A new model for the formation of a spaced crenulation (shear band) cleavage in the Dalradian rocks of the Tay Nappe,SW Highlands,Scotland |
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| Institution: | 1. Programa de Pós-Graduação em Geociências, Universidade Federal de Mato Grosso (UFMT), Avenida Fernando Correa da Costa, s/n, CEP: 78.060-900, Cuiabá, MT, Brazil;2. Faculdade de Geociências (FAGEO-UFMT), Avenida Fernando Correa da Costa, s/n, CEP: 78.060-900, Cuiabá, MT, Brazil;3. Grupo de Pesquisa em Evolução Crustal e Tectônica-Guaporé (FAGEO-UFMT), Avenida Fernando Correa da Costa, s/n, CEP: 78.060-900, Cuiabá, MT, Brazil;4. Facultad de Ciencias Exactas y Naturales, Departamento de Geologia, FACEN, Universidad Nacional de Asunción, San Lorenzo, Paraguay;5. Laboratório de Geocronologia, Instituto de Geociências, Universidade de Brasília (UnB), s/n, CEP: 70919-970, Brasília, DF, Brazil;6. Programa de Pós-Graduação em Geologia, Universidade de Brasília (UnB), Campus Universitário Darcy Ribeiro ICC, Asa Norte, CEP: 70919-970, Brasília, DF, Brazil;1. Laboratoire Géoazur, Université Nice-Sophia Antipolis, Nice, France;2. Géosciences Montpellier, Université Montpellier 2, Montpellier, France;3. Laboratoire de Géosciences Appliquées, Faculté des Sciences, Université Mohammed 1, Oujda, Morocco;4. GeoEcoMar Bucharest, 23-25 Dimitrie Onciul Street, RO-024053 Romania;5. National and Kapodistrian University of Athens, Faculty of Geology and Geoenvironment, Athens, Greece;6. Laboratoire de géologie de Lyon, Terre, Planètes et Environnement, UMR 5276, Université Lyon 1, Lyon, France;7. Département de Géologie, Université Abdelmalek Esaadi, 93003 Tetuán, Morocco;1. Technical University Munich, Tectonics and Material Fabrics Section, Arcisstr. 21, 80333 Munich, Germany;2. Università della Basilicata, Dipartimento di Scienze, Via dell’Ateneo Lucano 10, 85100 Potenza, Italy;3. Università degli Studi di Bari, Dipartimento di Scienze della Terra e Geo-Ambientali, Via Orabona 4, 70126 Bari, Italy;4. Technische Universität München, Lehrstuhl für Ingenieurgeologie, Arcisstr. 21, 80333 Munich, Germany;1. CONICET, IESGLO, Instituto de Sedimentología, Facultad de Ciencias Naturales y Fundación Miguel Lillo, Universidad Nacional de Tucumán, Miguel Lillo 205-251, T4000JFE, San Miguel de Tucumán, Argentina;2. IGEBA, CONICET-Depto. Cs. Geológicas, FCEyN, Univ. Buenos Aires, Pabellón 2, Ciudad Universitaria, C1428EHA, Buenos Aires, Argentina;1. State Key Laboratory for Geomechanics and Deep Underground Engineering, School of Mechanicals and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, China;2. School of Civil Engineering, Hefei University of Technology, Anhui 230009, China |
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| Abstract: | The main conclusion of this study is that non-coaxial strain acting parallel to a flat-lying D1 spaced cleavage was responsible for the formation of the D2 spaced crenulation (shear band) cleavage in Dalradian rocks of Neoproterozoic-Lower Ordovician age in the SW Highlands, Scotland. The cm-dm-scale D2 microlithons are asymmetric; have a geometrically distinctive nose and tail; and show a thickened central portion resulting from back-rotation of the constituent D1 microlithons. The current terminology used to describe crenulation cleavages is reviewed and updated. Aided by exceptional 3D exposures, it is shown how embryonic D2 flexural-slip folds developed into a spaced cleavage comprising fold-pair domains wrapped by anastomosing cleavage seams. The bulk strain was partitioned into low-strain domains separated by zones of high non-coaxial strain. This new model provides a template for determining the sense of shear in both low-strain situations and in ductile, higher strain zones where other indicators, such as shear folds, give ambiguous results. Analogous structures include tectonic lozenges in shear zones, and flexural-slip duplexes. Disputes over the sense and direction of shear during emplacement of the Tay Nappe, and the apparently intractable conflict between minor fold asymmetry and shear sense, appear to be resolved. |
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| Keywords: | Microlithon Vergence Non-coaxial Flexural-slip Back-rotation |
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