How Fault Rocks Form and Evolve in the Shallow San Andreas Fault

Williams, Randolph T.; Rowe, C. D.; Okamoto, Kristina; Savage, H. M.; Eves, Erin

doi:10.1029/2021gc010092

Cited by 10 publications

(15 citation statements)

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“…We infer that both seismic and aseismic mechanisms are integral in shallow fault processes and must therefore be accounted for in fault and earthquake models. This is supported by analyses of fault-related rocks that formed at 2-3 km depth in the San Andreas Fault (Studnicky et al, 2021), the San Gabriel Fault (Anderson et al, 1983), and other faults that recorded similar results (Gratier et al, 2003;Bradbury et al, 2015;Marone and Saffer, 2007;Boulton et al, 2017;Holdsworth et al, 2011;Jeffries et al, 2006;Mitchell and Faulkner, 2009;Faulkner et al, 2003;Williams et al, 2021).…”

Section: Discussionmentioning

confidence: 84%

Shallow Composition and Structure of the Upper Part of the Exhumed San Gabriel Fault, California: Implications for Fault Processes and Seisimic Properties

Crouch¹,

Evans²,

JANECKE³

2023

Preprint

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Analyses of the composition, properties, and structure of the upper parts of fault zones constrain the mechanical and seismological behavior of faults. We examine macro- to microstructures of fault-related rocks across the San Gabriel Fault zone, California, which was active 5-12 mya and exhibits evidence of Quaternary slip. The steeply plunging ALT-B2 geotechnical borehole encountered an upper ~60 m thick damage zone and a 20-m thick lower damage zone, and two principal slip zones composed of cohesive cataclasite, ultracataclasite, and intact clay gouge within them. The upper ~6.5 m thick principal slip zone separates Mendenhall Gneiss and Josephine Granodiorite, and a lower 11-m thick principal slip is enclosed within the Josephine Granodiorite. Microstructures recorded in core and field samples document overprinted brittle fractures, cohesive cataclasites, veins, sheared clay-rich rocks, and folded foliated horizons in the damage zones. Carbonate veins are common in the lower fault zone, and alteration and mineralization assemblages consist of clays, epidote, calcite, zeolites, and chloritic minerals. Highly altered, mineralized, and deformed rocks in the principal slip zones have very low rock mass rating values. This indicates that portions of the fault zones have very low Young’s moduli and Vs and Vp values as low as 500 and 1000m/sec, respectively, well below the protolith values recorded in a nearby borehole. These data show syntectonic grain-scale alteration, mineralization, and deformation result in low seismic velocities and weak fault-related rocks in the shallow crust, and reduced moduli may impact the distribution of seismic energy and slip to the surface.

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Section: Discussionmentioning

confidence: 84%

Shallow Composition and Structure of the Upper Part of the Exhumed San Gabriel Fault, California: Implications for Fault Processes and Seisimic Properties

Crouch¹,

Evans²,

JANECKE³

2023

Preprint

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“…In the Mojave region, the active San Andreas Fault [Figure 1] was preceded by slip on the Punchbowl and San Gabriel Faults (SGF) (Powell, 1993;Powell and Weldon, 1992;Nourse, 2002). The Elizabeth Lake study site on the San Andreas Fault (LADWP, 2019;Williams et al, 2021;Studnicky et al, submitted) consists of seven northeast-plunging boreholes drilled to depths of 144 m across the San Andreas Fault [Figure 1; Figure 2a]. These holes were drilled by the Los Angeles Department of Water and Power [LADWP, 2019] where the Los Angeles Aqueduct crosses the fault (Mulholland, 1918;Sutherland et al, 2013).…”

Section: Geologic Settingmentioning

confidence: 99%

Fluid-rock interactions, hydrothermal processes, and accommodation of slip in shallow parts of the San Andreas and San Gabriel Faults, southern California

