ASSESSMENT OF FAULT ZONE ARCHITECTURE ALONG A BIMATERIAL INTERFACE, A CASE STUDY SAN JACINTO FAULT, SOUTHERN CA, USA

Here we report the results of our work along a segment of the San Jacinto fault (SJF) located near Anza, CA that was exhumed from a depth of ~220 m. The studied segment is a NE-verging oblique-slip thrust that comprises part of a relic flower structure and places pre-mid Cretaceous gneiss of the Burnt Valley complex over alluvial sandstones of the Pleistocene Bautista Fm. The fault zone architecture at the study site consists of, from NE to SW, microscopically damaged sandstones of the Bautista Fm, an ~40-42 cm thick composite fault core, and an ~ 15 m thick damage zone within the Burnt Valley complex. Damage within sandstones with < 18% matrix is dominated by in situ fragmentation of some quartz grains. The principal slip surface (PSS) separates ~10-12 cm of cataclasite derived from the Bautista Fm on the NE from ~30 cm of black ultracataclasite derived from the Burnt Valley complex on the SW. The damage zone within the Burnt Valley complex consists of a 5-m thick inner part that grades to a 10-m thick outer part. The inner part is characterized by alternating lenses and seams of mesocataclasite and ultracataclasite, while the outer part is mostly fault breccia. Porosity values progressively increase across the inner damage zone toward the PSS, varying from 12% to 18%. In contrast, porosity within damaged sandstones varies from 4% to 25%, and from 13% to 17% within cataclasites derived from the Bautista Fm. Mass balance studies indicate that little to no chemical alteration occurred within the damaged sandstones of the Bautista Fm, while very minor losses in V and Nb mass occurred within mesocataclasites. In contrast, samples of the inner and outer damage zone on the Burnt Valley side are compositionally like relatively unaltered blocks collected from breccia and the undamaged wall rocks. The asymmetry of the fault zone at the study site is consistent with recent split Hopkinson pressure bar experiments reported in the literature, and clearly suggests that less damage has occurred in the more porous compliant side. We conclude that the observed asymmetry of damage is the result of a different response of lithology to similar transient loading conditions produced by large earthquakes on the SJF.