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_Journal of Structural Geology 33 (2011) 132-144_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Fault rocks from the SAFOD core samples: Implications for weakening at shallow depths along the San Andreas Fault, California R.E. Holdsworth a,*, E.W.E. van Diggelen b, C.J. Spiers b, J.H.P. de Bresser b, R.J. Walker a, L. Bowen a,c a Reactivation Research Group, Department of Earth Sciences, University of Durham, Durham DH1 3LE, UK b HPT Laboratory, Utrecht University, Utrecht, The Netherlands c Durham GJ Russell Microscopy Facility, Durham University, Durham DH1 3LE, UK Article info Article history: Received 9 September 2010; Received in revised form 11 November 2010; Accepted 18 November 2010; Available online 3 December 2010 Keywords: San Andreas Fault SAFOD Fault zone weakening Smectite Phyllosilicate Fluid-assisted alteration Abstract The drilling of a deep borehole across the actively creeping Parkfield segment of the San Andreas Fault Zone (SAFZ), California, and collection of core materials permit direct geological study of fault zone processes at 2-3 km depth. The three drill cores sample both host and fault rocks and pass through two currently active, narrow (1-2 m wide) shear zones enclosed within a broader (ca. 240 m wide) region of inactive foliated gouges. The host rocks preserve primary sedimentary features and are cut by numerous minor faults and small, mainly calcite-filled veins. The development of Fe-enriched smectitic phyllosilicate networks following cataclasis is prevalent in the presently inactive foliated gouges of the main fault zone and in minor faults cutting clay-rich host rocks. Calcite, anhydrite and minor smectitic phyllosilicate veins are interpreted to have formed due to local fluid overpressuring events prior to, synchronous with and after local gouge development. By contrast, the active shear zone gouges lack mineral veins (except as clasts) and contain numerous clasts of serpentinite. Markedly Mg-rich smectitic phyllosilicates are the dominant mineral phases here, suggesting that the fault zone fluids have interacted with the entrained serpentinites. We propose that weakening of the SAFZ down to depths of at least 3 km can be attributed to the pervasive development of interconnected networks of low friction smectitic phyllosilicates and to the operation of stress-induced solution-precipitation creep mechanisms. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Most geological faults are likely to be weak in a relative sense compared to nearby regions of intact host rock. However, geophysical measurements of surface heat flow (e.g., Brune et al., 1969; Lachenbruch and Sass, 1980) and stress orientation data (Mount and Suppe, 1987; Zoback et al., 1987) adjacent to crustal-scale faults, especially those sections of such faults undergoing creep, have demonstrated the existence of anomalously low frictional strengths, i.e. they are weak in an absolute sense, with friction coefficients (m) significantly less than 0.6. The causes, global importance and very existence of such anomalous weakness along faults remain highly controversial issues despite over forty years of research (e.g., see Scholz, 2000; Zoback, 2000). The recent deep drilling into an active segment of the San Andreas Fault Zone (SAFZ) at seismogenic depths (Zoback et al., 2007, 2010) provides an opportunity to directly sample and study core materials to assess the * Corresponding author. Tel.: +44 0 1913342299; fax: +44 0 1913342301. E-mail address: r.e.holdsworth@durham.ac.uk (R.E. Holdsworth). 0191-8141 $ e see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.11.010 mineralogy, deformation mechanisms and laboratory-measured constitutive properties of fault rocks. Unlike fault rocks in surface exposures, materials recovered from such in-situ sampling at depth have not experienced recent surface alteration. This means that they potentially preserve a more accurate record of deformation and associated mineralogical changes that occur during faulting. The present paper outlines a geological description and interpretation of the lithologies, meso- and micro-structures found in the three sections of core from the San Andreas Fault Observatory at Depth (SAFOD) (Fig. 1). Unlike most studies published to date, which have focussed on isolated samples, the findings presented here are based on a visual assessment of all Phase 3 core materials, and on detailed optical microscope- and SEM-based studies of 38 representative thin sections taken from those cores. These include all of the thin sections prepared for the Phase 3 core atlas (SAFOD, 2010). The observations made using the SEM are supplemented by use of Energy Dispersive X-ray (EDX) analyses to identify specific mineral grains and produce elemental maps of selected regions. The overview presented here is intended to provide some important preliminary geological constraints for many of the more detailed studies being carried out. R.E. Holdsworth