Residual stress preserved in quartz from the San Andreas Fault Observatory at Depth

We report on measurements of residual stress up to 300 MPa with a microfocused synchrotron X-ray beam in quartz fragments in a cataclasite from the damage zone of the San Andreas fault, California (USA). Samples were extracted from the San Andreas Fault Observatory at Depth drill core at a depth of 2.7 km. Stresses were derived from lattice distortions observed on Laue diffraction images. These stresses are distributed nonhomogeneously at the micron scale and are much higher than bulk-rock strengths of fault gouge, suggesting different processes at the microscopic and macroscopic scales. Our results indicate that residual lattice strain in quartz is a potential paleopiezometer to estimate stress in deformed rocks. INTRODUCTION Stresses that are released during earthquakes have been of major interest in seismology, engineering, structural geology, and mineral physics (Byerlee, 1978; van der Elst and Brodsky, 2010). In an effort to better understand earthquake mechanisms, several deep drilling projects have been undertaken to retrieve material from seismically active zones of major faults, such as the San Andreas Fault Observatory at Depth (SAFOD) for the San Andreas fault in California (Zoback et al., 2011), the Nojima fault in Japan (Ohtani et al., 2000), the Taiwan Chelungpu-Fault Drilling Project (Ma et al., 2006), the Deep Fault Drilling Project for the Alpine fault in New Zealand (Townend et al., 2009), the Wenchuan earthquake Fault Scientific Drilling Project in China (Xue et al., 2013), and the Japan Trench Fast Drilling Project for the plate boundary thrust off the coast of Japan (Chester et al., 2013). Material retrieved from drill cores can be studied in the laboratory and thus provides direct information about physical and chemical processes that occur at depth within a seismically active zone. Data obtained from such projects can then be compared with records of active seismicity to advance our understanding of the way different mechanisms of brittle failure in Earth’s crust are recorded at microscopic to macroscopic scales and how these influence the type of earthquake produced. Of particular interest from a rock mechanics point of view is fault gouge formed during seismic displacements. Fault gouge is composed of mineral fragments, typically including quartz, feldspar, and phyllosilicates resulting from comminution and alteration reactions. Some of the phyllosilicates are authigenic, suggesting an environment with aqueous components (Gratier et al., 2011; Janssen et al., 2012). In SAFOD material we also find extensively fractured quartz cataclasites, such as the sample studied here (hole E, run 1, section 6, Dressen, at 3141 m; www.earthscope.org/science /data/data-access/safod-data/) that stems from the Great Valley Formation in the damage zone, ~50 m from the currently active southwest deforming zone (e.g., Zoback et al., 2011, their figures 3 and 5). Because the shear strength of quartz at low temperature and pressure is between 4 and 10 GPa (LaLone and Gupta, 2009), rocks containing fractured quartz grains must have undergone such high stresses at least locally. This optical evidence of brittle failure of quartz grains raises the question whether some residual stresses connected to the stress field causing the brittle failure are still preserved in the crystal lattice. In this study we used synchrotron X-ray Laue microdiffraction as a probe to measure residual deviatoric lattice strain with a high spatial resolution (<1 mm). Laue microdiffraction was developed by materials scientists to characterize texture, strain, and stress on a subgrain level (e.g., Chen et al., 2009; Ice et al., 2011; Jiang et al., 2013). Residual stress has been investigated in metals for a long time, documenting lattice distortions through diffraction evidence (e.g., Noyan and Cohen, 1987). Here we present a first application of this powerful technique to investigate residual stress in quartz from an active earthquake zone.