Competence inversion in high-grade metamorphic rocks: Insights from mullions
Final Report Abstract
The present project was focusing on the development of mullions in two and three dimensions and their role as indicator for rheological inversion in high-grade metamorphic rocks. Natural examples collected in the Odenwald and in the southern Bohemian Massif were investigated in detail. Experimental studies on mullion formation in 2 and 3D were carried out using rock analogues. Quartzmonzodiorite of the Weschnitz pluton (Odenwald) was cut by a spessartite sill, which led to contact melt at ca. 15 km depth. The migration of contact melt into shrinkage cracks of the sill was very fast (< 4 months) and led to thin felsic veins within the sill. Subsequent constriction led to (1) subvertical prolate grain-shape fabrics, (2) increase of the sill’s dip, (3) doubling of the sill’s thickness, (4) mullions with cusps pointing into the host, and (5) boudinage of felsic veins cutting through the mafic sill. The sigmoidal shape of the foliation/lineation within the sill was caused by sill rotation, which led to shear stresses along the contacts with the host resulting in antithetic rotation of the grain-shape fabric that is subvertical in its central part and subparallel along the margins. The shape of the mullions, the boudinage of the felsic veins and the sigmoidal foliation/lineation indicate that the mafic sill was incompetent and the felsic host and veins were competent. This inversion in rheology was caused by phase-boundary diffusion that was more effective in the fine grained mafic sill. Evidence for phase-boundary diffusion in form of strongly lobate phase boundaries is widespread and concerns all of the constituent phases present within the sill, host and felsic veins. Similar results were obtained from metagranitoids and mafic sheets of the southern Bohemian Massif, where rheological inversion is indicated by (1) a stronger foliation in the mafic sheets, (2) development of mullions at shortened contacts and of inverse folds at extended contacts with the mafic cusps pointing into the felsic host, and (3) folding and boudinage of felsic veins in amphibolite matrix. Scale-model experiments, involving pure shear shortening of a single incompetent layer embedded in a competent matrix, were carried out to study the influence of the bulk strain geometry and an inclined layer on the growth rate and shape of mullions. The viscosity ratio between non-linear viscous layer and matrix varied from 1/4 – 1/35. If layer parallel shortening (eZ) is the same, the growth rate of mullions is highest under bulk constriction, moderate under bulk plane strain, and lowest under bulk flattening. Initial layer inclination under bulk constriction led to antisymmetric mullions. Increasing the viscosity ratio between layer and matrix results in (1) faster rotation of the layer towards the main stretching axis, X, (2) earlier attainment of a stable mullion wavelength, and (3) earlier formation of flames. The style of deformation (coeval folding and boudinage vs. shearing) of a thin competent sheet, inserted into the incompetent layer, is controlled not only by the viscosity ratio, but also by the bonding between competent and incompetent material. The experimental results shed light on the deformation behavior of Sederholm-type dikes and sills at deep crustal levels.
Publications
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Development of a synorogenic composite sill at deep structural levels of a continental arc (Odenwald, Germany). Part 1: Sederholm-type emplacement portrayed by contact melt in shrinkage cracks. Tectonophysics, 805, 228774.
Zulauf, G.; Gerdes, A.; Hattingen, E.; Hoefer, H.E.; Loeckle, F.; Marschall, H.R.; Nesbor, H.-D.; Petschick, R.; Reischmann, T.; Schmeling, H. & Zulauf, J.
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Development of a synorogenic composite sill at deep structural levels of a magmatic arc (Odenwald, Germany). Part 2: Rheological inversion and mullion formation under bulk constriction. Journal of Structural Geology, 155, 104525.
Zulauf, G.; Zulauf, J.; Linckens, J.; Gerdes, A.; Hattingen, E.; Loeckle, F.; Marschall, H.R. & Steckenreiter, L.
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Poster contribution, International conference DRT Deformation Mechanisms, Rheology and Tectonics, Catania 2022 Rheological inversion and mullion formation of a composite sill under bulk constriction (Odenwald, Germany)
Zulauf. J.; Linckens. J.; Gerdes, A.. Marschall, H.. Loeckle, F.; Schmeling, H. & Zulauf, G.
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Scientific Talk, International conference, 4th Bohemian Massif Meeting in Freistadt/Austria, 2022. Sederholm-type emplacement and deformation of a synorogenic composite sill at deep structural levels of a continental arc (Odenwald, Germany).
Zulauf, G.; Gerdes, A.; Linckens, J.; Loeckle, F.; Marschall, H.R.; Nesbor, H.-D.; Petschick, R.; Schmeling, H. & Zulauf, J.
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Formation of mullions in two and three dimensions: Results from analogue modelling. Journal of Structural Geology, 172, 104890.
Zulauf, J.; Zulauf, G. & Hattingen, E.
