Comparison of SAFOD Pilot Hole phyllosilicate mineral assemblages to the Punchbowl fault: Recognizing post-faulting alteration in exhumed fault zones

The chlorite assemblages in cuttings from two sampled intervals of the SAFOD Pilot Hole (488-914m and 1585-2012 m) can be separated into two populations based on X-ray diffraction characteristics. The first is characterized by peaks with a width of ∼0.1-0.3 ° 2θ and a ratio of the area of the 002 chlorite peak to the 001 chlorite peak of ∼0.3-4.4, while the second exhibits peak widths of ∼0.2-1.0 ° 2θ and a peak area ratio of ∼0.1-0.6. The first population occurs in the deeper interval, and rarely in the shallower, while the second population occurs only in the shallower interval. The difference in peak area ratio indicates a difference in chemistry (most likely in octahedral iron and magnesium), and the difference in peak width indicates that the deeper samples have larger crystallites and fewer expandable interlayers than the shallower population. The X-ray characteristics for the deeper population match those for samples from protolith and cataclasite for the exhumed Punchbowl fault, while samples from the intensely-deformed ultracataclasite are dissimilar to that population. This supports previously published findings, based on scanning and transmission electron microscopy as well as X-ray techniques, that the mineral assemblages in the ultracataclasite formed after the cessation of motion along the fault, and that fault-related mineral assemblages have been overprinted by post-faulting alteration. This illustrates how detailed characterizations of the phyllosilicate mineral assemblages in the SAFOD Pilot Hole can be used to constrain interpretations of exhumed fault zones by establishing a baseline characterization of phyllosilicate mineral assemblages at depth. This allows for the recognition of fault-related and non-fault-related phyllosilicates in exhumed fault zones, aiding interpretations of fault strength and permeability structure, both of which can be strongly influenced by phyllosilicates.