Fluid-Rock Interaction and Weakening in Faults of the San Andreas System: Inferences from San Gabriel Fault-Rock Geochemistry and Microstructures

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Contribution to Book

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U. S. Geological Survey "Redbook" Fluids and Faulting

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Optical and scanning electron microscopy and whole rock geochemical analyses are used to investigate variations in deformation mechanisms and fluid-rock interactions in rocks at three sites on the San Gabriel fault, southern California: Pacoima Canyon, Bear Creek, and North Fork. At Bear Creek, unaltered and undeformed granite, granodiorite, and diorite protolith bound a fault core several meters thick that consists of foliated cataclasite on either side of 2–20 cm thick ultracataclasite layer. The foliated cataclasite contains clays and zeolite veins which developed by alteration of protolith during slip. The ultracataclasite consists of 20–100 μm diameter feldspar and quartz fragments embedded in a clay-zeolite matrix. The matrix consists of grains <10 μm and is enriched in Fe, Mg, Mn, and Ti relative to the average composition of the protolith. In contrast, ultracataclasite at Pacoima Canyon contains little clay and zeolite and apparently evolved with little fluid-rock interaction. Whole rock geochemical analyses of the fault rock compositions at both sites are best explained as a result of mechanical mixing with local redistribution of some elements in a closed system relative to fluids. At both sites the ultracataclasite compositions can be modeled as the result of mixing of the bounding foliated cataclasites. Similarly, the foliated cataclasites were derived by mixing the protoliths on the same side of the ultracataclasite layer. Whole rock analyses for rocks from the North Fork site, which lies on a major splay of the San Gabriel fault, suggest an open system relative to fluids. The concentration of immobile elements in the fault core relative to all protoliths is best explained by fluid-assisted volume loss of 37%±10%. Overall, the results imply local and regional-scale variations in the hydrologic setting along the San Gabriel fault that produced contrasting styles of deformation and fluid-rock interactions.

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