SCEC Award Number 13105 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Precise Fault Slip Rates on the Oak Ridge fault: New age constraints on the Saugus Formation using 36C/10Be isochron burial dating
Investigator(s)
Name Organization
Duane DeVecchio University of California, Santa Barbara Dylan Rood University of California, Santa Barbara
Other Participants
SCEC Priorities 1, 4 SCEC Groups Geology, WGCEP, SDOT
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
 With our 2013 SCEC funding, we successfully achieved our primary goal, which was to produce the first radiometric ages for the Saugus Formation using cosmogenic isochron burial dating. The Saugus Formation is an important Quaternary strain marker in southern California because it is variably deformed across numerous major structures in the Western Transverse Ranges. Our specific focus is the rate of deformation on the Oak Ridge fault where current estimates of the slip rate are extremely high and uncertain, ranging between 5.9 mm/yr and 12.5 mm/yr. Rates are based on the inferred age of the Saugus Formation, with the 2-fold range in the rate reflecting the uncertainty in the upper age of the Saugus. We produced two new ages for the lower and upper Saugus Formation. The lower Saugus Formation near Moorpark yields a high-precision 26Al-10Be isochron burial age of 1.01 +0.16/-0.14 Ma (1-sigma, N=4). Samples from the upper Saugus give an isochron burial age of 0.35 +0.71/-0.53 Ma (1-sigma, N=4). Geologic mapping and structural data were used with subsurface data to construct a cross-section across the Oak Ridge fault and hangingwall anticline, and quantify the magnitude of deformation. Preliminary estimates from this section suggest ~1550 m of south-side-up displacement of the top of the Saugus Formation across the Oak Ridge fault. This is ~70% of that estimated by previous researchers and when combined with our preliminary upper age limit for the Saugus formation in this region, a fault-slip rate of ~4.4 m/ka is suggested.
Intellectual Merit This project directly addresses the following SCEC4 Fundamental Problems of Earthquake Physics: Problem 1 by gathering well-constrained slip-rates on a major structure within the southern California fault system, and Problem 4a by contributing to the understanding of the Ventura Special Fault Study Area. Our work will also contribute to the Earthquake Geology Disciplinary Activity by 1) assisting in the quantification of along-strike variations in strain localization, specifically as it applies to multi-fault ruptures on major reverse faults in the WTR, including the ORF (i.e. proposed SCEC 2014 work on additional sites where the Saugus is deformed could help resolve spatial patterns on neighboring faults and near fault intersections) and 2) utilizing the Geochronology Infrastructure. It also directly contributes to the following Interdisciplinary Focus Areas: A. Unified Structural Representation (USR) and C. Stress and Deformation Over Time (SDOT) by providing information on geologic fault slip rates and subsurface fault geometry that can be used to evaluate 1) the Community Fault Model (CFM) and 2) deformation and earthquake cycle models, respectively. It has importance to the USGS EHP programs (i.e. National Seismic Hazard Mapping Program) and Special Projects and Initiatives A. Working Group on California Earthquake Probabilities (WGCEP) (specifically, UCERF3).
Broader Impacts This work supported two junior SCEC researchers and early-career PIs, DeVecchio and Rood. It also provided two undergraduate students a unique opportunity to actively participate in earthquake geology field work, and learn the theory, laboratory procedures, and application of cosmogenic isotopes in the Earth sciences.
Exemplary Figure Figure S1 in Supplemental Materials. Cosmogenic 26Al-10Be isochron results for the lower Saugus Formation on the hangingwall of the Oak Ridge fault showing sample data (grey 2-sigma error ellipses), best fit regression (dotted line), and 2-sigma uncertainty on regression (thin solid lines) with summary statistics (boxes). (Courtesy of D. Rood & D. DeVecchio)