QUATERNARY GEOLOGIC MAPPING, GEOCHRONOLOGY, AND STRUCTURAL MODELING OF QUATERNARY ACTIVE FAULTS IN THE SANTA MARIA BASIN, SAN LUIS OBISPO AND SANTA BARBARA COUNTIES CALIFORNIA

The western Transverse Ranges (WTR) are an active fold-and-thrust belt in central California with high rates of north-south shortening (7 to 12 mm/yr). In onshore the Santa Maria Basin, at the western end of the fold-and-thrust belt, several large, seismogenic blind faults are present that deform Quaternary sediments and are inferred to produce >M7.0 events. Despite the potential for large-magnitude events, there is large uncertainty regarding the structural character, slip rate, and interconnectivity of these faults. To refine fault source parameterization for seismic hazard assessments, we evaluated several of the major faults by combining new and previously published stratigraphic, structural, and geochronologic data and then used these data to construct fault models to calculate deformation rates and assess the degree of fault connectivity. Our compilation of basin -wide Quaternary terrace mapping and geochronology are now publicly available.

Our new data include deformation measurements of coastal terraces that cross two major faults, the Lions Head and the Honda fault and the age of these terraces based on seven (7) infrared-stimulated luminescence (IRSL) ages. These data show that the lowest emergent marine terrace along the western Santa Maria Basin developed at approximately 45 ka. The lowest emergent terrace is not deformed across the Honda Fault, indicating that this fault is not active. This terrace is faulted and folded across the Lions Head fault, producing 16 meters of vertical displacement for which we calculated a deformation rate of 0.36 mm/yr. We note, however, that kinematic data collected in the field from surficial fault strands suggest that the Lions Head fault also has a significant component of lateral slip, which we have not yet been able to quantify.

We also used subsurface well data, previously published geologic maps and Quaternary terrace mapping and chronology to construct three area-balanced cross-sections across the eastern Santa Maria Basin. We developed iterations of fault-related fold models to quantify shortening across regional structures in the eastern basin and calculate ranges of fault displacements for a given modeled structure. Our new models provide a more detailed depiction of the fault geometries at depth and indicate a dip-slip rate of 0.3 to 2.2 mm/yr on the regional, blind Los Alamos-Baseline fault over the past 150 kyr.