The composition and structure of shallow portions of the San Andreas and San Gabriel Faults

James P. Evans, Rebekah Reimann, Caroline Studnicky, & Kelly K. Bradbury

Published August 14, 2018, SCEC Contribution #8551, 2018 SCEC Annual Meeting Poster #169

We investigate the composition and structure of the upper 2 km of strike-slip faults by examining core from two geotechnical investigations in the San Andreas (SAF) and San Gabriel Fault (SGF) zones. Seven steeply north inclined cores from Lake Elizabeth intersect steep south-dipping SAF zones up to 30 m thick of fault-related rocks, narrow slip surfaces, and wide zones of alteration to a depth of 140 m. We also examine a core acquired in a steeply plunging borehole across the steep north- dipping San Gabriel Fault in the western San Gabriel Mountains to a depth of ~ 500 m. The SGF zone is up to 100 m wide here.

Standard optical petrographic study of the SAF samples reveals a range of brittle deformation processes and evidence for syntectonic hydrothermal alteration. Narrow slip surfaces lie within bands of cataclasite and ‘gouge’ zones that consist of fractured and altered granitic rocks subsequently sheared. Mineralogy of the deformed rocks includes a host of alteration produces from the protolith gneiss, including epidote, chlorite, zeolites (laumontite, nontronite), clay minerals, and palygorskite. Brecciated epidote in veins document brittle overprint on higher temperature mineralization. Core from the SGF consists of granodiorite gneiss that contains indurated foliated cataclasite, narrow slip surfaces, and sheared and brecciated chlorite+zeolite zones, and injection related breccia. We used a 60 cm section of the Lake Elizabeth core to develop a new analytical method to map the geochemistry of the fault related rocks. We examined the core with macroscopic X-ray Fluorescence Spectrometry mapping to document fine-scale alteration patterns over a relatively large region of core at the SSRL, Stanford. Rapid elemental mapping of the core documents the mobility of alteration-related elements K, Na, Mg, and the concentration of less mobile Cr, Ni, and Mn.

The patterns observed in the core support a model of syntectonic hydrothermal alteration in the fault zone that represent either 1) the fault –related rocks we observe formed at depths > than 1.5 km and the fault zone was exhumed, or 2) elevated thermal gradients produce alteration in the shallow portion of the faults. The distributed deformation and alteration in these shallow levels may help explain how slip is diffused in the upper several km of a fault zone, and in part explains the shallow slip deficits observed in seismogenic faults.

Key Words
fault rocks, alteration, slip distribution

Citation
Evans, J. P., Reimann, R., Studnicky, C., & Bradbury, K. K. (2018, 08). The composition and structure of shallow portions of the San Andreas and San Gabriel Faults. Poster Presentation at 2018 SCEC Annual Meeting.

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