Pure Appl. Geophys. Ó 2014 Springer Basel DOI 10.1007/s00024-014-0896-6 Pure and Applied Geophysics Composition, Alteration, and Texture of Fault-Related Rocks from Safod Core and Surface Outcrop Analogs: Evidence for Deformation Processes and Fluid-Rock Interactions 1 1 1 1 1 KELLY K. BRADBURY, COLTER R. DAVIS, JOHN W. SHERVAIS, SUSANNE U. JANECKE, and JAMES P. EVANS Abstract—We examine the fine-scale variations in mineralogi- 1. Introduction cal 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 Well-constrained geological, geochemical, and Observatory at Depth (SAFOD) borehole near Parkfield, California. geophysical models of active fault zones are needed if This work provides insight into the physical and chemical properties, we are to understand fault zone behavior and earth- structural architecture, and fluid-rock interactions associated with the actively deforming traces of the San Andreas Fault zone at depth. quake deformation, constraining the factors that affect Exhumed outcrops within the SAF system comprised of serpentinite- the distribution of earthquakes, and the nature of slip bearing protolith are examined for comparison at San Simeon, Goat in the shallow crust by developing realistic models of Rock State Park, and Nelson Creek, California. In the Phase 3 SAFOD drillcore samples, the fault-related rocks consist of multiple subsurface fault zone structure and ground motion juxtaposed lenses of sheared, foliated siltstone and shale with block- predictions. Earthquakes nucleate in rocks at depth in-matrix fabric, black cataclasite to ultracataclasite, and sheared (e.g., FAGERENG and TOY 2011;SIBSON 1977; 2003), serpentinite-bearing, finely foliated fault gouge. Meters-wide zones of sheared rock and fault gouge correlate to the sites of active yet until recently (see ANDO, 2001;BOULLIER et al. borehole casing deformation and are characterized by scaly clay 2009;CORNET et al. 2004;HEERMANCE et al. 2003; fabric with multiple discrete slip surfaces or anastomosing shear HICKMAN et al. 2004;HICKMAN et al. 2007;HIRONO zones that surround conglobulated or rounded clasts of compacted clay and/or serpentinite. The fine gouge matrix is composed of Mg- et al. 2007;OHTANI et al. 2000;TANAKA et al. 2002; rich clays and serpentine minerals (saponite ± palygorskite, and TOBIN and KINOSHITA 2006;TOWNEND et al. 2009;ZO- lizardite ± chrysotile). Whole-rock geochemistry data show BACK et al. 2007;ZOBACK et al. 2010), we have only increases in Fe-, Mg-, Ni-, and Cr-oxides and hydroxides, Fe-sul- fides, and C-rich material, with a total organic content of [1% had limited sampling of rocks or observations from locally in the fault-related rocks. The faults sampled in the field are within active plate-boundary fault zones where composed of meters-thick zones of cohesive to non-cohesive, ser- earthquake nucleation and rupture propagation has pentinite-bearing foliated clay gouge and black fine-grained fault rock derived from sheared Franciscan Formation or serpentinized occurred. Direct knowledge of rock properties from Coast Range Ophiolite. X-ray diffraction of outcrop samples shows active fault zones through integrated drilling, field, that the foliated clay gouge is composed primarily of saponite and and laboratory studies can provide significant insight serpentinite, with localized increases in Ni- and Cr-oxides and C-rich material over several meters. Mesoscopic and microscopic textures into fault processes, and can be used to: (1) identify and deformation mechanisms interpreted from the outcrop sites are systematic compositional and textural changes to infer remarkably similar to those observed in the SAFOD core. Micro- the chemical and physical processes involved in fault scale to meso-scale fabrics observed in the SAFOD core exhibit textural characteristics that are common in deformed serpentinites zone evolution; (2) delineate the dimensions of the and are often attributed to aseismic deformation with episodic structural and permeability fault zone architecture; seismic slip. The mineralogy and whole-rock geochemistry results and (3) consider the role of fluid migration and fluid- indicate that the fault zone experienced transient fluid–rock inter- actions with fluids of varying chemical composition, including related alteration throughout deformation (BRODSKY evidence for highly reducing, hydrocarbon-bearing fluids. et al. 2010; CAINE et al. 1996, 2010;CHESTER and LOGAN 1986;COWAN 1999;EVANS and CHESTER 1995; FAGERENG and SIBSON 2010;KNIPE et al. 1998;MARONE and RICHARDSON 2010;MENEGHINI and MOORE 2007; ROWE et al. 2009;SHIPTON AND COWIE and COWIE 2001; 1 Geology Department, Utah State University, Logan, UT 84322-4505, USA. E-mail: [email protected] SIBSON 1989;WIBBERLEY et al. 2008). K. K. Bradbury et al. Pure Appl. Geophys. The San Andreas Fault Observatory at Depth ZOBACK et al. 2011). Previous research by BRADBURY (SAFOD; HICKMAN et al. 2004;HICKMAN et al. 2007; et al.