Deformational behaviour of migmatites and problems of structural analysis in migmatite terrains

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Abstract
An understanding of the response of migmatites to deformation is crucial to an interpretation of their structures, and in anatectic and intrusive migmatite terrains due consideration must be given to the modification of deformation processes imposed by melts. In partially molten systems containing more than 30% melt the classical theories of fold formation are inapplicable; the folds produced are of the ‘viscous fold’ type. Their geometry is unpredictable and may mimic structures due to refolding; this is particularly true at very high degrees of melting where flow of melt carrying rafts of solids begins. Failure to distinguish ‘viscous folds’ from the products of refolding will lead to errors in interpreting the structural and metamorphic history of an area, and to consequent misinterpretation of the true structural relations between geometrically complex high-grade and more regularly deformed low-grade areas.