A STABLE AND AR-40 AR-39 ISOTOPE STUDY OF A MAJOR THRUST IN THE HELVETIC NAPPES (SWISS-ALPS) - EVIDENCE FOR FLUID-FLOW AND CONSTRAINTS ON NAPPE KINEMATICS/

Stable isotope and Ar-40/Ar-39 measurements,were made on samples assoc iated with a major tectonic discontinuity in the Helvetic Alps, the ba sal thrust of the Diablerets nappe (external zone of the Alpine Belt) in order to determine both the importance of fluids in this thrust zon e and the timing of thrusting. A systematic decrease in the delta(18)O values (up to 6 parts per thousand) of calcite, quartz, and white mic a 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 na ppe. A similar decrease in the delta(13)C values of calcite is observe d only in the deepest sections (up to 3 parts per thousand). The delta D-SMOW (SMOW = standard mean ocean water) values of white mica are -5 4 parts per thousand +/- 8 parts per thousand (n = 22) and are indepen dent of the distance from the thrust. These variations are interpreted to reflect syntectonic solution reprecipitation during fluid passage along the thrust. The calculated delta(18)O and delta D values (versus SMOW) for the fluid in equilibrium with the analyzed minerals is 12 p arts per thousand to 16 parts per thousand and -30 parts per thousand to +5 parts per thousand, respectively, for assumed temperatures of 25 0 to 450 degrees C. The isotopic and structural data are consistent wi th fluids derived from the deep-seated roots of the Helvetic nappes wh ere large volumes of Mesozoic sediments were metamorphosed to the amph ibolite facies, It is suggested that connate and metamorphic waters, o verpressured by rapid tectonic burial in a subductive system escaped b y upward infiltration along moderately dipping pathways until they rea ched the main shear zone at the base of the moving pile, where they we re channeled toward the surface, This model also explains the mechanis m by which large amounts of fluid were removed from the Mesozoic sedim ents during Alpine metamorphism. White mica Ar-49/Ar-39 ages vary from 27 Ma far from the Diablerets thrust to 15 Ma along the thrust. An ol der component is observed in micas far from the thrust, interpreted as a detrital signature, and indicates that regional metamorphic tempera tures were less than about 350 degrees 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, wh ich may have been enhanced in the zones of highest fluid flow. The 15 Ma Ar-40/Ar-39 age plateau measured on white mica sampled exactly on t he thrust surface dates the end of both fluid flow and tectonic transp ort.