Rock Properties and Internal Structure of the San Andreas Fault Near ~ 3 km Depth in the SAFOD Borehole Based on Meso- to Micro-scale Analyses of Phase III Whole Rock Core

We examine the relationships between rock properties and structure within ~ 41 m of PHASE III whole-rock core collected from ~ 3 km depth along the SAF in the San Andreas Fault Observatory at Depth (SAFOD) borehole, near Parkfield, CA

Composition and Structure of the San Andreas Fault Observatory at Depth (SAFOD) Phase III Whole-Rock Core: Implications for Fault Zone Deformation and Fluid-Rock Interactions

We examine the composition and texture of whole-rock core from ~ 3 km depth in the San Andreas Fault Observatory at Depth (SAFOD) borehole, which provides a unique opportunity to characterize in situ rock properties of the near-fault environment, and how these properties vary in an area where deformation is accommodated by aseismic creep and high-rates of microseismicity. Detailed petrography and microstructural analyses coupled with X-Ray Diffraction and X-ray Fluorescence techniques are used to describe composition, alteration, and textures. All samples record multiple generations of cataclastic deformation in a complexly deformed and altered sequence of fine-grained sheared rocks. Localized shears bound multi-layered zones of medium to ultra-fine grained cataclasite. Phacoidal clasts or porphyroclasts comprised of serpentinite, quartz, and older cataclasite are embedded within the comminuted phyllosilicate-rich gouge. The intensity of damage-related features and the development of a pervasive anastomosing fabric increases towards and within the two active slip zones near ~ 3192 and 3302 m MD. Foliated fabrics alternating with discrete fractures suggest a mixed-mode style of deformation including both ductile flow and brittle deformation processes during fault zone evolution. Deformation at high-strain rates is suggested by the presence of crack-seal veins in clasts, the presence of porphyroclasts, and the development of S-C fabrics in the phyllosilicate- rich gouge. Evidence for fluid-rock interaction across the fault zone is indicated by depletion of Si and enrichment of MgO, FeO, and CaO; with significant clay alteration and/or growth of neo-mineralized vein fillings and fracture surface coatings. Shear localization may decrease porosity and inhibit fluid flow whereas fracturing may locally facilitate fluid migration and/or chemical alteration within the fault zone. These results constrain hypotheses related to fault zone behavior and broaden our understanding of the processes controlling earthquake nucleation and/or energy adsorption within the SAF. Based on the similarity of our observations to previous results from surface exposures of the SAF, we emphasize the importance of exhumed fault zone studies as proxies for understanding deformation and seismicity in the shallow crust

Lithology and Internal Structure of the San Andreas Fault at Depth Based on Characterization of Phase 3 Whole-rock Core in the San Andreas Fault Observatory at Depth (SAFOD) Borehole

We characterize the lithology and structure of the spot core obtained in 2007 during Phase 3 drilling of the San Andreas Fault Observatory at Depth (SAFOD) in order to determine the composition, structure, and deformation processes of the fault zone at 3 km depth where creep and microseismicity occur. A total of approximately 41 m of spot core was taken from three separate sections of the borehole; the core samples consist of fractured arkosic sandstones and shale west of the SAF zone (Pacific Plate) and sheared fine-grained sedimentary rocks, ultrafine black fault-related rocks, and phyllosilicate-rich fault gouge within the fault zone (North American Plate). The fault zone at SAFOD consists of a broad zone of variably damaged rock containing localized zones of highly concentrated shear that often juxtapose distinct protoliths. Two zones of serpentinite-bearing clay gouge, each meters-thick, occur at the two locations of aseismic creep identified in the borehole on the basis of casing deformation. The gouge primarily is comprised of Mg-rich clays, serpentinite (lizardite ± chrysotile) with notable increases in magnetite, and Ni-Cr-oxides/hydroxides relative to the surrounding host rock. The rocks surrounding the two creeping gouge zones display a range of deformation including fractured protolith, block-in-matrix, and foliated cataclasite structure. The blocks and clasts predominately consist of sandstone and siltstone embedded in a clay-rich matrix that displays a penetrative scaly fabric. Mineral alteration, veins and fracture-surface coatings are present throughout the core, and reflect a long history of syn-deformation, fluid-rock reaction that contributes to the low-strength and creep in the meters-thick gouge zones

