A range of fault slip styles on progressively misoriented planes during flexural-slip folding,Cape Fold Belt,South Africa |
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| Institution: | 1. University of British Columbia, Department of Earth, Ocean, and Atmospheric Sciences, 2020-2207 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada;2. Geological Institute, Department of Earth Sciences, Swiss Federal Institute of Technology, Sonneggstrasse 5, Zurich 8092, Switzerland;3. University of New Brunswick, Department of Earth Sciences, 2 Bailey Dr., Fredericton, New Brunswick E3B 5A3, Canada;1. Universidade de São Paulo, Instituto de Geociências, Departamento de Geologia Sedimentar e Ambiental, Rua do Lago, 562, São Paulo, SP 05508-080, Brazil;2. Comissão Nacional de Energia Nuclear, Centro de Desenvolvimento da Tecnologia Nuclear, Serviço de Tecnologia Mineral, Rua Mário Werneck s/n, Campus UFMG, Belo Horizonte, MG 31270-901, Brazil;3. Universidade Federal de Minas Gerais, Instituto de Ciências Exatas, Departamento de Física, Avenida Antônio Carlos 6627, Campus da Pampulha, Belo Horizonte, MG 31270-000, Brazil;1. Department of Geology, University of Otago, PO Box 56, Dunedin, 9054, New Zealand;2. School of Environmental Sciences, University of Liverpool, Liverpool, L69 3GP, United Kingdom;3. School of Geography, Environment, and Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, 6012, New Zealand;1. Department of Earth and Planetary Sciences, McGill University, Montréal, QC, H3A 0E8, Canada;2. Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch, 7701, South Africa;3. School of Earth and Climate Sciences, University of Maine, Orono, ME 04469, United States;4. Council for Geoscience, 3 Oos Street, Bellville, 7530, South Africa;1. COMET, Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK;2. Université Grenoble Alpes, ISTerre, 38058 Grenoble, France;3. CNRS, ISTerre, UMR 5275, 38058 Grenoble, France;4. Research Institute for Earth Sciences, Geological Survey of Iran, Azadi Square, Meraj Blvd, Tehran, Iran;5. State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China |
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| Abstract: | Flexural slip folds are distinctive of mixed continuous-discontinuous deformation in the upper crust, as folding is accommodated by continuous bending of layers and localized, discontinuous slip along layer interfaces. The mechanism of localized, layer-parallel slip and the stress and fluid pressure conditions at which flexural slip occurs are therefore distinctive of shear localization during distributed deformation. In the Prince Albert Formation mudstone sequence of the Karoo Basin, the foreland basin to the Cape Fold Belt, folds are well developed and associated with incrementally developed bedding-parallel quartz veins with slickenfibers oriented perpendicular to fold hinge lines, locally cross-cutting axial planar cleavage, and showing hanging wall motion toward the fold hinge. Bedding-parallel slickenfiber-coated veins dip at angles from 18° to 83°, implying that late increments of bedding-parallel shear occurred along unfavorably oriented planes. The local presence of tensile veins, in mutually cross-cutting relationship with bedding-parallel, slickenfiber-coated veins, indicate local fluid pressures in excess of the least compressive stress.Slickenfiber vein microstructures include a range of quartz morphologies, dominantly blocky to elongate-blocky, but in places euhedral to subhedral; the veins are commonly laminated, with layers of quartz separated by bedding-parallel slip surfaces characterized by a quartz-phyllosilicate cataclasite. Crack-seal bands imply incremental slickenfiber growth, in increments from tens of micrometers to a few millimeters, in some places, whereas other vein layers lack evidence for incremental growth and likely formed in single slip events. Single slip events, however, also involved quartz growth into open space, and are inferred to have formed by stick-slip faulting. Overall, therefore, flexural slip in this location involved bedding-parallel faulting, along progressively misoriented weak planes, with a range of slip increments. |
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| Keywords: | Flexural slip Fold-and-thrust belts Brittle-ductile deformation Faulting Fluid pressure Hydrothermal veins |
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