Insights into fault processes and the geometry of the San Andreas fault system: Analysis of core from the deep drill hole at Cajon Pass, California

David H. Forand, James P. Evans, Susanne U. Janecke, & Joseph Jacobs

Published October 15, 2017, SCEC Contribution #7093

The Cajon Pass Deep Drillhole Project, drilled 4 km northeast of the San Andreas Fault, southern California, sampled 3013 vertical meters of deformed crystalline rocks with a range of borehole geophysical methods, and acquired 109.3 m of core in crystalline rocks at 50 spots along the borehole. Analyses of samples from outcrop and core show that the upper 2 km of the crustal column is deformed by narrow brittle fractures and faults. The lower 1700 m of the core is cut by at least 25 steeply dipping faults, 11 of which are newly identified here. Faults at 2500, 2800, 3200, and 3400 m measured depth have altered damage zones up to 20 m thick with thin mineralized slip surfaces. With increasing depth the fault zones contain indurated foliated laumontite and chlorite-rich catalasites with evidence for semi-brittle deformation mechanisms, evidence for diffusive mass transfer and solution transfer. Whole-rock geochemistry and microstructural analyses indicate that semi-brittle deformation was associated with low-temperature laumontite + chlorite ± epidote hydrothermal mineralization. Several of the altered fault zones are associated with low sonic velocity and higher porosity than the host rock. We examine the data in light of a new model for a dipping San Andreas Fault (Fuis et al., 2012), and suggest that the faults encountered by the borehole at 2500 to 3400 m might be related to the steeply dipping, slightly listric Cleghorn fault, and that the faulted rocks cored near the base of the borehole sampled the damage zone of hydrothermally altered rock of the northeast-dipping San Andreas Fault and/or Cleghorn faults. This interpretation, when combined with the SHmax orientation in the borehole 30 oriented at 57° ± 19°, is consistent with a weak dextral-reverse slip component on a northeast-dipping San Andreas Fault here. The low coefficient of friction for the San Andreas Fault required by the heat flow data and stress orientations here may be created by synkinematic hydrothermal zeolite ± chlorite alteration in the 2.2 -3.7 km deep fault zones on faults that are more optimally oriented for slip than a vertical fault.


Citation
Forand, D. H., Evans, J. P., Janecke, S. U., & Jacobs, J. (2017). Insights into fault processes and the geometry of the San Andreas fault system: Analysis of core from the deep drill hole at Cajon Pass, California . Geological Society of America Bulletin, 117. doi: https://doi.org/10.1130/B31681.1. https://doi.org/10.1130/B31681.1


Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)