Composition, Alteration, and Texture of Fault-Related Rocks from Safod Core and Surface Outcrop Analogs: Evidence for Deformation Processes and Fluid-Rock Interactions

We examine the fine-scale variations in mineralogical composition, geochemical alteration, and texture of the fault-related rocks from the Phase 3 whole-rock core sampled between 3,187.4 and 3,301.4 m measured depth within the San Andreas Fault Observatory at Depth (SAFOD) borehole near Parkfield, California. This work provides insight into the physical and chemical properties, structural architecture, and fluid-rock interactions associated with the actively deforming traces of the San Andreas Fault zone at depth. Exhumed outcrops within the SAF system comprised of serpentinite-bearing protolith are examined for comparison at San Simeon, Goat Rock State Park, and Nelson Creek, California. In the Phase 3 SAFOD drillcore samples, the fault-related rocks consist of multiple juxtaposed lenses of sheared, foliated siltstone and shale with block-in-matrix fabric, black cataclasite to ultracataclasite, and sheared serpentinite-bearing, finely foliated fault gouge. Meters-wide zones of sheared rock and fault gouge correlate to the sites of active borehole casing deformation and are characterized by scaly clay fabric with multiple discrete slip surfaces or anastomosing shear zones that surround conglobulated or rounded clasts of compacted clay and/or serpentinite. The fine gouge matrix is composed of Mg-rich clays and serpentine minerals (saponite ± palygorskite, and lizardite ± chrysotile). Whole-rock geochemistry data show increases in Fe-, Mg-, Ni-, and Cr-oxides and hydroxides, Fe-sulfides, and C-rich material, with a total organic content of \u3e1 % locally in the fault-related rocks. The faults sampled in the field are composed of meters-thick zones of cohesive to non-cohesive, serpentinite-bearing foliated clay gouge and black fine-grained fault rock derived from sheared Franciscan Formation or serpentinized Coast Range Ophiolite. X-ray diffraction of outcrop samples shows that the foliated clay gouge is composed primarily of saponite and serpentinite, with localized increases in Ni- and Cr-oxides and C-rich material over several meters. Mesoscopic and microscopic textures and deformation mechanisms interpreted from the outcrop sites are remarkably similar to those observed in the SAFOD core. Micro-scale to meso-scale fabrics observed in the SAFOD core exhibit textural characteristics that are common in deformed serpentinites and are often attributed to aseismic deformation with episodic seismic slip. The mineralogy and whole-rock geochemistry results indicate that the fault zone experienced transient fluid–rock interactions with fluids of varying chemical composition, including evidence for highly reducing, hydrocarbon-bearing fluids

Rock properties and structure within the San Andreas Fault Observatory at Depth (SAFOD) borehole northwest of Parkfield, California: In situ observations of rock deformation processes and fluid-rock interactions of the San Andreas fault zone at ~ 3 km depth

The San Andreas Fault Observatory at Depth (SAFOD) is a scientific drilling experiment situated along the central creeping segment of the San Andreas Fault, near Parkfield, California, and north of a segment of the fault that has experienced large historical earthquakes. Drilling into active fault zones allows scientist's to examine in situ rock samples and to record real-time data. The main goal of this study is to characterize the geologic setting and rock properties of the San Andreas fault at ∼ 3 km depth in the SAFOD borehole. In this region, the fault deforms nearly continuously through aseismic creep and small earthquakes. By sampling and characterizing the rocks from this location of the fault, we can begin to identify the features associated with fault-related deformation processes in the shallow crust; revealing the nature of the earth's crust in the near-fault environment and yields insight into the mechanisms associated with earthquake generation along an active strike-slip fault. It is also useful to seismologists for developing well-constrained, predictive earthquake models. Project costs are ∼ $175,000 funded primarily by NSF-Earthscope grant EAR- 0454527 to Dr. James P. Evans with additional support provided by the Geology Department and national scholarships to the student. Costs are associated with travel to examine core at the U.S.G.S. Core Lab in Menlo Park, CA and the IODP Gulf Coast Repository in College Station, TX; lab work, and sample processing and analyses at USU and Washington State University; field work travel plus an assistant, and collection and processing of field samples; and expenses associated with Teaching and Research Assistantships appointed to Kelly K. Bradbury during the course of this research

