Digital Appendix of Masters Thesis "Constraining deformation mechanisms of damage zones: A case study of the shallow San Andreas Fault at Elizabeth Lake, Southern California"

We performed macroscopic, optical petrographic, scanning electron microscopy, and geochemical analyses on rock core acquired across the San Andreas Fault at Elizabeth Lake, California, in order to understand the distribution and accommodation of fault-related slip and energy within the shallow damage zone of this continental scale strike-slip fault. We characterized the deformation structures, alteration textures, and elemental variabilities to constrain the properties of the uppermost ~2 km fault-related damage zone at this site. We identified evidence for coseismic slip in the form of pulverized rocks, injection veins, clay-clast aggregates, and pseudotachylyte, and aseismic slip through calcite twins, dilatant vein fills, and possibly by a network of aligned chlorite, biotite, and muscovite cleavages. Alteration assemblages indicate a temperature range of ~50-250°C with calcite, zeolite, chlorite, and pseudomorphic plagioclase as the most common mineral alterations. The geochemical data do not
exhibit trends as a function of distance from the fault core but pointed to distinct geochemical signatures for fault-rock types including the Si-rich pulverized rocks and a relative enrichment in the fault gouge Fe concentrations as compared to the quartzo-feldspathic protolith. The cross-cutting relationships of deformation microstructures and the distribution of the observed deformation and alteration textures indicate that coseismic and aseismic slip mechanisms are active throughout the shallow fault zone at this site. We document brittle-plastic cross cutting relationships within thin sections indicating that the slip mechanisms vary in their nature and style over multiple earthquake cycles. We show that the fault zone at Elizabeth Lake consists of a complex of superimposed fault strands that recycle and rework fault-related products and likely distribute the seismically generated energy throughout the damage zone through a combination of brittle and semi-brittle processes, and geochemical alterations. The observed deformation and alteration textures decrease the seismic velocities and reduce the overall rheologic strength of the fault-related rocks, and likely enable seismically radiated energy to be distributed throughout the fault damage zone. The distribution, nature, and degree of deformation and alteration found here may help explain how fault-related low-velocity zones form, and characterize the rocks within which shallow co-seismic slip deficits, if real, may develop.