(2007), BRADBURY et al.(2011), HOLDSWORTH ZOBACK et al. 2010; http://www.earthscope.org/ et al.(2011), SPRINGER et al. (2007), and THAYER and observatories/safod) near Parkfield, California, pro- ARROWSMITH (2005) illustrate the geologic and vides a nearly continuous record of rock cuttings and structural setting and provide detailed core observa- a limited amount of core from the subsurface through tions of the SAFOD site. the Pacific Plate northeastward across the San Serpentinite was identified in several SAFOD Andreas Fault and into the North American Plate samples, between a depth of 3 and 4 km (BRADBURY (Fig. 1). At SAFOD, in situ fault-related rocks from et al. 2011;MOORE and RYMER 2007;MOORE and *3 km depth were sampled along the central RYMER 2012;SOLUM et al. 2006; Phase 3 Core Photo creeping segment of the San Andreas Fault (SAF). Atlas, http://www.earthscope.org/observatories/ The surface creep rate of the SAF near SAFOD is 20 safod). This mineralogy and the associated alter- mm/yr (TITUS et al. 2006), and is interpreted to occur ation products may promote weak fault behavior on multiple parallel strands in the subsurface (HOLE (HIRTH and GUILLOT 2013;MOORE et al. 1997;MOORE et al. 2006;MCPHEE et al. 2004;THURBER et al. 2004; and RYMER 2012;REINEN et al. 2000; SCHLEICHER et al. THURBER et al. 2006;ZOBACK et al. 2005). Within the 2012). In the two *1–2 m thick gouge zones asso- SAF here, a series of repeating microearthquakes ciated with active creep demonstrated by borehole occur near 2,500–2,800 m vertical depth, or about casing deformation at SAFOD, serpentinite and ser- *50–300 m from the actively creeping fault strands pentinite-derived clays are present (SOLUM et al. intersected by the SAFOD borehole (Fig. 1;NADEAU 2006, 2007;BRADBURY et al. 2011;MOORE and RYMER et al. 2004;THURBER et al. 2004;ZOBACK et al. 2010; 2009, 2012). The two zones, referred to as the True Vertical Depth Cross Section N 45 E (K.B., m) Geologic Plate PA Boundary - 2600 Hole G Runs (1,2,3) 3157 m MD - 2650 NA North American Hole G Runs (4,5,6) Plate (NA) - 2700 Damage Zone Goat Rock - 2750 SDZ 3192 m MD CDZ 3413 m MD San Francisco 3302 m MD - 2800 Nelson Creek 1150 12001250 1300 1350 1400 SAFOD N 45 East (m) San Simeon San Andreas Fault Pacific Plate (PA) Los Angeles Figure 1 Location map of the SAFOD borehole and surface outcrops in California relative to the positions of the San Andreas Fault, Pacific Plate, and North American plate. The inset illustrates a simplified vertical profile of the SAFOD borehole, location of the targeted earthquakes, SDZ, CDZ, and the associated damage zone (JEPPSON et al. 2010;ZOBACK et al. 2010), and the Phase 3 core as sampled at depth (modified from Phase 3 Core Photo Atlas, http://www.earthscope.org) Evidence for Deformation Processes and Fluid-Rock Interactions a i 3312.3 m 3190.4 m b h 3192.9 m 3301.3 m c g 3195.8 m 3297.7 m d e f 3197.0 m 3197.8 m 3297.5 m Black cataclasite to Phyllosilicate-rich block-in-matrix melange ultracataclasite SDZ CDZ Phyllosilicate-rich block-in-matrix melange ~ 95 m Gap in Core a b c d efg h i 7 m m m m m 0 m 1 m 6 m 12. m m m .4 m 9.4 .9 m 5 m 1 m .6 33 8.3 9.8 .3 m 4.3 m 5.8 7.9 00 04. 05. 07. 08. 10. 329 33 33 3311 3186.7 318 318 3192.8 m3193 319 319 3197.43198.9 m 3199.5 m m 3294.9 3296m 329 33 3302.5 m 33 33 33 Extent of Low Velocity Zone (after Zoback et al., 2010) Location of thin-section photomicrographs shown in Figures 4,5, and 7 and Extent of gas rich zones (after Weirsberg & Erzinger, 2009) whole-rock geochemical analyses shown in Figures 4A & 4B Total Organic Carbon (TOC) sample results shown in Table 2 b Location of image shown above (may or may not correspond to sample location) Figure 2 Mesoscopic textures of fault-related rocks in SAFOD Phase 3 core: a foliated block-in-matrix fabric within damaged rocks west of the SDZ; b localized black (carbon-rich) staining parallel to fracture surface at 3,192.9 m MD; c Black fault-related rock consists of cataclasite to ultracataclasite at 3,195.8 m MD; d serpentinite and scaly clay gouge of the SDZ at *3,197 m MD; e foliated fault gouge with scaly clay fabric at 3,197.8 m MD; f clay-rimmed altered serpentinite and siltstone clasts within foliated fault gouge of the CDZ at 3,297.5 m MD; g foliated fault gouge with scaly clay matrix and numerous rounded and lens-shaped, altered serpentinite clasts and fragments at 3,297.7 m MD; h block of intensely sheared siltstone with abundant calcite veinlets to the immediate northeast of the CDZ fault boundary; and i intensely sheared black clay (organic-rich?) interval with shiny, polished slickensided surfaces at 3,312.3 m MD.