Fractured Dirt: Deformation Textures and Processes in Sediments and Other Unconsolidated Deposits

Structural geologists examine rock deformation to evaluate how Earth\u27s crust responds to tectonic loads in the brittle, mixed mode (or the schizosphere of Scholz, 2002) and plastic regimes of the crust (Rutter, 1986). Recently, some researchers have turned their attention to uncon-solidated and nonlithified deposits, elucidating how materials in the near-surface regime deform, and how the impact of this deformation may be felt in a variety of settings (Maltman, 1988; Rawling et al., 2001; Cashman et al., 2007, p. 611 of this issue)

Faulting and Fracturing of Nonwelded Bishop Tuff, Eastern California: Deformation Mechanisms in Very Porous Materials in the Vadose Zone

Field and microstructural studies were conducted on the basal, nonwelded and partially welded portions of the rhyolite Bishop Tuff in eastern California to examine the nature and processes of brittle deformation in these units. The nonwelded tuff consists of a variable sequence of finely laminated to massive pumice-rich deposits, fine-grained ash, and pyroclastic glass erupted from the Long Valley Caldera. The deposits are experiencing east–west extension in the hanging wall of the White Mountain fault, and small-displacement faults and fractures cut the tuff. Deformation in the Bishop Tuff occurred by fracturing associated with faults, and by slip along narrow faults with smooth, often mineralized surfaces. Localization of fracturing appears to be a function of welding. Units with a greater degree of welding have a greater abundance of fractures associated with faults, whereas nonwelded portions typically have a narrow deformation band–type faults with little or no associated damage. Microstructural observations show that transgranular fractures lie along grain boundaries of pumice and feldspar phenocrysts, and these fractures are often filled with calcite. These deposits appear to have behaved as an open-cell foam with a low strength, but with a cohesion that allowed the support of a differential stress to failure that resulted in subvertical open fractures and faults. These results demonstrate how brittle deformation may be manifested in nonwelded deposits in the vadose zone, and impart an anisotropy in which flow would be enhanced vertically and impeded horizontally. The Bishop Tuff is analogous to other nonwelded tuffs in the western USA. Thus, these results have implications for understanding deformation and flow in a variety of arid regions

Geophysical Properties Within the San Andreas Fault Zone at the San Andreas Fault Observatory at Depth (SAFOD) and Their Relationships to Rock Properties and Fault Zone Structure

We examine the relationships between borehole geophysical data and physical properties of fault‐related rocks within the San Andreas Fault as determined from data from the San Andreas Fault Observatory at Depth borehole. Geophysical logs, cuttings data, and drilling data from the region 3‐ to 4‐km measured depth of the borehole encompass the active part of the San Andreas Fault. The fault zone lies in a sequence of deformed sandstones, siltstone, shale, serpentinite‐bearing block‐in‐matrix rocks, and sheared phyllitic siltstone. The borehole geophysical logs reveal the presence of a low‐velocity zone from 3190 to 3410 m measured depth with Vp and Vs values 10–30% lower than the surrounding rocks and a 1–2 m thick zone of active shearing at 3301–3303 m measured depth. Seven low‐velocity excursions with increased porosity, decreased density, and mud‐gas kick signatures are present in the fault zone. Geologic data on grain‐scale deformation and alteration are compared to borehole data and reveal weak correlations and inverse relationships to the geophysical data. In places, Vp and Vs increase with grain‐scale deformation and alteration and decrease with porosity in the fault zone. The low‐velocity zone is associated with a significant lithologic and structural transition to low‐velocity rocks, dominated by phyllosilicates and penetratively foliated, sheared rocks. The zone of active shearing and the regions of low sonic velocity appear to be associated with clay‐rich rocks that exhibit fine‐scale foliation and higher porosities that may be a consequence of the fault‐related shearing of foliated and fine‐grained sedimentary rocks

