Project Abstract
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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. . 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.
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.
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