Mineralogical characterization of protolith and fault rocks from the SAFOD Main Hole

Abstract

Washed cuttings provide a continuous record of the rocks encountered during drilling of the main hole of the San Andreas Fault Observatory at Depth (SAFOD). Both protolith and fault rocks exhibit a wide variety of mineral assemblages that reflect variations in some combination of lithology, P‐T conditions, deformation mechanisms, and fluid composition and abundance. Regions of distinct neomineralization bounded by faults may record alteration associated with fluid reservoirs confined by faults. In addition, both smectites occurring as mixed‐layer phases and serpentine minerals are found in association with active strands of the San Andreas Fault that were intersected during drilling, although their rheological influence is not yet fully known. Faults containing these mineralogical phases are prime candidates for continuous coring during Phase 3 of SAFOD drilling in the summer of 2007.

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

“…Ca is highly mobile throughout the entire fault zone and likely contributed to the precipitation of Ca-zeolites, Ca-sulfates, and calcite. These precipitates are common in cuttings and core material [Solum et al, 2006;Bradbury et al, 2007;Draper et al, 2005], and their abundance fluctuates across the fault traces occurring as vein or adjacent pore fillings. Another distinct Ca source is the alteration of anorthite, whereas the loss of K generally reflects the dissolution of detrital K-feldspar, biotite, or authigenic illite-smectite minerals that were formed during prior diagenesis.…”

“…[10] The X-ray diffraction (XRD) analyses of randomly oriented bulk rock samples show abundant quartz, feldspar (both plagioclase and K-feldspar) and clay minerals (illite and illite-smectite/I-S) as main components, along with minor amounts of calcite and laumontite in some samples [see also Solum et al, 2006]. 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].…”

“…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].…”

Electron Microscopy of Clay Minerals in Mudrocks from the San Andreas Fault Observatory at Depth (SAFOD)

“…They are surrounded by a compacted fabric (presumably bedding parallel), perpendicular to the polished fractures and containing notably higher concentrations of K, Mg, and , and and Fe. Illite, chlorite, and inter-, and interand interstratifications of smectite with chlorite and illite have also been recognized by Solum et al (2006) in the whole-rock XRD patterns. Based on first analysis of illite diffraction patterns (SAED), ordered and disordered 1M polytypes can be distinguished, whereas a highertemperature 2M 1 illite polytype has not yet been recognized.…”

“…Recent 2004). Recent electron microscopy and x-ray diffraction (XRD) studies of (XRD) studies of studies of rock fragments and cuttings from this drill site show site show site show abundant clay minerals in the host rock, on fracture surfaces, and within mineralized veins (Schleicher et al, 2006;Solum , 2006;Solum 2006;Solum 2006;Solum Solum et al, 2006). It is, therefore, of importance to determine the , 2006).…”