Hematite accommodated shallow, transient Pleistocene slow slip in the exhumed southern San Andreas fault system, California, USA

Abstract

Slow slip is part of the earthquake cycle, but the processes controlling this phenomenon in space and time are poorly constrained. Hematite, common in continental fault zones, exhibits unique textures and (U-Th)/He thermochronometry data patterns reflecting different slip rates. We investigated networks of small hematite-coated slip surfaces in basement fault damage of exhumed strike-slip faults that connect to the southern San Andreas fault in a flower structure in the Mecca Hills, California, USA. Scanning electron microscopy shows these millimeter-thick surfaces exhibit basal hematite injection veins and layered veinlets comprising nanoscale, high-aspect-ratio hematite plates akin to phyllosilicates. Combined microstructural and hematite (U-Th)/He data (n = 64 new, 24 published individual analyses) record hematite mineralization events ca. 0.8 Ma to 0.4 Ma at <1.5 km depth. We suggest these hematite faults formed via fluid overpressure, and then hematite localized repeated subseismic slip, creating zones of shallow off-fault damage as far as 4 km orthogonal to the trace of the southern San Andreas fault. Distributed hematite slip surfaces develop by, and then accommodate, transient slow slip, potentially dampening or distributing earthquake energy in shallow continental faults.

Hematite Frictional Behavior and He Loss From Comminution During Deformation Experiments at Slow Slip Rates

“…In contrast, nm‐scale hematite platelets that deform by sliding along grain boundaries would not be expected to lose He during deformation unless deformation occurred at seismic slip rates. Thus, our experiments support prior inferences that (U‐Th)/He dates obtained from small (cm‐scale) hematite‐coated faults that formed and deformed at transient subseismic slip rates solely reflect the timing of initial mineralization (DiMonte et al., 2022).…”

“…In contrast to the microstructural evolution of specularite, hematite observed in some shallow fault zones exhibits an anisotropic texture and morphology of high‐aspect ratio platelets with an average plate thickness of ∼30 nm and plate length ranging from ∼300 nm to 1 μm (Calzolari et al., 2018; DiMonte et al., 2022; McDermott et al., 2021; Moser et al., 2017). Microstructural analysis of natural samples shows that this foliated, nm‐scale hematite does not appear to experience comminution during post‐mineralization slip (DiMonte et al., 2022; McDermott et al., 2021). These faults exhibit a scaly fabric and/or S‐C‐like structures, and deformation is accommodated by sliding between grain boundaries without comminution at subseismic rates (DiMonte et al., 2022).…”

“…Microstructural analysis of natural samples shows that this foliated, nm‐scale hematite does not appear to experience comminution during post‐mineralization slip (DiMonte et al., 2022; McDermott et al., 2021). These faults exhibit a scaly fabric and/or S‐C‐like structures, and deformation is accommodated by sliding between grain boundaries without comminution at subseismic rates (DiMonte et al., 2022). The difference in resulting deformation textures between those generated from μm‐scale versus nm‐scale hematite implies that initial hematite grain size and textures may influence subsequent deformation processes and, potentially, frictional behavior.…”

“…Coseismic temperature rise induces He loss in hematite via recrystallization or thermally activated volume diffusion, resulting in hematite He dates that postdate hematite formation (Ault et al., 2015; McDermott et al., 2017). Other hematite fault surfaces, composed of nanometer‐scale, foliated hematite platelets, develop and deform at subseismic slip rates, and most hematite He dates from such fault zones are interpreted to record the timing of the initial hematite formation (DiMonte et al., 2022; McDermott et al., 2021; Moser et al., 2017). Documenting the frictional behavior of hematite and textures that form over a range of slip rates and displacements in the lab is critical for refining these interpretations of slip rate and behavior, as well as (U‐Th)/He dates in nature.…”

Evaluating the Compatibility of Hematite (U‐Th)/He Data and Hematite‐Carried Secondary Magnetizations: An Example From the Colorado Front Range

“…Under certain conditions, hematite can retain most or all of its radiogenic He over geologic timescales. Hematite He thermochronometry has been used to document continental weathering reactions (Monteiro et al., 2022), fault slip (Ault et al., 2015; DiMonte et al., 2022; McDermott et al., 2017; Moser et al., 2017), and hydrothermal mineralization (Jensen et al., 2018; Wu et al., 2018).…”