Thermomechanics of an extensional shear zone,Raft River metamorphic core complex,NW Utah |
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| Institution: | 1. Instituto de Geociências, Universidade de São Paulo, Rua do Lago 562, Cidade Universitária, São Paulo, SP CEP 05508-080, Brazil;1. Department of Earth Sciences, ETH Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland;2. Department of Geological Sciences, University of Manitoba, 125 Dysart Rd, Winnipeg, Manitoba, R3T 2N2, Canada;3. Department of Geosciences, University of Padova, Via Gradenigo 6, 35131 Padova, Italy;1. Instituto de Geociências, Universidade de São Paulo, Rua do Lago 562, CEP 05508-080 São Paulo, SP, Brazil;2. CPRM – Geological Survey of Brazil, Rua Costa 55, Consolação, CEP 01304-010 São Paulo, SP, Brazil |
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| Abstract: | A detailed structural and microstructural analysis of the Miocene Raft River detachment shear zone (NW Utah) provides insight into the thermomechanical evolution of the continental crust during extension associated with the exhumation of metamorphic core complexes. Combined microstructural, electron backscattered diffraction, strain, and vorticity analysis of the very well exposed quartzite mylonite show an increase in intensity of the rock fabrics from west to east, along the transport direction, compatible with observed finite strain markers and a model of ``necking'' of the shear zone. Microstructural evidence (quartz microstructures and deformation lamellae) suggests that the detachment shear zone evolved at its peak strength, close to the dislocation creep/exponential creep transition, where meteoric fluids played an important role on strain hardening, embrittlement, and eventually seismic failure.Empirically calibrated paleopiezometers based on quartz recrystallized grain size and deformation lamellae spacing show very similar results, indicate that the shear zone developed under stress ranging from 40 MPa to 60 MPa. Using a quartzite dislocation creep flow law we further estimate that the detachment shear zone quartzite mylonite developed at a strain rates between 10?12 and 10?14 s?1. We suggest that a compressed geothermal gradient across this detachment, which was produced by a combination of ductile shearing, heat advection, and cooling by meteoric fluids, may have triggered mechanical instabilities and strongly influenced the rheology of the detachment shear zone. |
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| Keywords: | Thermomechanics Rheology Metamorphic core complex Extensional detachment Quartz deformation Deformation lamellae Paleopiezometry |
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