through a detailed fault kinematic analysis. We collected structural data from 151 minor faults including fault orientation and, where possible, slip vector orientation (n=52), sense of slip, displacement, and timing of slip. Quantitative fault kinematic analyses were conducted to evaluate the homogeneity of strain and to determine the orientations of the principal strain axes responsible for faulting. Minor faults strike predominantly NNW-SSE, have an average dip of ~67°, and show normal or oblique dextral-normal slip, similar to mapped structures. Kinematic analyses indicate a spatial variation in extension direction from NE- or ENE-directed extension farther inland in the SW to more E- or ESE-directed extension near the coast in the NE. Similar kinematics determined from faults cutting the Comondú, Boleo, and Tirabuzón Fms suggest the orientations of the principal strain axes remained relatively constant from the late Miocene to Pliocene. Local evidence of growth strata is unrelated to recent mining activity and indicates active faulting throughout deposition of the Boleo Fm (ca. 7–6 Ma; Holt et al., 2000), supporting a causal link with mineralization. Structural data from the Comondú and Boleo Fms reveal similar dips of ~5–45° in both units, implying little or no tilting prior to Boleo deposition. Our results suggest that most faulting occurred since ~7–6 Ma and was associated with active oblique rifting in the adjacent Guaymas pull-apart basin. We interpret the change to ESE-directed extension along the modern coast to reflect the partitioning of strain into (1) a strike-slip component on faults close to the dextral, SE-striking Carmen transform fault, and (2) NE-directed extension farther inland along the southwest margin of the SRB.