Reaction progress during mylonitization of basaltic dikes along the S�rv thrust, Swedish Caledonides

Abstract

The mylonite zone at the base of the Sarv thrust sheet, Swedish Caledonides, contains diabase dikes which record intense deformation and syntectonic greenschist facies metamorphism. An angular shear strain of γ≈100 is calculated for a single dike which can be followed for 50 m in the mylonite zone and abundant centimeter thick greenschist layers imply shear strains in excess of 1000. This extraordinary amount of deformation is comparable to the largest strains attained during experimental superplastic deformation of metals and alloys and, by analogy, suggests that dike deformation was macroscopically superplastic. The progress of five syntectonic reactions was measured as a function of increasing strain for the continuously exposed dike in order to assess the contribution of reactionenhanced ductility and fluid-rock interactions to strain localization along the thrust. Reaction progress calculations suggest that the breakdown of amphibole to form weaker phyllosilicates (which are added to the incompetent matrix fraction) is the important strain softening mechanism below γ≈100. The ultimate tectonite is a stable biotite-epidote schist comprised of a uniformly fine grain size (< 200 μm), constant grain shapes and strain free grains. Below γ≈40, metamorphism was isochemical and shear strain was independent of H2O in the reactions. Petrologic fluid:rock ratios are low and suggest that deformation could have occurred under relatively dry conditions.