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Structural Architecture of the Western Transverse Ranges and Potential for Large Earthquakes – Trishear Forward Models

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

Published August 14, 2017, SCEC Contribution #7631, 2017 SCEC Annual Meeting Poster #123

Fold-and-thrust belts 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 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 thrust is interpreted as the current thrust front structure, and spatially stable back thrusts accommodate 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 thrust, and that the thrust front propagated south with time to the Red Mountain fault and eventually to the currently active thrust front, the southward-vergent Pitas Point-Ventura fault. Our interpretation is based on combining various sources of data and previous models suggested by others. We used the observed rate of subsidence in the basin, assuming it represents the regional rate, shortening rates from different studies and from our own estimates, and finally, we compared these to the regional rate of uplift in the hanging wall where folding has ceased in order make an estimate for the dip of the underlying “flat” or decollement. Based on these rates and assumption we estimate that the deep fault dips north at around 20º. This result seems to match previous seismologic observations. To test our interpretations of the evolution and structure of the WTR, we used Trishear forward modeling. We compared our results to the observed geology and the Trishear models are a good first-order match. While our solution is non-unique, it is consistent with all of the currently available data. We believe that this model resolves much of the ongoing debate regarding the dip direction of the primary structure at depth, and modeling of multiple cross-sections argues that all of the observed deformation can be explained by an evolved fold and thrust belt, which includes a regionally extensive decollement underlying the observed thrust fronts. In addition, our model predicts the loading signal to be north of the Santa Ynez Mountain range. This prediction provides an opportunity to test the model trough geodetic observations.

Levy, Y., Rockwell, T. K., Shaw, J. H., Plesch, A., Driscoll, N. W., & Perea, H. (2017, 08). Structural Architecture of the Western Transverse Ranges and Potential for Large Earthquakes – Trishear Forward Models . Poster Presentation at 2017 SCEC Annual Meeting.

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