Mylonites form from rock flour ground up along faults. The rocks have received a good deal of study but progress has been difficult because of their fine, almost impalpable grain size. There is considerable current interest in mylonites not only because they may yield information on earthquake mechanisms but also because old faults may act as deep conduits for the flow of metamorphic and hydrothermal fluids. The proposed research includes: (1) determination of the sizes of domains over which chemical and isotopic equilibrium is achieved in mylonites; (2) measurement of reaction progress and chemical metasomatism to estimate fluid-rock ratios; and (3) adapting Rb-Sr and U-Pb isotopic methods to dating mylonitization. These methodologies will be applied to mylonites from the Brevard fault zone, Georgia, the Hope Valley sheartone, Connecticut, and the Borrega Springs mylonite zone, California. The comparison of detailed studies of mylonites from a variety of P-T fluid environments should make it possible to isolate important factors controlling the development of mylonites.
