Current Abstract

June 11, 2019 Dinner Meeting Abstract

“Rate and character of Late Quaternary folding in the Santa
Maria Basin, CA and implications for active faulting

Presented by: Ian McGregor, CSU Long Beach


The onshore Santa Maria area in central California is an inverted basin with several kilometers of estimated shortening that has folded, faulted, and uplifted Miocene deep-water rocks [Woodring and Bramlette, 1950;
Namson and Davis, 1990; McCrory,1995]. Abundant outcrop and subsurface data from active oil fields in the area describe the basin stratigraphy and general kinematics through time, but do not estimate the late Quaternary activity of these structures and Quaternary deformation rates, pertinent for seismic risk assessment. A detailed quantitative analysis on the structures within the Santa Maria Basin, in terms of the recent amount and rate of uplift, shortening, and fault slip that has occurred, was conducted using the basal contact of a regional late Pleistocene fluvial deposit, the Orcutt Formation, as a marker of deformation. Results also aim to evaluate conflicting structural models proposed for present uplift and folding [ Seeber and Sorlien, 2000; Lettis et al, 2004].

Structure contour maps of the late Pleistocene horizon show north-east verging asymmetric folding of distinct anticlines arranged in a left-stepping en-echelon pattern within the Casmalia Hills uplifted area. Folding within the Purisima Hills to the south contains a south-west verging asymmetry with decreasing structural relief to the west. Maximum vertical uplift potentially accentuated by forelimb subsidence, occurs along the crest of the folds and ranges from 295 – 316 meters with maximum regional uplift along the Casmalia Hills fold-trend. Line-length balanced horizontal shortening estimates range from 22 – 47 meters for the restored base Orcutt Formation horizon. Fault related fold models constrained from subsurface, outcrop, and structure contour data calculate area-balanced fault displacement that ranges from 684 – 730 meters of reverse displacement for the base-Orcutt horizon.

Numerical ages from optically stimulated luminescence dating of the Orcutt Formation range from 125.7 – 86.1 Ka, coincident with the 5e-a paleo sea-level high stands and regional depositional events [DeVecchio et al, 2012]. The range and distribution of ages suggests expansive relatively synchronous fluvial deposition across the fold trends after a period of erosion and planation. Using the maximum and minimum ages of
the Orcutt Formation to calculate a range of the deformation rates, we estimate uplift rates of 1.9 – 3.67 mm/yr, horizontal shortening rates of 0.18 -0.55 mm/yr, and fault slip rates of 5.4 – 8.5 mm/yr.


Ian McGregor is a Masters Candidate at CSU Long Beach with thesis completion expected for summer 2019. He earned his B.S. in 2016 from San Francisco State University completing a senior research thesis on fault kinematics in the Santa Lucia Range, central California. Professional experience includes geophysical work, interpreting seismic data, and building structural models for basins world-wide. Ian was an intern for the CSULB Office of Research and Sponsored Projects, Geological Society of America graduate grant recipient, and CSULB College of Natural Sciences and Mathematics Richard D. Green Fellowship award recipient in 2018.

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