Structural modeling of the Western Transverse Ranges: An imbricated thrust ramp architecture

Y Levy,J.H Shaw,N.W Driscoll,H Perea,T.K Rockwell,A Plesch

doi:10.1130/l1124.1

Abstract

AbstractActive fold-and-thrust belts can potentially accommodate large-magnitude earthquakes, so understanding the structure in such regions has both societal and scientific importance. Recent studies have provided evidence for large earthquakes in the Western Transverse Ranges of California, USA. However, the diverse set of conflicting structural models for this region highlights the lack of understanding of the subsurface geometry of faults. A more robust structural model is required to assess the seismic hazard of the Western Transverse Ranges. Toward this goal, we developed a forward structural model using Trishear in MOVE® to match the first-order structure of the Western Transverse Ranges, as inferred from surface geology, subsurface well control, and seismic stratigraphy. We incorporated the full range of geologic observations, including vertical motions from uplifted fluvial and marine terraces, as constraints on our kinematic forward modeling. Using fault-related folding methods, we predicted the geometry and sense of slip of the major faults at depth, and we used these structures to model the evolution of the Western Transverse Ranges since the late Pliocene. The model predictions are in good agreement with the observed geology. Our results suggest that the Western Transverse Ranges comprises a southward-verging imbricate thrust system, with the dominant faults dipping as a ramp to the north and steepening as they shoal from ∼16°–30° at depth to ∼45°–60° near the surface. We estimate ∼21 km of total shortening since the Pliocene in the eastern part of the region, and a decrease of total shortening west of Santa Barbara down to 7 km near Point Conception. The potential surface area of the inferred deep thrust ramp is up to 6000 km2, which is of sufficient size to host the large earthquakes inferred from paleoseismic studies in this region.

Highlights

Active fold-and-thrust belts produce destructive earthquakes, such as the M 7.9 Wenchuan earthquake in 2008 and the M 7.3 Gorkha earthquake in 2015 (Hayes et al, 2016)
Considering that our goal was to model the overall rst-order, Pliocene and younger contractional deformation of the Western Transverse Ranges, we suggest that the resulting mismatches from not incorporating these stratigraphic details are minor
Because we have no way to constrain the paleorelief of these units, we cannot quantify this uncertainty, there are no clastic deposits eroded from the Eocene strata in the Miocene units, which argues against subaerial exposure of the Eocene strata to the north. Considering those issues, and accepting the limitations of our inherent assumptions, we suggest that our structural models reproduce the geometry and kinematics of the major thrust sheets, and they are consistent with all large-scale observations along the transects (Figs. 9 and 10)

Summary

INTRODUCTION

Active fold-and-thrust belts produce destructive earthquakes, such as the M 7.9 Wenchuan earthquake in 2008 and the M 7.3 Gorkha earthquake in 2015 (Hayes et al, 2016). Considering the complex structural geology in the upper several kilometers of the Western Transverse Ranges, the observed uplifts were suggested to be the result of multisegment thrust fault ruptures (Hubbard et al, 2014). This explanation has been questioned based on arguments of fault complexity (Sorlien and Nicholson, 2015). Los Angeles cross section, with the intent of developing a crustal-scale model of the entire seismogenic portion of the crust Incorporated into this modeling was information on the local and regional vertical motions, which aided in constraining the fault dip at depth.

F Point f

RESULTS

E O M Shore line

Findings

CONCLUSIONS