Competence inversion in high-grade metamorphic rocks: Insights from mullions

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Mafic sheets in gneissose host rocks behave generally as competent layers, which are deformed by folding and boudinage. In some cases, however, the mafic sheets are incompetent and show outward pointing cusps related to mullions in shortened and inverse folds in extended contacts. This inversion in competence is hardly understood. Published results indicate that mullions in mafic sheets are particularly common if deformation occurred under upper amphibolite to granulite facies conditions. However, microfabric data of the constituent minerals of sheet and host, which could explain the competence inversion by changes in deformation mechanisms, are largely lacking. To shed more light on the growth of mullions and inverse folds in mafic sheet/gneissose host rock associations, and to explain competence inversion, we studied a natural example from the Weschnitz pluton (Odenwald) and carried out first analogue experiments. Microfabric and computertomographic (CT) studies of the natural example revealed that the mafic sheet in gneissose host rock was mullioned in a constrictional environment at T > 650°C. Both the sheet and the host show evidence for solid-state diffusion creep, which is supported not only by high temperature, but also by small grain size. As the grain size of the sheet is one order of magnitude smaller than that of the host, the strain rate in the sheet must have been faster resulting in competence inversion. This working hypothesis will be tested in the present proposal by studying further mafic sheet/gneissose host examples exposed in the southern Bohemian Massif and in the Montagne Noire. Microscopic, EBSD and CT analyses will reveal the deformation microfabrics as well as lattice- and shape-preferred orientation of the constituent minerals. Our preliminary experimental studies suggest that mullions grow particularly fast under bulk constriction. These studies will be continued using Polydimethylsiloxan (PDMS) as rock analogue to ensure linear viscous behavior, which is inherent to solid-state diffusion creep. Incompetent single layers with varying thickness, embedded in a competent matrix, will be deformed at varying viscosity ratios using a new deformation apparatus. A 3D scanning system and the use of transparent PDMS as matrix will allow see-through experiments and a continuous monitoring of the growing mullions with progressive strain. The geometric data of the experimental mullions can be used as a paleoviscosimeter to natural mullions, which can be observed not only in mafic sheet/gneissose host associations, but also in many other rocks deformed under various metamorphic conditions in different geodynamic settings. Examples of economic significance of mullions include incompetent layers of potash salt embedded in rock salt, and massive sulfide bodies with mullioned boundaries and economic concentrations of ore in the cusps.