Fluid circulation, progressive deformation and mass-transfer processes in the upper crust: the example of basement-cover relationships in the External Crystalline Massifs, Switzerland

Basement and cover rocks of the external zone in the Swiss Alps are affected by Tertiary ductile deformation under greenschist facies conditions (300–450°C; 3–4.5 kbar). The analyses of stable isotope systematics of veins and shear zones, as well as variation profiles of chemical elements across major shear zones, distinguish two types of fluid-rock interactions. (i) Closed systems: most syntectonic veins within the Helvetic carbonate cover have δ¹⁸O compositions depending on the adjacent wall rock compositions and varying with respect to the initial chemical heterogeneity of each sedimentary layer. Within the granitic basement, chemical profiles across minor granite shear zones show equal volume gains and losses for each oxide Na₂O or K₂O. (ii) Open systems: variation profiles in major cover thrusts show a variable increase of ⁸⁷Sr/⁸⁶Sr ratios combined with a strong decrease in δ¹⁸O approaching an isotopic equilibrium with the basement rocks. Within the major basement shear zones, decrease in CaO and increase in MgO content are observed with progressive deformation increase. A two-step tectonic and geochemical model is proposed to explain the coexistence of open and closed systems. Fluid sources and transport mechanisms of chemical elements are discussed.