Fluid flow and rock alteration along the Glarus thrust

Abstract

Chemical alteration of rocks along the Glarus overthrust reflects different stages of fluid rock interaction associated with thrusting. At the base of the Verrucano in the hanging wall of the thrust, sodium was largely removed during an early stage of fluid-rock interaction, which is ascribed to thrust-parallel fluid flow in a damage zone immediately above the thrust. This alteration leads to the formation of white mica at the expense of albite-rich plagioclase and potassium feldspar. This probably enhanced mechanical weakening of the Verrucano base allowing for progressive strain localization. At a later stage of thrusting, fluid-mediated chemical exchange between the footwall and the hanging wall lithologies produced a second generation of alteration phenomena. Reduction of ferric iron oxides at the base of the Verrucano indicates fluid supply from the underlying flysch units in the northern section of the thrust. Fluid supply from the footwall may have kept pore fluid pressure close to lithostatic and enhanced cataclastic deformation. The chemical characteristics of the Lochseiten calc-tectonite suggest its derivation from Mesozoic limestone. In the southern sections of the thrust, the major element and stable isotope compositions show continuous trends from the Cretaceous limestone in the footwall of the thrust up to the contact with the Verrucano, indicating that the calc-tectonite developed due to progressive deformation from the footwall units. In the northern sections of the thrust, the Lochseiten calc-tectonite has a distinct chemical and stable isotope signature, which suggests that it is largely derived from Infrahelvetic slices, i.e. decapitated fragments of the footwall limestone from the southern sections of the thrust, which were tectonically emplaced along the thrust further north. Only at the Lochseiten type locality the original chemical and stable isotope signatures of the calc-tectonite were completely obliterated during intense reworking by dissolution and re-precipitation.