Mylonitization in the lower plate of the Buckskin-Rawhide detachment fault,west-central Arizona: Implications for the geometric evolution of metamorphic core complexes |
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| Institution: | 1. Department of Earth Sciences, University of Ottawa, Ottawa, Canada;2. Department of Geodynamics and Sedimentology, University of Vienna, Vienna, Austria;3. Department of Lithospheric Research, University of Vienna, Vienna, Austria;1. Tectonophysics Laboratory, School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, Republic of Korea;2. MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China;1. GET, Université de Toulouse, CNRS, IRD, UPS, (Toulouse), France;2. Boston College, Dept Earth & Environmental Sciences, 140 Commonwealth Avenue, Chestnut Hill, MA 02467, USA;3. School of Earth and Environment, The University of Western Australia, Perth, Australia;4. CRPG-CNRS, 15 rue Notre Dame des Pauvres, 54501 Vandoeuvre les Nancy, France |
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| Abstract: | In metamorphic core complexes it is commonly unclear whether lower plate mylonites formed as the down-dip continuation of a detachment fault, or whether they represent a subhorizontal shear zone that was captured by a more steeply dipping detachment fault. Detailed microstructural, fabric, and strain data from mylonites in the Buckskin-Rawhide metamorphic core complex, west-central Arizona, constrain the structural development of the lower plate shear zone. Widespread exposures of ～22–21 Ma granitoids of the Swansea Plutonic Suite enable us to separate Miocene strain coeval with core complex extension from older deformation. Mylonites across the lower plate consistently record top-to-the-NE-directed shear. Miocene quartz and feldspar deformation/recrystallization mechanisms indicate ～450–500 °C mylonitization temperatures that were relatively uniform across a distance of ～35 km in the extension direction. Quartz dynamically recrystallized grain sizes do not systematically vary in the extension direction. Strain recorded in the Swansea Plutonic Suite is also relatively uniform in the extension direction, which is incompatible with models in which lower plate mylonites form as the ductile root of a major detachment fault. Altogether these data suggest the mylonitic shear zone initiated with a ≤4° dip and was unroofed by a more steeply dipping detachment fault system. Lower plate mylonites in the Buckskin-Rawhide metamorphic core complex thus represent a captured subhorizontal shear zone rather than the down-dip continuation of a detachment fault. |
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