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Lessons from SAFOD : Mineralogical Composition, Mechanical Properties, and

Lessons from SAFOD : Mineralogical Composition, Mechanical Properties, and Microstructural Fabric of Fault Zone Rocks. By: Ryan D. Witkosky (with help from Jean-Philippe Avouac ) Ge 277, Active Tectonics Seminar, Winter 2014 California Institute of Technology. Questions:

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Lessons from SAFOD : Mineralogical Composition, Mechanical Properties, and

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  1. Lessons from SAFOD: Mineralogical Composition, Mechanical Properties, and Microstructural Fabric of Fault Zone Rocks By: Ryan D. Witkosky (with help from Jean-Philippe Avouac) Ge 277, Active Tectonics Seminar, Winter 2014 California Institute of Technology

  2. Questions: 1. Why is the central portion of the San Andreas Fault creeping? 2. Mechanical properties (i.e. friction) of fault zone rocks dictate creep versus stick-slip behavior, but are the mechanical properties dependent on composition, penetrative fabrics, or both? 3. Can micro-textural observations help explain observed behavior?

  3. San Francisco, 1906, M 7.8 Creeping Section Parkfield, 2004, M 6.0 Figure 2 modified from Irwin and Barnes, 1975.

  4. Parkfield Creeping Section “Serpentinite at base of Great Valley Sequence.” Figure 3 modified from Irwin and Barnes, 1975.

  5. “Migration of fluid toward permeable fault zone.” E W Figure 5 modified from Irwin and Barnes, 1975.

  6. A new, better picture from SAFOD. Figure 1a from Carpenter et al., 2011.

  7. Where is the talc in the surface outcrop? THESE ARE CUTTINGS FROM DRILL MUD! Dubious…? Figures 1a & 3c from Moore and Rymer, 2007.

  8. “Anastomosing shear zones in clay-rich foliated gouge.” Figure 1c from Carpenter et al., 2011.

  9. Experiments on POWDERED drill cuttings from WALL ROCK. HEALING Figure 2 from Carpenter et al., 2011.

  10. “Friction coefficient and healing rate increase away from the fault zone.” μ= ~0.20 for powdered fault zone rocks 3297.7 m Figure 3 from Carpenter et al., 2011.

  11. Mechanical Properties for INTACT (not powdered) fault rocks. NO HEALING μ< 0.20 for intact fault zone rocks. Figure 2A,B (modified),C from Carpenter et al. 2012.

  12. Hold experiment on intact core material from SAFOD shows the same general results when comparing fault zone versus wall rock. HEALING NO HEALING Figure 3B modified from Carpenter et al., 2012.

  13. Back to the Serpentinite… Figure 1A from Moore and Rymer, 2012.

  14. Does the clayey gouge at the surface correlate with the gouge at depth from SAFOD? Figure 1B from Moore and Rymer, 2012.

  15. Evidence: both surface outcrop and SAFOD core show mineralogical similarities, believed to result from mixing between mafic Franciscan ophiolites and Great Valley sedimentary rocks. SURFACE OUTCROP SAFOD CORE SAFOD CORE Figures 4A,C,F modified from Moore and Rymer, 2012.

  16. “Pressure Solution Evidence.” Figures 2E & 2C (modified) from Gratier et al., 2011.

  17. Smaller grain size or higher temperature produces a larger strain rate. Equation (2), Figures 3C & 3D from Gratier et al., 2011.

  18. SAFOD samples from within creeping section of central deforming zone. Figure 3E & 3F from Gratier et al., 2011.

  19. The end point: do we understand what’s going on at depth? 1. Mineralogy certainly plays a key role in rock mechanics, but penetrative fabrics also contribute significantly. 2. Of all the minerals discussed, it appears that the rate-strengthening, Mg-bearing clays (i.e. saponite) are ALWAYS present in creeping faults. 3. Is there a minimum volume of clay required to induce creep?

  20. The end. Please see me if you have any further questions regarding the references cited in this slide show. Thank you.

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