Date of Award:

1997

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

James P. Evans

Abstract

Three traverses across the exhumed trace of the Punchbowl fault zone in the 11 Pelona Schist, southern California, were examined at the millimeter to kilometer scales to determine the morphology, deformation mechanisms, and geochemistry of the fault zone in schistose rocks. The Pelona Schist is predominantly a quartz-albite-muscovite-actinolite schist with associated minor metabasalts. The Punchbowl fault zone, which is exhumed 2-4 km, has 44 km of right lateral slip, and is composed of a fault core enveloped by a damaged zone.

The fault core is a region of extreme slip localization that records most fault displacement. Deformation in the fault core is dominated by grain-size reduction accompanied by fluid-dominated alteration, in contrast to the damage zone, where deformation is dominated by brittle and semi-brittle fracturing, cataclasis, and slip. Portions of the Punchbowl fault have multiple fault cores, with each fault core less than 10 cm thick. The thickness of a fault depends upon the type of deformation measured and the scale of observation. Mesoscopic fractures begin 50 m from the fault core, whereas the onset of fault-related microfractures occurs at approximately 40 m from the fault core . A geochemical signature based on whole-rock geochemistry suggests a fault thickness of less than 10 m. Grain-size reduction occurs over a 10 m thick region and mineralogic changes occur over a region 20-30 m thick. Reorientation of preexisting foliation occurs over a zone 30 m thick.

Fault-core morphology and textures are similar to those in the Punchbowl fault to the northwest, where it displaces sedimentary rocks, and the San Gabriel fault, which formed in crystalline rocks. This suggests that the processes that form foliated, finegrained, cataclasite-dominated fault cores of large displacement faults are similar for diverse lithologies. The composition of the fault core is variable along strike, with little geochemical or mineralogical homogenization. Whereas processes that form fault cores are similar, localized interaction of fluids caused compositional variability within the core. Changes in fault composition and fluid-rock interactions may result in different fault properties and behaviors.

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