Asymmetrical Fault Zone Architecture: The case for a different response of lithology to similar transient loading conditions along the San Jacinto fault, SE California, USA
Advisor: Dr. Gary H. Girty
May 4th, 2017 – CSL 422, 1:15 am
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Here we report the results of our work along a segment of the San Jacinto fault located near Anza, California 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 paragneiss of the Burnt Valley complex over alluvial variably porous (~4% – ~25%) sandstones of the Pleistocene Bautista Formation. The fault zone architecture at the study site consists of, from NE to SW, microscopically damaged sandstones of the Bautista Formation, an ~10-12 cm thick zone of cataclasite series rocks, an ~18 cm thick black fault core, and an ~ 15 m thick damage zone within the Burnt Valley complex. The ~18 cm thick black fault core separates the NE less stiff and more compliant sandstone side of the fault zone, from the SW more stiff and less compliant metamorphic rock side. Damage within sandstones is non-pervasive and dominated by in-situ intragranular fragmentation of some framework quartz grains. Protocataclasite and mesocataclasite derived from sandstones dominate the narrow relatively porous (~13%-17%), ~10-12 cm wide zone of cataclasite series rocks. The volume of the < 2 µm fraction extracted from multiple samples of the black fault core was not large enough for clay mineral XRD analysis. These results along with microscopic observations suggests that the black fault core is characterized by microcrystalline to cryptocrystalline solid granular material. Lying along the NE side of the black fault core is the ~5-meter-wide inner damage zone of the SW more stiff and less compliant metamorphic rock side. Centimeter-scale fragments of leucocratic pulverized plutonic material occur within the inner most ~25-30 cm wide part of this zone which is characterized by ultracataclasite, mesocataclasite, and protocataclasite. Porosity values progressively decrease from a high of ~30% in the outer part of the inner damage zone to an average value of ~14% in the inner part adjacent to the black fault core. The outer, ~10 m thick, damage zone is composed mostly of crackle and chaotic breccia. Porosities in breccia blocks vary from ~3% – ~7%, and are like those measured in undamaged wall rocks. Mass balance studies indicate that little to no chemical alteration occurred within the damaged sandstones of the Bautista Formation. In contrast, very minor losses in V and Nb mass occurred within mesocataclasites adjacent to the black fault core. In contrast, within the inner damage zone of the stiffer and less compliant metamorphic rock side, the masses of Fe, Mg, Ti, and V were increased while the mass of Co was decreased relative to analyzed undamaged blocks from fault breccia and wall rock samples. Moreover, a well-defined clay mineral assemblage of smectite (saponite), kaolinite, and illite is characteristic of the inner damage zone. Hence, the water/rock ratios within the inner damage zone on the less compliant side were likely greater than in any other part of the studied fault zone. 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 non-pervasive damage has occurred in the more porous and compliant sandstone side. In contrast, more pervasive damage and chemical alteration occurred in the stiffer less compliant metamorphic rock 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 San Jacinto fault.