Fluids, and water in particular, play an integral role in deformation within the Earth’s crust over a wide range of physical conditions. At low temperatures (<150°C), the effect of water is dominantly mechanical, largely through the effects of pore fluid pressure. At higher temperatures (>300°C), water-related chemical processes, such as diffusive mass transfer, advective mass transfer, and hydrolytic weakening dominate. In the transition between high and low temperature regimes, both mechanical and chemical processes operate and interact in complex ways.
The Cove fault zone in south central Pennsylvania contains several mapscale blocks of quartz arenite, which display a wide range of brittle and ductile microstructures. Abundant evidence of fluids is present from map- to latticescales, including quartz precipitated in microveins, fluid inclusion planes, and zones of cataclasis, and the removal of quartz along dissolution surfaces. Three different aqueous fluids were identified based on the cathodoluminescence color of quartz precipitated by that fluid, and the homogenization (Th), last ice melting temperatures (Tm), and salt species from aqueous, two-phase fluid inclusions. Fourier Transform Infrared (FTIR) spectroscopy data indicate that a large amount of water has become incorporated in the lattice of deformed detrital grains, especially those in the proximity of fluid conduits (e.g., microveins). The fluids, based on the oxygen isotopic composition of the quartz precipitated, appear to be derived from basinal brines or metamorphic fluids that were probably driven northwestward along regional aquifers and fault zones.
The microstructures in the Cove fault zone indicate that water was important in controlling the operative deformation mechanisms. Although brittle deformation is prevalent, a significant amount of ductile deformation is present. The formation of ductile microstructures at temperatures of less than 250° C is attributed to the presence of water that lowered the temperature threshold for ductile deformation. These fluids not only promoted diffusive mass transfer, but penetrated grain interiors, thereby promoting hydrolytic weakening and crystalplastic deformation. Furthermore, the chemistry of these fluids may account for the extreme amount of quartz dissolution found in portions of this fault zone.
|School:||Bowling Green State University|
|School Location:||United States -- Ohio|
|Source:||MAI 57/05M(E), Masters Abstracts International|
|Keywords:||Deformation, Ductile, Fluid, Fluid pathways, Quartz, Quartz arenites|
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