Fluid regimes in the deformation of the Helvetic nappes, Switzerland, as inferred from stable isotope data

Abstract

The stable isotope composition of veins, pressure shadows, mylonites and fault breccias in allochthonous Mesozoic carbonate cover units of the Helvetic zone show evidence for concurrent closed and open system of fluid advection at different scales in the tectonic development of the Swiss Alps. Marine carbonates are isotopically uniform, independent of metamorphic grade, where δ 13C=1.5±1.5 (1 σ) and δ 18O=25.4±2.2 (1 σ). Total variations of up to 2‰ in δ 13C and 1.5‰ in δ 18O occur over a cm scale. Calcite in pre- (Type I) and syntectonic (Type II) vein arrays and pressure shadows are mostly in close isotopic compliance with the matrix calcite, to within ±0.5‰, signifying isotopic buffering of pore fluids by host rocks during deformation, and closed system redistribution of carbonate over a cm to m scale. This is consistent with microstructural evidence for pressure solution — precipitation deformation. Type III post-tectonic veins occur throughout 5 km of structural section, extend several km to the basement, and accommodate up to 15% extension. Whereas the main population of Type III veins is isotopically undistinguishable from matrix carbonates, calcite in the largest of these veins is depleted in 18O by up to 23‰ but acquired comparable δ 13C values. This generation of veins involved geopressurized hydrothermal fluids at 200 to 350° C where δ 18O H2O=−8 to +20‰, representing variable mixtures of 18O enriched pore and metamorphic fluids, with 18O depleted meteoric water. Calc-mylonites (δ 18O=25 to 11‰) at the base of the Helvetic units, and syntectonic veins from the frontal Pennine thrust are characterized by a trend of 18O depletion relative to carbonate protoliths, due to exchange with an isotopically variable reservoir (δ 18O H2O=20 to 4‰). The upper limiting value corresponds to carbonate-buffered pore fluid, whereas the lower value is interpreted as 18O-depleted formation brines tectonically expelled at lithostatic pressure from the crystalline basement. Carbonate breccias in one of the large scale late normal faults exchanged with infiltrating 18O-depleted meteoric surface waters (δ 18O=−8 to −10‰). During the main ductile Alpine deformation, individual lithological units and associated tectonic vein arrays behaved as closed systems, whereas mylonites along thrust faults acted as conduits for tectonically expelled lithostatically pressured reservoirs driven over tens of km. At the latest stages, marked by 5 to 15 km uplift and brittle deformation, low 18O meteoric surface waters penetrated to depths of several km under hydrostatic gradients.