The onshore Santa Maria Basin in central California is an inverted basin with several kilometers of estimated shortening that has folded, faulted, and uplifted Miocene through Quaternary rocks. Abundant outcrops and subsurface data from oil fields in the area have been used by previous studies to determine deformation styles and amounts recorded in Miocene through Pleistocene rocks. The late Quaternary activity of major structures and Quaternary deformation rates, however, has not been systematically evaluated. This information is critical for earthquake risk assessment and an understanding of the topographic evolution of the area. In this study, we present the first measurements of Quaternary deformation in the form of uplift and shortening rates across active folds, and slip rates along the deeper faults that are controlling fold growth.

The Orcutt Formation is a late Quaternary fluvial deposit that formed during regional planation by a broad fluvial system flowing across growing folds. This unit provides a young datum for analyzing recent crustal deformation and fault activity. We did a geometric analysis of recent structural development within the basin using the basal contact of the Orcutt Formation as a marker. Luminescence dating of the Orcutt Formation provided numerical ages to calculate uplift amounts and rates, horizontal shortening amounts and rates, and fault displacement and slip rate. Our results tie structural data from the older geology to the Quaternary deformation patterns and help to evaluate conflicting structural models proposed for present uplift and folding that can address fault connectivity, seismogenic area of active faults, basin inversion and history of fluid flow.

Structure contour maps of the late Pleistocene, base-Orcutt horizon show north-east verging asymmetric folding of distinct northwest-trending anticlines arranged in a left-stepping en-echelon pattern within the Casmalia hills uplifted area. Folding within the western Purisima hills to the south contains a south-west verging asymmetry along a west-northwest fold-trend with decreasing structural relief to the west. Maximum vertical displacement of the base-Orcutt horizon, potentially accentuated by forelimb subsidence, occurs along the crest of the folds and ranges from 295 – 316 meters with the greatest regional uplift occurring along the crest of the Casmalia hills fold-trend. Line-length balanced horizontal shortening estimates range from 22 – 47 meters for the restored base Orcutt Formation horizon. Fault displacement needed to match these horizontal and vertical displacements, computed from area-balanced forward models, range from 728 – 730 meters of reverse displacement on the Casmalia and Solomon structures (south dipping Casmalia-Orcutt detachment fault) for the base-Orcutt horizon.

Numerical ages from luminescence dating of the Orcutt Formation range from 112.9 – 81.4 ka, coincident with the 5e-a paleo sea-level high stands and regional depositional events. The range and spatial distribution of ages indicates regional fluvial deposition across the area after a period of erosion and planation, and prior to late-Quaternary folding. Using the maximum and minimum ages of the Orcutt Formation to calculate the range of deformation 0.20-0.85 mm/yr rates, we estimate uplift rates of 2.1 – 4.0 mm/yr, horizontal shortening rates of 0.20-0.85 mm/yr, and fault slip rates of 6.5 – 9.0 mm/yr. Structural analysis indicates an apparent periodicity of contractional events into the Quaternary since the onset of late Miocene-Pliocene convergence.