Pervasively pulverized pink potassium feldspar along the Mojave section of the SAF shows evidence for large-scale heat transfer by post-seismic fluid flow

Franciscus M. Aben, Kathryn Farrington, Boris G. Sanchez, Thomas M. Mitchell, Stephen A. Miller, Thomas K. Rockwell, Gary H. Girty, Giles Ostermeijjer, & Neta Wechsler

In Preparation 2022, SCEC Contribution #11872

Fault zones provide conduits for (transient) fluid flow and heat transfer in the upper crust, and may leave only traces within the geological record of what now appears to be a dry fault zone. Here we report on such traces in damage zone rock samples at two locations along the Mojave section of the San Andreas Fault (SAF). The damage zone consists of pulverized and damaged granitic rock with a pink hue, and were exhumed from a depth not greater than 1 to 2 km. The SAF at the two field locations experienced different structural histories, with the Lake Hughes site situated along the long-lived single strand of the fault, while the second site (Littlerock) is located along a younger strand in an old step-over structure between older strands of the SAF. The Littlerock site exhibits a more complex damage zone structure and experienced more intense alteration, with measured microstructures, petrophysical properties, mineral content, and geochemistry all indicating extensive hydrothermal alteration. The observed pink hue in Littlerock samples resulted from alteration of plagioclase to K-feldspar and hematite, suggesting a minimum temperature of 150ºC for hydrothermal alteration of the fault damage zone rocks. We focus on episodic post-seismic fluid flow from depth to explain the presence of the hot fluids required for the observed alteration, given that the maximum exhumation depth of 2 km limits rock temperature to about 50°C. Numerical fluid flow simulations suggest that post-seismic fluid flow is a viable mechanism to explain our observations, and also provide minimum values for fault zone permeability and overpressure.
 

Citation
Aben, F. M., Farrington, K., Sanchez, B. G., Mitchell, T. M., Miller, S. A., Rockwell, T. K., Girty, G. H., Ostermeijjer, G., & Wechsler, N. (2022). Pervasively pulverized pink potassium feldspar along the Mojave section of the SAF shows evidence for large-scale heat transfer by post-seismic fluid flow. Journal of Geophysical Research, (in preparation).