Timing and deformation conditions of the western Idaho shear zone, West Mountain, west-central Idaho
N. BraudyRichard M. GaschnigDiane WilfordJeffery D. VervoortCarlicia NelsonC. DavidsonMaureen KahnBasil Tikoff
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Abstract:
The western Idaho shear zone is a major, lithospheric-scale structure separating accreted terranes of the Blue Mountains from continental North America. We document the occurrence of the western Idaho shear zone in West Mountain, west-central Idaho. Rocks deformed by the western Idaho shear zone at West Mountain are dominantly orthogneisses, although exposures on West Mountain containing screens of metamorphosed sedimentary rocks are also present. Steeply E-dipping, N-NNE–oriented foliations and downdip lineations characterize the fabric in the orthogneisses, consistent with dextral transpressional kinematics. The foliation orientation changes from 005° to 024° from the northern to the southern part of the field area, and this is interpreted to reflect a primary along-strike variation in the orientation of the western Idaho shear zone. The westernmost unit in West Mountain (Four Bit Creek tonalite) has a U-Pb zircon age of 101 ± 3.0 Ma, yet it is only weakly deformed. We interpret this unit to have been emplaced pretectonically, thus constraining the initiation of the western Idaho shear zone. The youngest unit at West Mountain is the undeformed Rat Creek granite (88.2 ± 3.3 Ma). U-Pb analyses of zircons from orthogneisses at West Mountain span ages of 111–91 Ma, indicating both precursory and continuous magmatism coeval with western Idaho shear zone deformation. Two Lu-Hf garnet isochron ages, 97.3 ± 0.7 Ma and 99.5 ± 1.4 Ma, are interpreted to indicate peak metamorphism during western Idaho shear zone deformation. Geochemical analyses suggest that the westernmost exposed orthogneiss units are dominantly derived from continental material in West Mountain, and yet there is also evidence for a component of accreted terrane rocks at depth east of the western Idaho shear zone.Keywords:
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The north-north-east-trending (NNE) shear zones, which are defined by the Jiekou shear zone and Huangmao shear zone, are located in the eastern Jiangnan Orogen. This article provides kinematics, geochronological data, and dynamics on the Jiekou and Huangmao shear zones and serves as a reference for understanding the tectonic evolution in the South China Block during the Early Palaeozoic orogeny. All shearing signatures show that the Huangmao shear zone is dextral, and the Jiekou shear zone is sinistral. Both shear zones have certain genetic connections. The rocks from the Jiekou and Huangmao shear zones are metamorphosed and deformed into protomylonite and mylonite. Results of the deformation fabrics of feldspar and quartz and quartzc-axis fabric analysis indicate that the ductile deformation temperatures of the Jiekou shear zone sinistral shearing are similar to 400-500 degrees C. Dextral shearing with medium-high temperature (450-550 degrees C) is mainly recorded in the Huangmao shear zone. In combination with the previous geochronology, NNE-trending sinistral and dextral shearing in the eastern Jiangnan Orogen occurred at 465-430 Ma, coeval with the Early Palaeozoic orogeny. Therefore, the geodynamics of the NNE-trending shear zones is related to a major strain transformation from NW to WNW, which is remotely affected by the intracontinental convergence of Yangtze and Cathaysia blocks during the Early Palaeozoic orogeny.
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geodynamics
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Three large transcurrent systems, with related magmatism, are identified in southern Peru. A -40-80km wide, N150-trending, dextral corridor (CECLLA =Cusco-Lagunillas-Laraqueri-Abaroa structural corridor) is characterized by thick Oligocene basic lavas and numerous intrusions. The other two systems are oriented -N12S 130 and sinistral, and cartographically related to the CECLLA. Miocene to Quaternary basic magmas are associated to the northeastern system (SFUACC = Urcos-Ayaviri-Copacabana-Coniri faultsystem). In thesouthwestern region, the abundant Late Cretaceous-Paleocene arc magmatism seems related to the Incapuquio-EI Castillo fault system (SFIEC).
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The AMS studies were done on magnetite type granite, the Lavadores granite which is an example of late to post-orogenic Varsican granite in Northern Portugal (NW Iberian Peninsula). The post tectonic emplacement is supported by the magmatic nature of the microstructures. In the northern region Magnetic lineations are sub-vertical and sub-horizontal in the southern region. Magnetic foliations display a general WNW-ESE sub-vertical pattern consistent with a dextral kinematic. These AMS data suggest that the emplacement of the Lavadores granite was controlled by a major mechanical anisotropy formed by trans-tensional structures associated with the Porto-Tomar shear zone.
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