A stable and 40Ar/39Ar isotope study of a major thrust in the Helvetic nappes (Swiss Alps): Evidence for fluid flow and constraints on nappe kinematics

Abstract

Stable isotope and 40 Ar/ 39 Ar measurements were made on samples associated with a major tectonic discontinuity in the Helvetic Alps, the basal thrust of the Diablerets nappe (external zone of the Alpine Belt) in order to determine both the importance of fluids in this thrust zone and the timing of thrusting. A systematic decrease in the δ 18 O values (up to 6‰) of calcite, quartz, and white mica exists within a 10- to 70-m-wide zone over a distance of 37 km along the thrust, and they become more pronounced toward the root of the nappe. A similar decrease in the δ 13 C values of calcite is observed only in the deepest sections (up to 3‰). The δD SMOW (SMOW = standard mean ocean water) values of white mica are −54‰ ± 8‰ ( n = 22) and are independent of the distance from the thrust. These variations are interpreted to reflect syntectonic solution reprecipitation during fluid passage along the thrust. The calculated δ 18 O and δD values (versus SMOW) for the fluid in equilibrium with the analyzed minerals is 12‰ to 16‰ and −30‰ to +5‰, respectively, for assumed temperatures of 250 to 450 °C. The isotopic and structural data are consistent with fluids derived from the deep-seated roots of the Helvetic nappes where large volumes of Mesozoic sediments were metamorphosed to the amphibolite facies. It is suggested that connate and metamorphic waters, overpressured by rapid tectonic burial in a subductive system escaped by upward infiltration along moderately dipping pathways until they reached the main shear zone at the base of the moving pile, where they were channeled toward the surface. This model also explains the mechanism by which large amounts of fluid were removed from the Mesozoic sediments during Alpine metamorphism. White mica 40 Ar/ 39 Ar ages vary from 27 Ma far from the Diablerets thrust to 15 Ma along the thrust. An older component is observed in micas far from the thrust, interpreted as a detrital signature, and indicates that regional metamorphic temperatures were less than about 350 °C. The plateau and near plateau ages nearest the thrust are consistent with either neocrystallization of white mica or argon loss by recrystallization during thrusting, which may have been enhanced in the zones of highest fluid flow. The 15 Ma 40 Ar/ 39 Ar age plateau measured on white mica sampled exactly on the thrust surface dates the end of both fluid flow and tectonic transport.