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

Abstract

We examine drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes to determine the lithology and deformational textures inthe 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, interbedded with siltstone sequences, from 1920 to ~3150 m measured depth; and interbedded siltstones, mudstones, and shales from 3150 m to 3987 m measured depth. We also infer the presence of at least fi ve fault zones, which include regions of damage zone and fault core on the basis of percent of cata-clasite 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.

Faulting processes in active faults – Evidences from TCDP and SAFOD drill core samples

“…1a; Zoback et al, 2011). Within the SAFOD drill holes four major geological units have been identified (Bradbury et al, 2007) with Quaternary and Tertiary sediments, Salinian granite and arkosic sediments beneath the Buzzard Canyon fault. Approximately 1200 m NE of the drill site the arkosic sediments of the Salinian block (Pacific plate) are replaced by shales and siltstones of the Great Valley/Franciscan sequence (North American plate).…”

“…The main drill hole crosscuts multiple faults including two actively creeping strands, the Southwest Deforming Zone (SDZ) and the Central Deforming Zone (CDZ) (Fig. 1b;Zoback et al, 2010), which were revealed by pronounced and ongoing casing deformation (Bradbury et al, 2007;Zoback et al, 2010). Detailed descriptions of SAFOD Phase 3 core material from Holes E and G (Fig.…”

Constraints on mineralization, fluid‐rock interaction, and mass transfer during faulting at 2–3 km depth from the SAFOD drill hole

“…This drilling extended into interbedded mudstone, siltstone and shale lithologies at the bottom of the drill hole (Figure 1c) [Draper et al, 2005;Hickman et al, 2005]. Bradbury et al [2007] investigated the bulk mineralogy of the SAFOD drill cuttings, showing an arkosic rock sequence from 1920 to 2550 m, separated by a clay rich zone at 2530-2680 m and a fine-grained sequence at 2680-3150 m MD. A lithologic change occurs at $3150 m MD, where arenites are replaced by fine-grained siltstone and shaly fragments.…”

“…The appearance of chlorite and chlorite-smectite (C-S) minerals at $3150 m MD reflects the lithologic change from finer grained arkosic sandstones to a fine-grained quartz-feldspar-rich siltstone that alternates with sandy and shaly sections [Draper et al, 2005;Bradbury et al, 2007]. The picked fault-related grain material comprises fine-grained mudrock lithologies containing abundant illite and I-S along with quartz and plagioclase components in the shallow fault trace.…”

“…Two areas of primary interest were targeted for this study ( Figure 1c): a shallow fault trace (fault trace 1) between 3066 m and 3169 m measured depths (MD) and the main trace between 3292 m and 3368 m MD (fault trace 2). The $100 m wide shallow fault section is interpreted as a fault zone on the basis of rock deformation and alteration features [Bradbury et al, 2007;Solum et al, 2006]. This zone lays $25 m above a level of active casing deformation located at 3194 m [Hickman et al, 2008].…”