Geomorphic evidence for the geometry and slip rate of a young, low-angle thrust fault: Implications for hazard assessment and fault interaction in complex tectonic environments

Abstract

We present surface evidence and displacement rates for a young, active, low-angle (~20°) reverse thrust fault in close proximity to major population centers in southern California (U.S.A.), the Southern San Cayetano fault (SSCF). Active faulting along the northern flank of the Santa Clara River Valley displaces young landforms, such as late Quaternary river terraces and alluvial fans. Geomorphic strain markers are examined using field mapping, high-resolution lidar topographic data, 10Be surface exposure dating, and subsurface well data to provide evidence for a young, active SSCF along the northern flank of the Santa Clara River Valley. Displacement rates for the SSCF are calculated over 1,000-10,000 year timescales with maximum slip rates for the central SSCF of 1.9 +1.0/-0.5 mm/yr between ~19-7 ka and minimum slip rates of 1.3 +0.5/-0.3 mm/yr since ~7 ka. Uplift rates for the central SSCF have not varied significantly over the last ~58 ka, with a maximum value of 1.7 +0.9/-0.6 mm/yr for the interval ~58-19 ka, and a minimum value 1.2 +/-0.3 mm/yr since ~7 ka. The SSCF is interpreted as a young, active structure with onset of activity at some point after ~58 ka. The geometry for the SSCF presented here, with a ~20° north-dip in the subsurface, is the first interpretation of the SSCF based on geological field data. Our new interpretation is significantly different from the previously proposed model-derived geometry, which dips more steeply at 45-60° and intersects the surface in the middle of the Santa Clara River Valley. We suggest that the SSCF may rupture in tandem with the main San Cayetano fault. Additionally, the SSCF could potentially act as a rupture pathway between the Ventura and San Cayetano faults in large-magnitude, multifault earthquakes in southern California. However, given structural complexities, including significant changes in dip and varying Holocene displacement rates along strike, further work is required to examine the possible mechanism, likelihood, and frequency of potential through-going ruptures between the Ventura and San Cayetano faults. Confirmation of the SSCF in a previously well-studied area, such as the southern California, Highlights • Young faults often undetected but potentially key for seismic hazard assessments. • First geomorphic evidence for the Southern San Cayetano fault (SSCF). • 10 Be dating on offset terraces records Holocene slip rate of 1. 3 +0.5 /-0.3 mm yr-1. • SSCF has major implications for seismic hazard in southern California.

Quantifying Wind Erosion During the Late Quaternary in the Qaidam Basin, Central Asia

“…The exposure age and associated uncertainty were calculated by simulating the depth profile of measured 10 Be concentrations (Hidy et al, 2010). The associated key parameters were determined from our observations and published results (Balco et al, 2008;Chmeleff et al, 2010;Hidy et al, 2010;Hughes et al, 2018;Lal, 1991;Lifton et al, 2015;Stone, 2000) and included in Table S1 in the supporting information. We used a Monte Carlo method to simulate the secular variation in wind erosion rates within one glacial-interglacial cycle based on available erosion-time constraints.…”

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

“…Hubbard et al's (2014) flat was interpreted to connect a high-angle deep ramp of the San Cayetano fault to the Pitas Point/ Ventura fault system via the "southern San Cayetano fault." Recent work has established that the southern branch of the San Cayetano exists, and it is the likely surface linkage between the San Cayetano and Ventura faults (Hughes et al, 2018). The geometry presented by Hubbard et al (2014), which includes an ~30°N-dipping thrust ramp that extends downward from the flat, was favored over the high-angle reverse faults model by mechanical modeling (Marshall et al, 2017).…”

“…It might be expected that the marginal cross section would exhibit a different geometry close to the lateral boundaries of the system. That said, a young fold emerges out of the late Quaternary alluvium west of Fillmore and south of the San Cayetano fault (Rockwell, 1988;Hughes et al, 2018) that was too small to model at the scale of the model, but it does represent the very recent southward propagation of the San Cayetano fault, as demonstrated by Hughes et al (2018).…”

Inferring the timing of abandonment of aggraded alluvial surfaces dated with cosmogenic nuclides

“…We explore this idea by comparing the surface age data with two palaeoclimate proxy records ( Fig. 8d): the GRIP ice core δ 18 O record from Greenland (Johnsen et al, 1997) and the LR04 global benthic δ 18 O stack (Lisiecki and Raymo, 2005). These records primarily reflect the growth and decay of continental ice sheets, which are generalized into marine isotope stages (MIS).…”

“…For illustrative purposes, probability distributions of τ are shown if uncertainty in a min is not incorporated (red dashed). (d) Selected palaeoclimate proxies: the GRIP ice core δ 18 O record from Greenland (blue; Johnsen et al, 1997) and the LR04 global benthic δ 18 O stack (black; Lisiecki and Raymo, 2005). Marine isotope stages (MIS) are indicated with boxes.…”