Deformation induced quartz-fluid oxygen isotope exchange during low-grade metamorphism: an example from the Glarus thrust, E Switzerland

The Glarus overthrust is a large thrust fault in the Helvetic Alps of eastern Switzerland, where Helvetic decollement nappes were thrust northwards over Infrahelvetic units at subgreenschist (in the north) to greenschist (in the south) facies conditions during the Miocene. In the northern section of the thrust, a minimum of 35 kilometers of displacement was largely accommodated by deformation of a calc-mylonite layer (about one to three meter thick) between the Tertiary flysch in the footwall and the siliciclastic Verrucano formation in the hangingwall of the thrust and by deformation of the lowermost two to five meters of the Verrucano formation. Pressure solution, cataclastic flow and limited dynamic recrystallization were the prime deformation mechanisms in the siliciclastic rocks. Oxygen isotope exchange between quartz and a pore fluid that ascended from the dewatering flysch in the footwall was effective only in the highly strained portions at the base of the Verrucano, In contrast, quartz was largely inreactive with respect to oxygen isotope exchange at distances of more than about 2 meters above the thrust, where thrust related deformation is practically absent. The correlation between deformation intensity and the extent of quartz oxygen isotope alteration suggests that deformation was the prime control for quartz-fluid oxygen isotope exchange. Fracturing during cataclastic flow probably generated permeability, which allowed fluids to migrate from the flysch in the footwall into the Verrucano in the hangingwall of the thrust. Dissolution-reprecipitation, grain size reduction by fracturing and limited grain boundary migration associated with bulging recrystallization were the mechanisms that allowed for significant quartz-fluid oxygen isotope exchange at the base of the Verrucano.