Influence of melt induced mechanical anisotropy on the magnetic fabrics and rheology of deforming migmatites, Central Vosges, France

Abstract

In the Central Vosges (France), we describe and model the structural relationships existing between orthogneiss domains and their surrounding viscous migmatites. These domains show a structural and anisotropy of magnetic susceptibility (AMS) zonation associated with an increasing amount of melt from the core to their margins. A pre-extension oblate to plane strain fabric is preserved in the core of the orthogneiss domains where the granite veins were emplaced along a dilated mechanical anisotropy. The internal margin of the orthogneiss bodies exhibits development of a prolate fabric and active buckling of an orthogneiss/granite multilayer. The external margin of the orthogneiss bodies is characterized by oblate fabrics resulting from a layer perpendicular shortening of a melt-orthogneiss multilayer, pinch-and-swell structures and extensional kink bands along which granite veins have been injected. The surrounding metasedimentary migmatites show a similar structural history but the degree of fabric resetting is significantly higher. This systematic fabric and structural succession is due to contrasting rheological evolution between orthogneiss and metasedimentary migmatites during progressive melting and continuously evolving degree of mechanical anisotropy in the stronger orthogneiss. Numerical modeling confirms the role of the relationship between initial mechanical anisotropy and superimposed deformation overprints on finite AMS fabrics in migmatites.