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.

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