et al. Journal of Structural Geology 33 (2011) 132-144 Fig. 1. a) General location map of the SAFZ and SAFOD borehole, Parkfield, California. Topography based on the ASTER GDEM. (b) Highly simplified NW-SE cross-section showing steep SW dip of SAFZ and its bifurcation into at least 3 strands at depth, at least 2 of which (shown in red) are actively creeping. Box shows location of (c). (c) Cross-section showing inclined borehole crossing SAFZ and approximate location of Phase 3 cores. These are color-coded to the logs shown in Fig. 1eeg. (d) Left-hand image shows simplified geophysical logs and geology with depth along the Phase 2 borehole (after Zoback et al., 2010). Dashed red lines represent faults, whilst thick red zones correspond to the SDZ and CDZ where active creep has deformed the borehole casing. The broader damage zone is also shown corresponding to the region of lower resistivity and seismic velocities. Right-hand images show detail of the geophysical log properties in the region of the SDZ (left) and CDZ (right). The measured depths shown are for the Phase 2 borehole (see note below). (e)-(g) Simplified geological logs of the Phase 3 cores based on the descriptions given in the Core Atlas (SAFOD, 2010) together with a preliminary visual inspection of the core (in 2007) and a more detailed visual inspection using the On-line Core Viewer (http://www.earthscope.org/data/safod_core_viewer). Important note: The measured depths shown here are for the Phase 3 drilling, but the cores here were drilled as a series of multilaterals off the Phase 2 hole. As a result, the measured depths do not necessarily coincide. However, Zoback et al. (2010) have shown that the markedly anomalous geophysical signatures of the active SDZ and CDZ allow a synchronization of measured depths for the two sets of core runs taken from Hole G. Thus it is suggested that in core runs 1-3 (SDZ), 5.03 m should be subtracted from the Phase 3 m Md values to synchronize them with the Phase 2 values, whilst 3.96 m should be added to the Phase 3 m Md values in core runs 4-6 (CDZ). Locations of thin sections examined during the present study are shown as yellow dots. 134 R.E. Holdsworth et al. Journal of Structural Geology 33 (2011) 132-144 out by other geoscientists focussed on individual samples and short sections of the core material. 2. SAFOD: geological setting and location of borehole The San Andreas Fault forms the steeply dipping to sub-vertical dextral transcurrent boundary between the Pacific plate to the west and the North American plate to the east (Fig. 1a and b). The fault zone is over 1000 km long, extends at depth to at least 15 km, and is manifested at the surface by a complex zone of linked faults and associated brittle deformation ranging from a few hundred metres to several kilometres wide (Allen, 1981). Geophysical data have repeatedly demonstrated that the direction of maximum horizontal stress in the crust lies at a high angle to the San Andreas Fault tract throughout central California and remains at a high angle to within 200 m of the active fault traces within the SAFOD boreholes (e.g., Hickman and Zoback, 2004; Zoback and Hickman, 2005; Boness and Zoback, 2006). This evidence, together with the absence of any measurable heat flow anomaly, either regionally or in any of the SAFOD boreholes (e.g., Sass et al., 1997; Williams et al., 2005) strongly suggests that active creep (at w2.5 cm/year; Titus et al., 2006) along the Parkfield segment of the San Andreas Fault (Fig. 1a) occurs at very low values of resolved shear stress. Following the drilling of the main SAFOD borehole, it became apparent that two narrow (1-2 m wide) sections were undergoing active deformation by creep due to localised fault movements at 3192 m and 3302 m Md (? Measured depth, in this case along the length of the Phase 2 borehole). Following Zoback et al. (2010), these active regions are referred to here as the southwest deforming zone (SDZ) and the central deforming zone (CDZ) (Fig. 1b and c). During the final stage of drilling, Phase 3, a series of multilaterals were cored off from and adjacent to the Phase 2 hole in order to sample the rocks lying close to the trace of the geological San Andreas Fault and across the SDZ and CDZ (Fig. 1c). In Central California, the San Andreas Fault separates rocks located to the west belonging to the Salinian block from rocks located to the east referred to here as the Great Valley block, comprising units belonging to both the highly deformed, subduction-related Franciscan Complex and a Cretaceous forearc sedimentary sequence known as the Great Valley Group (Wakabayashi, 1999; Draper-Springer et al., 2009 and references therein). Close to the SAFZ at depth, the NE-dipping Phase 2 borehole cuts a thick sequence of arkosic sandstones and conglomerates inferred from detailed petrological, zircon fission track analyses and regional geological studies by Draper-Springer et al. (2009). 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