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

Abstract

Active 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.

Late Pleistocene rates of rock uplift and faulting at the boundary between the southern Coast Ranges and the western Transverse Ranges in California from reconstruction and luminescence dating of the Orcutt Formation

“…This regional uplift rate is supported by research conducted for several master's theses set in the Santa Maria Basin that used fluvial or marine terraces to determine incision and uplift rates of ~1 mm/yr in the footwall blocks of the reverse faults (Kelty, 2020;Farris, 2017;Tyler, 2013). The cause of this regional uplift rate is beyond the scope of this study, but it is likely related to deeper crustal thickening, possibly along the detachment at 12-15 km depth that has been interpreted by multiple studies (Levy et al, 2019;Huang et al, 1996;Namson and Davis, 1990). Our rock uplift rates along the crests of the Casmalia Hills and Purisima Hills are likely superposed on this regional uplift, such that the rate of rock uplift due to the underlying blind faults modeled in this study are anywhere from near 0 mm/yr at the western nose of the Purisima Hills to a little less than 4 mm/yr in the eastern Purisima Hills.…”

“…Marshall et al (2013) measured an average geodetic shortening rate of 7 mm/yr, while other geodetic studies have reported rates as high as 12 mm/yr (Donnellan et al, 1993;Hagar et al, 1999). Measurements of geologic shortening rates since Miocene time vary from 6.5 to 9.1 mm/yr (Levy et al, 2019) to as high as 25 mm/yr (Yeats, 1983;Huftile and Yeats, 1995). Slip rates on faults in the western Transverse Ranges have mainly been determined in the Ventura area, where upper limits range from 7 to 11 mm/yr along the Ventura, San Cayetano, and Red Mountain faults (Fig.…”

Three‐Dimensional Structure, Ground Rupture Hazards, and Static Stress Models for Complex Nonplanar Thrust Faults in the Ventura Basin, Southern California

“…SPP = Santa Paula peak, SSCF = Southern San Cayetano fault, LMA = Lion Mountain anticline, JCF = Javon Canyon fault, LCF = Lion Canyon fault, WSCF = Western San Cayetano fault, ESCF = Eastern San Cayetano fault, SAF = San Andreas Fault, TC = Timber Canyon, OC = Orcutt Canyon, SPC = Santa Paula Creek. (Hubbard et al, 2014;Hughes et al, 2018;Levy et al, 2019;Namson & Davis, 1988;Nicholson, Sorlien, et al, 2017;Rockwell, 1988;Sorlien et al, 2000;Sorlien & Nicholson, 2015;Rockwell et al, 2016;Yeats et al, 1988).…”

“…While modeled static Coulomb stress change on dip‐slip receiver faults is highly sensitive to changes in fault strike, dip‐slip faults also demonstrate sensitivity to changes in fault dip (Madden et al, 2013; Mildon et al, 2016; Mohammadi et al, 2019). Therefore, the ability to include down‐dip changes in fault dip is important, particularly in fold and thrust belts where faults are often interpreted to have ramp‐flat geometry (e.g., Hubbard et al, 2016; Levy et al, 2019; Shaw & Suppe, 1996; Suppe, 1983) and where ramp‐flat geometry can potentially control coseismic slip patterns (Hubbard et al, 2016). Furthermore, previous work has combined paleoseismic data and subsurface data with static Coulomb stress modeling to test feasible rupture scenarios between imbricate faults in the South Island, New Zealand (Stahl et al, 2016).…”

“…However, this suggestion has not been tested. Furthermore, the potential degree of multifault connectivity partly depends on the subsurface geometry of the Southern San Cayetano fault (SSCF) (Figure 1) and the nature of its potential connection with the neighboring San Cayetano, Pitas Point, and Ventura faults, which remains unresolved (Hubbard et al, 2014; Hughes et al, 2018; Levy et al, 2019).…”