SAFOD: Mineral Transformations and Fabrics

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Mineral transformations and fabrics, particularly involving phyllosilicates, can critically affect the mechanical behavior of shallow faults. Fabric development in fault rocks may also significantly influence the permeability/porosity and role of fluids in fault zones. Thus, detailed mineralogy, mineral transformation history and fabric development are central to the study of fault rocks and our understanding of fault mechanics. The SAFOD Hole of the Earthscope Program provides the opportunity to study the in situ change from undeformed protolith to fault rock, thereby establishing a reference for the characterization of the type and magnitude of changes in buried fault zones. The SAFOD hole samples also allow for direct comparison with nearby exhumed protolith and fault rocks (e.g., the Punchbowl Fault), which have been used as representative for fault processes at depth. The research approach on SAFOD hole samples consists of X-ray diffraction (for mineral identification, quantification and crystallinity), X-ray texture goniometry (to quantify phyllosilicate fabrics), scanning electron microscopy (micron to millimeter scale mineral distribution, microstructure and qualitative chemistry), transmission electron microscopy (nanometer-scale microstructure and semi-quantitative chemistry), and ICP-OES (to quantify elemental composition and material transport). These complementary techniques provide a thorough characterization of the physico-chemical changes from protolith and fault rock.The results from SAFOD Hole samples (1) offer the mineralogic framework for interpreting fault zone transformation, particularly in seismically active settings, (2) test the relevance of the processes inferred from the study of exhumed fault rocks, and (3) provide constraints on the thermo-mechanical behavior of seismogenic faults. The work complements studies by other geologic and geophysical teams on Earthscope's SAFOD hole, and allows comparison with results from exhumed fault zones nearby and elsewhere.