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

The Glarus thrust accommodated at least 30 km of northward displacement str ongly 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 feature s indicate the alternate activity of brittle and ductile deformation mechan isms. In contrast to the classical view that grain boundary sliding (superp lasticity) is the dominant deformation mechanism, it is advocated that flui ds, derived from the footwall and expelled along the thrust, are responsibl e for hydrofracturing and cataclastic deformation. In periods between fract ure events, deformation was ductile. In this new interpretation, a substant ial amount of the total thrust displacement was accommodated by numerous sh ort-lived and strongly localized fracture events at the base of the Verruca no thrust sheet, rather than a permanently weak decollement lithology.