CHARACTERIZATION OF LATE PLEISTOCENE DEFORMATION AT THE NORTHERN BOUNDARY OF THE TRANSVERSE RANGES IN CALIFORNIA FROM RECONSTRUCTION, KINEMATIC MODELING, AND LUMINESCENCE DATING OF THE ORCUTT FORMATION

The western Transverse Ranges (WTR) in southern California are undergoing active contraction and numerous studies have documented rapid shortening and uplift rates within the compressional mountain belt. However, at the boundary between the WTR and slower deforming southern Coast Ranges to the north, deformation rates and the kinematic history of faults and folds that deform Miocene to late Quaternary sedimentary deposits have not been constrained. To address this, we used a regional fluvial deposit, the Orcutt Formation, as a stratigraphic marker to constrain the timing, amount, and style of late Quaternary deformation in the Santa Maria Basin along the northwestern boundary of the WTR.

Post-infrared-infrared stimulated luminescence (pIR-IRSL) dating of the Orcutt Formation indicate that it was deposited between of 119 ± 8 and 85 ± 6 ka, coincident with the Marine Isotope stage 5 paleo sea level high stand. During this time, sediments accumulated on a low-relief fluvial plain that graded to a wave-cut platform at the coast. Basal surface reconstructions of the Orcutt Formation and two-dimensional structural models constrained by surficial and subsurface data indicate that the Orcutt Formation is deformed by a combination of fault propagation and fault-bend folding across west northwest-trending structures interpreted as deep, dipping detachment faults. Kinematic forward modeling results in a dip-slip rate of 5.6 –6.7 mm/yr for the buried faults beneath the fold trends. These relatively high rates of fault slip and uplift are consistent with resolved geodetic rates across faults in the Santa Maria Basin and similar to the high deformation rates documented from the Ventura Basin in the WTR. Punctuated deformation is indicated by the recent topographic growth and deformation signal within the Orcutt Formation that followed a period of tectonic quiescence, which allowed the erosion and planation of topography prior to its deposition. Angular unconformities in older stratigraphic horizons also display higher and variable degrees of deformation through time. Preliminary structural models incorporating early-to mid-Pleistocene surfaces suggest incremental fold growth during this interval.