Brittle-ductile deformation in the Glarus thrust Lochseiten (LK) calc-mylonite

The Glarus thrust accommodated at least 30 km of northward displacement strongly localized within a 1-m layer of 'Lochseiten' (LK) calc-mylonite. This layer displays veins in various states of plastic deformation and a wildly refolded foliated gouge texture. Lattice- and shape-preferred orientations are observed within the fine-grained, recrystallized matrix. These features indicate the alternate activity of brittle and ductile deformation mechanisms. In contrast to the classical view that grain boundary sliding (superplasticity) is the dominant deformation mechanism, it is advocated that fluids, derived from the footwall and expelled along the thrust, are responsible for hydrofracturing and cataclastic deformation. In periods between fracture events, deformation was ductile. In this new interpretation, a substantial amount of the total thrust displacement was accommodated by numerous short-lived and strongly localized fracture events at the base of the Verrucano thrust sheet, rather than a permanently weak décollement lithology.