Fluid Flow and Deformation Along the Glarus Thrust, Eastern Switzerland

The Glarus thrust is a prominent tectonic feature in the eastern Helvetic Alps. It accommodated an at least 40 kilometre northward displacement of the Helfvetic nappe system in the hangingwall of the thrust with respect to the Infrahelvetic units in the footwall. The Glarus thrust was active at sub-greenschist to greenschist facies conditions during Miozene times. Oxygen, carbon and strontium isotope patterns as well as bulk rock chemical compositions give evidence of both flow along and across the thrust. The oxygen isotope compositions of the Lochseite calc mylonite, a metre thick mylonite layer at the thrust contact, are generally low (about 12 per mil relative to V-SMOW) in the south and relatively high (about 21 per mill) in the north. This may be interpreted as an isotopic front that resulted from the northwards migration of isotopically light fluid along the thrust. This fluid was most likely derived from dehydration reactions associated with progressive metamorphism in the root zone some 20 kilometres south of the southernmost present day outcrops of the thrust. Data from sampling profiles across the thrust indicate that cross thrust fluid flow also contributed to this regional pattern. In the southern section of the thrust, where the footwall is represented by Mesozoic limestone, a downwards directed sub-vertical transport component is identified. In contrast, cross thrust flow was upwards directed in the northern section, where the footwall is represented by Tertiary flysch, from which water was expelled during thrusting. In the north, relatively high fluid pressures caused by ponding of the ascending fluids at the thrust contact favoured fracturing and strain localisation in the Lochseiten calc mylonite and in the lowermost one to two meters of the Verrucano in the hangingwall of the thrust. In the south this fluid pressure effect is missing. Nevertheless there is evidence for fracturing, but thrust related deformation migrated up into the hangingwall over several tens of meters. Microstructures and textures indicate that, apart from fracturing, pressure solution and limited crystal plasticity were active probably intermittently during deformation. The different fluid flow regimes in the northern and southern parts of the thrust appear to have had a major influence on the degree of strain localisation. Strain localisation was pronounced in the northern section, where the thrust was lubricated by fluids that ascended from the flysch in the footwall, and it was less pronounced in the southern section, where fluid supply from the footwall was missing.