SCEC Award Number 16049 View PDF
Proposal Category Individual Proposal (Special Fault Study Area)
Proposal Title Crustal Architecture of the Western Transverse Ranges, Southern California
Name Organization
Thomas Rockwell San Diego State University
Other Participants Yuval Levy
SCEC Priorities 1a, 4a, 4b SCEC Groups Geodesy, Geology, USR
Report Due Date 03/15/2017 Date Report Submitted 03/14/2017
Project Abstract
The deep subsurface structure of the Western Transverse Ranges (WTR) of southern California is still under ongoing debate. Recent work found evidence for large earthquakes. This evidence combined with the community’s agreement that existing models can be improved, motivated us to study further the structure in the region. Our goal is to construct a balanced retro-deformable 3D model, which will be in agreement with the full range of geologic, geodetic, and seismicity data. For achieving this goal we constructed several preliminary cross-sections by compiling data and reviewed existing models. We then expanded these sections based on regional geologic mapping. These cross sections will be used with additional surface data to interpolate the 3D model. Our current interpretation is that the WTR, during the period of shortening, developed thrust fronts which propagated southward in time, similar to other fold-and-thrust belts worldwide. We interpret the nearly continuous overturned Tertiary stratigraphy of the Santa Ynez Mountains as a large anticlinorium that formed as an early 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. We further suggest that the steep dip angle and continued activity of the Red Mountain fault, as observed near the surface, is a result of northward rotation of the fault in the backlimb of the Ventura Avenue anticline, which has caused it to flexurally slip in the near surface.

Intellectual Merit The potential for great earthquakes in the Western Transverse Ranges is a critical question to resolve, as such an earthquake would likely produce broad regional damage, including in the Los Angeles Basin. The results of this work will be incorporated into the SCEC CFM, to be used to model the observed deformation field in the Western Transverse Ranges. An additional contribution will be furthering the understanding of the geologic evolution of the Western Transverse Ranges, as well as fold and thrust belts worldwide.
Broader Impacts This project supports one Ph.D. student (Yuval Levy). This project has enhanced the partnership between Shaw's group at Harvard and Rockwell's group at SDSU. Benefits to society include an awareness of the potential for great earthquakes along the coastal cities of the Transverse Ranges, including Santa Barbara, Goleta, Ventura and Oxnard.
Exemplary Figure The exemplary figure is provided as such in the PDF of the report. Caption: Map of the western Transverse Ranges showing the Santa Ynez Mountains anticlinorium, a continuous upright to overturned fold that extends from Point Conception to Fillmore. The continuity of the fault requires a continuous fault at depth, which is blid except where the San Cayetano is emergent in the east. The lower panel is a preliminary crustal-scale cross-section (location in the top panel). Compiled in MOVE (of Midland Valley) using surface geology (1986 Dibblee maps), offshore seismic data (USGS seismic survey repository), and well data (from Jackson and Yeats, 1982). Note the very large overturned anticline that comprises the Santa Ynez Mountains. This continuous fold strongly demonstrates that the deep thrust ramp is a continuous structural feature, and that it is located beneath (now north of) the Santa Ynez Mountains. This provides a framework within which to construct a series of balanced, kinematically consistent cross-sections from which the 3-dimensional character of the North Channel – Pitas Point – Ventura – San Cayetano thrust can be characterized and implemented in the SCEC CFM.