Lithology and Internal Structure of the San Andreas Fault at Depth Based on Characterization of Phase 3 Whole-rock Core in the San Andreas Fault Observatory at Depth (SAFOD) Borehole

We characterize the lithology and structure of the spot core obtained in 2007 during Phase 3 drilling of the San Andreas Fault Observatory at Depth (SAFOD) in order to determine the composition, structure, and deformation processes of the fault zone at 3 km depth where creep and microseismicity occur. A total of approximately 41 m of spot core was taken from three separate sections of the borehole; the core samples consist of fractured arkosic sandstones and shale west of the SAF zone (Pacific Plate) and sheared fine-grained sedimentary rocks, ultrafine black fault-related rocks, and phyllosilicate-rich fault gouge within the fault zone (North American Plate). The fault zone at SAFOD consists of a broad zone of variably damaged rock containing localized zones of highly concentrated shear that often juxtapose distinct protoliths. Two zones of serpentinite-bearing clay gouge, each meters-thick, occur at the two locations of aseismic creep identified in the borehole on the basis of casing deformation. The gouge primarily is comprised of Mg-rich clays, serpentinite (lizardite ± chrysotile) with notable increases in magnetite, and Ni-Cr-oxides/hydroxides relative to the surrounding host rock. The rocks surrounding the two creeping gouge zones display a range of deformation including fractured protolith, block-in-matrix, and foliated cataclasite structure. The blocks and clasts predominately consist of sandstone and siltstone embedded in a clay-rich matrix that displays a penetrative scaly fabric. Mineral alteration, veins and fracture-surface coatings are present throughout the core, and reflect a long history of syn-deformation, fluid-rock reaction that contributes to the low-strength and creep in the meters-thick gouge zones

Mineralogic and Textural Analyses of Drill Cuttings from the San Andreas Fault Observatory at Depth (SAFOD) Boreholes: Initial Interpretations of Fault Zone Composition and Constraints on Geologic Models

We examine drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes to determine the lithology and deformational textures in the fault zones and host rocks. Cutting samples represent the lithologies from 1.7-km map distance and 3.2-km vertical depth adjacent to the San Andreas Fault. We analyzed two hundred and sixty-six grain-mount thin-sections at an average of 30-m-cuttings sample spacing from the vertical 2.2-km-deep Pilot Hole and the 3.99-km-long Main Hole. We identify Quaternary and Tertiary(?) sedimentary rocks in the upper 700 m of the holes; granitic rocks from 760–1920 m measured depth; arkosic and lithic arenites, interbed-ded with siltstone sequences, from 1920 to ∼3150 m measured depth; and interbed-ded siltstones, mudstones, and shales from 3150 m to 3987 m measured depth. We also infer the presence of at least five fault zones, which include regions of damage zone and fault core on the basis of percent of cataclasite abundances, presence of deformed grains, and presence of alteration phases at 1050, 1600–2000, 2200–2500, 2700–3000, 3050–3350, and 3500 m measured depth in the Main Hole. These zones are correlated with borehole geophysical signatures that are consistent with the presence of faults. If the deeper zones of cataclasite and alteration intensity connect to the surface trace of the San Andreas Fault, then this fault zone dips 80–85° southwest, and consists of multiple slip surfaces in a damage zone ∼250–300 m thick. This interpretation is supported by borehole geophysical studies, which show this area is a region of low seismic velocities, reduced resistivity, and variable porosity