Several types of amorphous material in ultracataclastic core samples recovered from 3194 m and 3294 m depth of the main bore hole of the San Andreas Fault Observatory at Depth are identified and described with transmission electron microscopy and scanning electron microscopy. We observed (1) amorphous material on a slickenside surface, (2) glassy bands contained in an ultracataclastic matrix and (3) amorphous rims surrounding quartz or feldspar clasts. Chemical analyses of the amorphous material reveal that silica content is slightly enriched or similar as in the adjacent matrix. We suggest that all amorphous material was formed by comminution of clasts (crush‐origin pseudotachylytes) rather than by melting (melt‐origin pseudotachylytes). The observed amorphous phases may act as lubricating layers that reduce friction in the San Andreas Fault.
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Janssen et al. (2010) studied this question.
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