Late Pleistocene rates of folding and faulting in the western Transverse Ranges, California, from reconstruction and luminescence dating of the late Pleistocene Orcutt formation

Ian McGregor, & Nate W. Onderdonk

Published August 14, 2019, SCEC Contribution #9571, 2019 SCEC Annual Meeting Poster #099

The onshore Santa Maria Basin in central California is an inverted basin with up to 9 kilometers of estimated shortening that has folded, faulted, and uplifted Miocene through Quaternary rocks. Previous studies have used subsurface data from oil fields to determine deformation styles and amounts recorded in Miocene through Pliocene rocks. 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 performed 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.

Three-dimensional point data derived from geologic maps georeferenced to digital elevation models, oil and gas wells, and ground water wells were used to contour structural surfaces and as subsurface geometric constraints on stratigraphic horizons for forward modeling. Structure contour maps of the late Pleistocene, base-Orcutt horizon and subsurface data show asymmetric folding of northwest-trending anticlines that are a combination of fault-propagation and fault-bend-folding controlled by deep thrust faults.

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 deformation rates, we estimate localized uplift rates of 2.1 – 4.0 mm/yr, regional horizontal shortening rates of 0.20-0.85 mm/yr, and fault slip rates of 6.5 – 9.0 mm/yr along the reverse faults that underlie the southwestern Santa Maria Basin.

Key Words
Quaternary deformation rates, tectonic geomorphology, structural geology, western transverse ranges

Citation
McGregor, I., & Onderdonk, N. W. (2019, 08). Late Pleistocene rates of folding and faulting in the western Transverse Ranges, California, from reconstruction and luminescence dating of the late Pleistocene Orcutt formation . Poster Presentation at 2019 SCEC Annual Meeting.

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