The relationship between foliation and strain: an experimental investigation |
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| Authors: | BE Hobbs WD Means PF Williams |
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| Institution: | Department of Earth Sciences, Monash University, Clayton, Victoria 3168, Australia;Department of Geological Sciences, State University of New York at Albany, Albany, NY, U.S.A.;Department of Geology, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada |
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| Abstract: | A multilayered salt/mica specimen with embedded strain markers was shortened to produce a fold and the distribution of strain was subsequently mapped out over the profile plane. On a fine scale the initial foliation, which is parallel to the undeformed layers, is folded by tight kinks to produce two new foliations; one is defined by the preferred orientation of kink boundaries and the other by the preferred orientation of (001) of mica. In the hinge region of the fold the first of these new foliations is parallel to the local λ1λ2-principal plane of strain whereas the preferred orientation of mica is bimodal and is symmetrical about the λ1λ2-plane. Elsewhere the two new foliations are not parallel to the principal plane of strain and angular divergencies of up to 30–35° are measured. If a March model with initial random mica orientation is assumed for the development of mica preferred orientation then the correct value of strain is predicted but the orientation of the principal plane of strain can be grossly in error. A theoretical analysis of the angular relationships to be expected between kink boundaries and the λ1λ2-plane of strain confirms that for the type of geometries experimentally developed, large divergences of up to 35° should be common. In rocks where the foliation has developed by processes similar to those recorded here, large angular divergencies between the foliation and the λ1λ2-principal plane of strain should be expected as the rule. |
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