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Extension in the Basin and Range Province and East Pacific Margin |
Department of Geosciences, The University of Arizona, Tucson, AZ 85721, USA
Some of the world’s best exposed and most thickly developed mylonites, ultramylonites, and cataclasites are found in parts of a number of mountain ranges in southeastern Arizona. These fault rocks and associated structures occupy metamorphic core complexes, which I view as mountain-size geological exposures of regional, upward-tapering ductile-brittle shear zones. The fundamental characteristics of the core complexes were fashioned by regional crustal extension in the Tertiary. Normal-slip shearing served to ‘telescope’ fault rocks and structures which had formed originally at different depth levels in the crust. The result of normal-slip simple shear within the shear zones was the translation of hanging wall crust by tens of kilometres. The shear zones and linked fault zones are interpreted to have dipped at 45° or more initially, rotating progressively to more gentle inclinations and shallower depths as the crust thinned and stretched. Progressive simple-shear under conditions of a steadily decreasing confining pressure and temperature is dramatically recorded in suites of structures and fabrics: for example mylonite gneiss is converted to microbrecciated mylonite gneiss, which in turn is transformed to cataclasite and ultracataclasite. The physical nature of this progressive deformation is beautifully exposed in the gently dipping Catalina-Rincon shear zone, whose once-deep and once-shallow parts are now exposed to view over broad expanses E and NE of Tucson, Arizona.
