Structural Architecture of the Western Transverse Ranges and Potential for Large Earthquakes – Initial Results of 3D Trishear Forward Modeling

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

The Western Transverse Ranges (WTR) is an active fold and thrust belt. Fold-and-thrust belts usually evolve over time, can involve 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. The evidence for large thrust events in the area of Ventura and the existence of competing structural models led us to test new models that incorporate the full range of geological observations, and apply different methods than ones previously used in that region. We propose an imbricated Thrust-Ramp-Thrust architecture, which has propagated southward since the inception of shortening in the Pliocene. Based on the stratigraphic and topographic relief, the ages of structures, geometry of the folds and faults, and observed type of deformation (uplift and folding), we interpreted 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 and South Sulfur Mountain faults and eventually to the currently active, southward-verging Pitas Point-Ventura fault. We completed 7 cross-sections based on published geologic maps and subsurface data from the upper several kms and forwarded-modeled these in 2D with Trishear. The regional cross-sections have been linked in 3D by interpolation in MOVE and we now test our proposed model by using 3D Trishear forward modeling of the interpolated fault surfaces. The underlying inferred master fault has a surface area of about 6000 km², consistent with recently published observations of large coastal uplift that suggest earthquake magnitudes in the high M7s. While these are initial results, our intention is to compare in 3D the observed first order structures to the deformation produced in the model and then tune the model against geodetic data.

Presenting Author: Yuval Levy

Authors