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

Abstract

Abstract-We examine the fine-scale variations in mineralogical composition, geochemical alteration, and texture of the faultrelated 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 serpentinitebearing 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 blockin-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 Mgrich 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 [1 % 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. Microscale 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.

How phyllosilicate mineral structure affects fault strength in Mg‐rich fault systems

“…Evidence of sepiolite and palygorskite transformation to smectites as a result of fluid-rock interaction in surface outcrops indicates that they could exist widely in their fibrous configuration at Geophysical Research Letters 10.1002/2017GL073055 seismogenic depths [Callen, 1984;Krekeler et al, 2005]. Sepiolite and palygorskite have been observed in cores recovered from the Mariana fore-arc region [Natland and Mahoney, 1982], and minor amounts of palygorskite were recognized in cores recovered from the San Andreas Fault [Bradbury et al, 2011[Bradbury et al, , 2015, demonstrating that fibrous clay minerals can indeed constitute important amounts of fault gouges in Mg-rich fault systems and influence frictional sliding at seismogenic depths.…”

Serpentinite‐Rich Gouge in a Creeping Segment of the Bartlett Springs Fault, Northern California: Comparison With SAFOD and Implications for Seismic Hazard

“…The Central Deforming Zone (CDZ) is located in the middle of the~200-m-wide damage zone, and the Southwest Deforming Zone (SDZ) forms the southwest boundary of the fault (Zoback et al, 2010(Zoback et al, , 2011. Both the CDZ and SDZ are foliated gouges composed of porphyroclasts of serpentinite and a variety of quartzofeldspathic crustal rock types in a matrix rich in saponitic clays (e.g., Bradbury et al, 2011Bradbury et al, , 2015Holdsworth et al, 2011;Moore & Rymer, 2012;Moore, 2014;Richard et al, 2014). Whole-rock chemical analyses of the gouges confirm that they contain a large ultramafic component (Bradbury et al, 2011(Bradbury et al, , 2015Janssen et al, 2014;Moore et al, 2017).…”

“…Both the CDZ and SDZ are foliated gouges composed of porphyroclasts of serpentinite and a variety of quartzofeldspathic crustal rock types in a matrix rich in saponitic clays (e.g., Bradbury et al, 2011Bradbury et al, , 2015Holdsworth et al, 2011;Moore & Rymer, 2012;Moore, 2014;Richard et al, 2014). Whole-rock chemical analyses of the gouges confirm that they contain a large ultramafic component (Bradbury et al, 2011(Bradbury et al, , 2015Janssen et al, 2014;Moore et al, 2017). The wall rocks are quartzofeldspathic sedimentary rocks, and the contacts with the foliated gouge zones are marked by abrupt transitions in strength (Carpenter et al, 2012;Lockner et al, 2011) and mineralogy (Moore, 2014).…”

“…Specifically, as discussed in this section, the bulk chemistry of the gouges, their frictional properties, and their inferred modes of emplacement into the faults are essentially the same. Figure 10 is an enlargement of the lower-right corner of the ACF plot in Figure 8, to which XRF analyses of SAFOD gouge samples from a number of sources (Bradbury et al, 2011(Bradbury et al, , 2015Janssen et al, 2014;Moore et al, 2017) were added for comparison with BSF mixed-mineralogy gouge samples #2-4 ( Table 2). Compositions of the BSF and SAFOD gouges are closely similar, in particular the CDZ and BSF samples; the SDZ gouges are shifted slightly toward the metagraywacke compositional field in Figure 8.…”

“…Comparison of BSF (#2-4 in Table 2) and SAFOD foliated gouge compositions; the plot is the lower-right corner of the ACF diagram in Figure 8, and it retains the BSF mineral composition data. SAFOD CDZ and SDZ gouge compositions are from Bradbury et al (2011Bradbury et al ( , 2015, Janssen et al (2014), and Moore et al (2017); all are X-ray fluorescence analyses. Compositions of saponite from the CDZ and SDZ and corrensite from the SDZ are from Moore (2014) Constentius et al (2000); the western half incorporates shallow structure from cross section A-A' of McLaughlin et al (2018).…”