Oxygen, carbon and strontium isotope systematics in two profiles across the Glarus thrust: implications for fluid flow

Abstract

The Glarus thrust is a prominent tectonic feature in the eastern Helvetic Alps. It has been recognized as a potential major pathway for syntectonic crustal scale fluid flow. The oxygen, carbon and strontium isotope patterns obtained from two vertical profiles across the thrust indicate fundamentally different flow regimes in the southern section of the thrust, where the footwall is represented by Mesozoic limestones, and in the northern section, where the footwall is represented by Tertiary flysch. At the Grauberg locality in the south, the observed isotope patterns give evidence of a net mass transport component from the hanging wall Verrucano to the footwall limestone with a maximum time-integrated volumetric fluid flux of 6.1 m3/m2 In the south, the hydration of the lowermost 10 to 20 m of the hanging wall Verrucano requires introduction of an aqueous fluid by subhorizontal flow along the thrust with a minimum time integrated flux of 240 m3/m2. At the Lochseite locality in the north, the isotope patterns indicate a vertical mass transport component from the footwall flysch to the hanging wall Verrucano with a time-integrated fluid flux of 2.6 m3/m2. In the north, the fluids were probably derived from compaction and dehydration of the footwall flysch during thrusting. The ascending fluids were ponded below the Verrucano and 'lubricated' the thrust. Short-term pressure drops associated with seismic motion along the thrust led to the precipitation of calcite in veins at the thrust surface contributing material to the Lochseiten calc-mylonite, a thin calc-mylonite layer at the thrust contact. Although cross thrust fluid flow may have been two to three orders of magnitude smaller than flow along the thrust, it had a major impact on the isotopic composition of the Lochseiten calc-mylonite. In particular, it buffered the oxygen isotope composition of the calc-mylonite towards the relatively 18O-depleted composition of the hanging wall Verrucano in the south and towards the relatively 18O-enriched compositions of the footwall flysch in the north. By this mechanism a regional south to north 18O-enrichment trend was simulated within the Lochseiten calc-mylonite.