Evans¹,

Crouch²,

Studnicky³

et al. 2023

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We examine fault-related rocks from the upper parts of the active San Andreas and ancient San Gabriel Faults, southern California, to determine the nature and origin of micro-scale composition and geochemistry of fault-related rocks. These data constrain the nature and extent of fluid-rock interactions and processes by which slip is accommodated on shallow portions of these faults. The steeply dipping San Gabriel Fault (SGF) was sampled in a steeply inclined borehole to a depth of 400 m, and the San Andreas Fault (SAF) was sampled by seven northeast-plunging boreholes to a depth of 250 m. Fault damage zones 100m+ wide exhibit narrow fault core zones within broad damage zones. Petrographic, mineralogic, whole-rock geochemical analyses and synchrotron-based X-ray fluorescence mapping of whole drill core and thin sections reveal evidence for repeated syntectonic hydrothermal alteration, Fe-Mn rich mineralization, shearing, and brecciation that resulted in the formation of foliated cataclasites and clay and chlorite-rich shear zones in fractured network fault zones. Mineralization and alteration include clay and chlorite development, carbonate and zeolite mineralization, and the mobility of trace and transition elements in the deformed rocks. Textural evidence for repeated shearing, alteration, vein formation, brittle deformation fracture, fault slip, pressure solution, and re-lithification of faulted rocks suggests that hydrothermal alteration occurred during deformation at shallow levels. The rock assemblages likely represent significantly weakened rocks that have the potential to slip at low shear stresses, experience creep, distribute seismic energy within and near the fault, and show that hydrothermal conditions in faults may exist at very shallow levels of active faults.

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“…Coseismic temperature rise depends on a variety of factors including ambient temperature, stress, displacement, slip velocity, and the intrinsic properties (frictional strength, thermal properties, thickness, surface roughness) of a deforming zone that evolve in space and time (Cardwell et al., 1978; Lachenbruch, 1986; Platt et al., 2014; Rice, 2006). Lithologies incorporated into a fault zone and hydrothermal alteration impart mechanical heterogeneity (Bruhn et al., 1994; Chester et al., 1993; Goddard & Evans, 1995; Williams et al., 2021). The thickness of the slipping zone that accommodates the bulk of deformation may vary from microns to meters within a single fault strand (Chester & Logan, 1986; Faulkner et al., 2003; Kirkpatrick et al., 2018).…”

Section: Introductionmentioning

confidence: 99%

Microscale Spatial Variations in Coseismic Temperature Rise on Hematite Fault Mirrors in the Wasatch Fault Damage Zone

et al. 2023

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Coseismic temperature rise activates fault dynamic weakening that promotes earthquake rupture propagation. The spatial scales over which peak temperatures vary on slip surfaces are challenging to identify in the rock record. We present microstructural observations and electron backscatter diffraction data from three small‐displacement hematite‐coated fault mirrors (FMs) in the Wasatch fault damage zone, Utah, to evaluate relations between fault properties, strain localization, temperature rise, and weakening mechanisms during FM development. Millimeter‐ to cm‐thick, matrix‐supported, hematite‐cemented breccia is cut by ∼25–200 μm‐thick, texturally heterogeneous veins that form the hematite FM volume (FMV). Grain morphologies and textures vary with FMV thickness over μm to mm lengthscales. Cataclasite grades to ultracataclasite where FMV thickness is greatest. Thinner FMVs and geometric asperities are characterized by particles with subgrains, serrated grain boundaries, and(or) low‐strain polygonal grains that increase in size with proximity to the FM surface. Comparison to prior hematite deformation experiments suggests FM temperatures broadly range from ≥400°C to ≥800–1100°C, compatible with observed coeval brittle and plastic deformation mechanisms, over sub‐mm scales on individual slip surfaces during seismic slip. We present a model of FM development by episodic hematite precipitation, fault reactivation, and strain localization, where the thickness of hematite veins controls the width of the deforming zones during subsequent fault slip, facilitating temperature rise and thermally activated weakening. Our data document intrasample coseismic temperatures, resultant deformation and dynamic weakening mechanisms, and the length scales over which these vary on slip surfaces.

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How Fault Rocks Form and Evolve in the Shallow San Andreas Fault

Abstract: The physical properties of fault rocks govern the stability and strength of upper crustal faults in addition to the magnitude and distribution of radiated energy during earthquakes (e.g.,

Cited by 10 publications

References 93 publications

Shallow Composition and Structure of the Upper Part of the Exhumed San Gabriel Fault, California: Implications for Fault Processes and Seisimic Properties

Shallow Composition and Structure of the Upper Part of the Exhumed San Gabriel Fault, California: Implications for Fault Processes and Seisimic Properties

Fluid-rock interactions, hydrothermal processes, and accommodation of slip in shallow parts of the San Andreas and San Gabriel Faults, southern California

Microscale Spatial Variations in Coseismic Temperature Rise on Hematite Fault Mirrors in the Wasatch Fault Damage Zone

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