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Determination of volume loss and element mobility patterns associated with the development of the Copper Basin fault, Picacho State Recreation Area, SE California, USA
Colby, Thomas Anthony
Girty, Gary H.
Since it was originally mapped in the 1970’s, the structure referred to herein as the Copper Basin fault has been incorrectly interpreted as an “arcuate”, north-dipping normal fault. Detailed structural analysis reveals that the ~EW trending Copper Basin fault dips between 35° and 62° to the south and displays a reverse-sense of slip, placing amphibolite-grade rocks as old as 161.3 ± 2.4 Ma in the hanging wall over the 25.2 ± 0.4 Ma unmetamorphosed Quechan volcanics in the footwall. The fault core and adjacent damage zones of the Copper Basin fault are well preserved in a vertical bank cut by stream incision. We collected 48 samples, from four distinct architectural zones (fault core, inner damage zone, outer damage zone, and protolith) within the footwall block. Each sample was analyzed for major, trace, and rare earth element (REE) compositions, thin section petrography, bulk and grain densities, and clay mineralogy. Changes in elemental mass, bulk mass, volumetric strain, and porosity were then calculated for the fault core, inner damage zone, and outer damage zone relative to the protolith. A number of statistical tools and plots suggest that the masses of the REE, and in particular Yb and Tm, were conserved during the production of the four architectural zones within the footwall block. Utilizing Yb as the framework element, bulk mass changes within the fault core were calculated to be on the order of -26% ± 5%. Significant elemental mass changes were documented across all four architectural zones, with the greatest changes in the fault core. Porosity and volumetric strain both increase toward the slip surface reaching a maximum in the inner damage zone and then dropping significantly in the fault core where volumetric strains reach as low as -20% ± 6%. In the damage zone and unaffected wall rocks, smectite is the most prominent clay species, whereas illite dominates the fault core. The conversion of smectite to illite suggests that temperatures may have reached up to ~150°C during the production of the fault core. A number of studies have interpreted mass changes and clay mineral reactions, similar to those documented in this paper, to indicate high water/rock ratios within the fault core. Such interpretations imply that during rupture, permeability must have increased within the fault core allowing it to act as a pathway for hot, chemically active fluids. During and after rupture, CO2 and SiO2 rich fluids moved into the inner and outer damage zones where calcite and quartz precipitation healed the fractured and permeable rock. Such processes are characteristic of fault-valve behavior and indicate that the Copper Basin fault may have formed through the reactivation of an older structure as the western North American plate-margin changed from a convergent to transform setting.
San Diego State University
Master of Science (M.S.) San Diego State University, 2012
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