Structural Architecture of the Western Transverse Ranges and Potential for Large Earthquakes - New Results of Trishear Forward Models

Yuval Levy, Thomas K. Rockwell, John H. Shaw, Andreas Plesch, Neal W. Driscoll, & Hector Perea

Published August 14, 2018, SCEC Contribution #8497, 2018 SCEC Annual Meeting Poster #252

Fold-and-thrust belts usually evolve over time, can produce large-scale faults and potentially accommodate large magnitude earthquakes. The thrust fronts of these structures typically form large fold structures in their hanging walls, and they tend to propagate forward (in sequence) over time to form new thrust fronts. In the Santa Barbara and Ventura region of the Western Transverse Ranges (WTR) of southern California, the Pitas Point-Ventura fault is interpreted as the current frontal thrust structure of this system, with spatially stable back thrusts accommodating deformation in the hanging wall block of the thrust sheet (More Ranch fault, Rincon Creek fault, other faults). We interpret the nearly continuous, overturned Tertiary stratigraphy of the Santa Ynez Mountains as a large anticlinorium that formed as the first thrust front over the (mostly) blind San Cayetano fault, and that the thrust front propagated south with time to the Red Mountain fault and eventually to the currently active, southward-verging Pitas Point-Ventura fault. Offshore Summerland, the Red Mountain fault is currently also active and may have been the dominant structure in the Holocene after sea level rise; the Red Mountain fault does not extend west of Santa Barbara. Our interpretation of the deep structure of the WTR is based on a combination of various data sources and previously published models. To test our interpretations of the evolution and structure of the WTR, we used Trishear forward modeling. The Trishear models are a good first-order match to published geologic maps and well data. While our solution is non-unique, it is consistent with all of the currently available observations. For this poster, we present new cross-sections westward to Point Conception and show that the amount of slip required to reproduce these structures is lower than the amount of slip required across the major structures to the east in Ventura Basin. This result is expected due to the location of the different faults relatively to the big bend of the San Andreas fault and is consistent with the geodetic signal. Further, modeling of multiple cross-sections argues that all of the observed deformation can be explained by an evolving fold and thrust belt, which includes a regionally extensive decollement underlying the observed thrusts and folds. In addition, our fault model supports the contention that the WTR are capable of generating large magnitude earthquakes, based on scaling relations.

Levy, Y., Rockwell, T. K., Shaw, J. H., Plesch, A., Driscoll, N. W., & Perea, H. (2018, 08). Structural Architecture of the Western Transverse Ranges and Potential for Large Earthquakes - New Results of Trishear Forward Models . Poster Presentation at 2018 SCEC Annual Meeting.

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Earthquake Geology