Transtensional folding |
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| Institution: | 1. Department of Earth Science, Museum of Natural History, University of Bergen, Postboks 7803, N-5007 Bergen, Norway;2. Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455, USA;1. South Australian Museum, GPO Box 234, Adelaide, SA, 5000, Australia;2. School of Earth Sciences, The University of Melbourne, Vic., 3010, Australia;3. School of Earth and Environmental Sciences, James Cook University, Qld., 4811, Australia;1. CNRS, CRPG-Université de Lorraine, UMR 7358, 15 rue Notre Dame des Pauvres, F-54501 Vandœuvre-lès-Nancy, France;2. Ecole Nationale Supérieure de Géologie-Université de Lorraine, 2 Rue du doyen Marcel Roubault, F-54518 Vandœuvre-lès-Nancy, France;1. Australian Research Council of Excellence for Core to Crust Fluid Systems/GEMOC, Department of Earth and Planetary Sciences, Macquarie University, NSW, 2109, Australia;2. Geological Survey of Western Australia, 100 Plain Street, East Perth, WA, 6004, Australia |
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| Abstract: | Strain modeling shows that folds can form in transtension, particularly in simple shear-dominated transtension. Folds that develop in transtension do not rotate toward the shear zone boundary, as they do in transpression; instead they rotate toward the divergence vector, a useful feature for determining past relative plate motions. Transtension folds can only accumulate a fixed amount of horizontal shortening and tightness that are prescribed by the angle of oblique divergence, regardless of finite strain. Hinge-parallel stretching of transtensional folds always exceeds hinge-perpendicular shortening, causing constrictional fabrics and hinge-parallel boudinage to develop.These theoretical results are applied to structures that developed during oblique continental rifting in the upper crust (seismic/brittle) and the ductile crust. Examples include (1) oblique opening of the Gulf of California, where folds and normal faults developed simultaneously in syn-divergence basins; (2) incipient continental break-up in the Eastern California-Walker Lane shear zone, where earthquake focal mechanisms reflect bulk constrictional strain; and (3) exhumation of the ultrahigh-pressure terrain in SW Norway in which transtensional folds and large magnitude stretching developed in the footwall of detachment shear zones, consistent with constrictional strain. More generally, folds may be misinterpreted as indicating convergence when they can form readily in oblique divergence. |
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| Keywords: | Transtension Folding Shear zones Oblique divergence Constrictional strain |
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