Stable isotope and Ar-40/Ar-39 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 delta(18)O
values (up to 6 parts per thousand) 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 delta(13)C values of calcite is observed 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 -54 parts
per thousand +/- 8 parts per thousand (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
delta(18)O and delta D values (versus SMOW) for the fluid in equilibrium
with the analyzed minerals is 12 parts per thousand to 16 parts per
thousand and -30 parts per thousand to +5 parts per thousand,
respectively, for assumed temperatures of 250 to 450 degrees 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 Ar-49/Ar-39 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 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, which 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 the thrust surface dates the end of
both fluid flow and tectonic transport.