AGES AND DEFORMATION OF MARINE TERRACES BETWEEN POINT CONCEPTION AND GAVIOTA, WESTERN TRANSVERSE RANGES, CALIFORNIA

Quaternary chronology and rock uplift recorded by marine terraces, Gaviota coast, Santa Barbara County, California, USA

Geological Society of America Bulletin ◽

10.1130/b35609.1 ◽

2021 ◽

Author(s):

Daniel L. Morel ◽

Kristin D. Morell ◽

Edward A. Keller ◽

Tammy M. Rittenour

Keyword(s):

Southern California ◽

Seismic Hazard Assessment ◽

Fault System ◽

The South ◽

Tectonic Model ◽

Transverse Ranges ◽

Marine Terraces ◽

Uplift Rates ◽

Regional Tectonic ◽

Source Models

The Transverse Ranges of southern California are a region of active transpression on the western margin of North America that hosts some of the world’s highest uplift rates at the Ventura anticline. Yet, the manner in which rock uplift rates change along strike from Ventura to the westernmost Transverse Ranges and the structures that may be responsible for this uplift remain unclear. Here, we quantified rock uplift rates within the westernmost 60 km of the Transverse Ranges by obtaining new age constraints from raised beach and shoreface deposits from marine terraces along the Gaviota coast. Twelve radiocarbon (seven sites) and eight luminescence (six sites) ages, ranging from ca. 50 to 40 k.y. B.P. and ca. 56 to 43 ka, respectively, consistently suggest that the first emergent terrace dates to marine isotope stage (MIS) 3, rather than MIS 5a as previously reported for the western Gaviota coast. These younger ages yield rock uplift rates between 0.8 ± 0.3 and 1.8 ± 0.4 m/k.y., i.e., over five times higher than previous estimates for this region. The spatial distribution of rock uplift rates and the abrupt along-strike changes in marine terrace elevations favor a regional tectonic model with a step-wise change in rock uplift across the south branch of the Santa Ynez fault. The south branch of the Santa Ynez fault appears to separate two regional tectonic blocks, characterized by rock uplift rates of ∼1.3−1.6 m/k.y. to the east and slightly lower rates to the west (∼0.8−1.4 m/k.y.). Our observations suggest that coastal rock uplift is primarily accommodated by deeply rooted far-field structures such as the offshore Pitas Point−North Channel fault system and the Santa Ynez fault, and that smaller through-going structures impart second-order controls and locally accommodate short-wavelength (<10-km-long strike length) deformation. These results imply that although the rates of rock uplift decline westward along strike, the westernmost portion of the western Transverse Ranges nonetheless accommodates relatively high (>1 m/k.y.) rock uplift rates at a significant distance (>50 km) from the rapidly uplifting (6−7 m/k.y.) Ventura anticline, and >100 km from the prominent restraining bend (“Big Bend”) in the San Andreas fault. The new constraints on the geometry of Quaternary-active structures and regional rates of fault-related deformation have implications for regional earthquake source models and seismic hazard assessment in the highly populated southern California coast region.

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

Lithosphere ◽

10.1130/l1124.1 ◽

2019 ◽

Vol 11 (6) ◽

pp. 868-883 ◽

Cited By ~ 2

Author(s):

Y. Levy ◽

T.K. Rockwell ◽

J.H. Shaw ◽

A. Plesch ◽

N.W. Driscoll ◽

...

Keyword(s):

Structural Model ◽

Full Range ◽

Order Structure ◽

The North ◽

Transverse Ranges ◽

Marine Terraces ◽

Thrust Belts ◽

Subsurface Geometry ◽

Surface Geology ◽

Large Earthquakes

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.

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