Shear sense |
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Authors: | T H BELL S E JOHNSON |
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Institution: | Department of Geology, James Cook University, Townsville, Queensland 4811, Australia |
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Abstract: | Investigation of microstructural relationships in major movement zones in metamorphic rocks, where the sense of displacement is known from regional geological relationships, indicates numerous problems with current concepts of shear-sense criteria and their application. The direction of apparent shearing commonly conflicts from one criterion to another (e.g. from the symmetry of quartz c -axis orientation diagrams to the asymmetry of extensional crenulation cleavages). This implies that interpretations of shear sense along foliations from some mesoscale and microscale criteria have been erroneous. A new approach to interpreting shear sense, involving the use of strain fields, resolves conflicts in mesoscopic and microscopic criteria and provides a method for determining coherent shear-sense histories extending back before the last shearing event for 'any foliated metamorphic rock'. It also provides a powerful tool for determining the structural/metamorphic path that a rock has followed within an orogen. For determination of the shear sense on the last foliation developed in a rock, this approach uses geometries developed around competent heterogeneities such as quartz pebbles, pegmatite pods, veins, porphyroclasts, porphyroblasts and breccia clasts. A shear-sense history is derived by applying this approach to earlier foliations preserved within the heterogeneities and their strain shadows. |
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Keywords: | deformation partitioning porphyroblasts porphyroclasts shear-sense history |
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