Coarse-grained gabbros from two different localities in the Gets nappe
(Upper Prealps) have been dated by U-Pb and Ar-40/Ar-39 isotopic
analyses. Zircons from both gabbros gave identical concordant U-Pb ages
of 166 +/- 1 Ma (Fig. 4). Amphibole from one of them gave an Ar-40/Ar-39
plateau age of 165.9 +/- 2.2 Ma (Fig. 5). This concordance implies that
166 +/- 1 Ma is the age of magmatic crystallization of these gabbros.
The Gets wildflysch with its mafic and ultramafic lenses is an
ophiolitic melange, that we infer to come from a proximal part of the
accretionary prism at the foot of the active SE margin of the Piemont
ocean. In this position we can expect to find remnants of the oldest
parts of the Piemont oceanic crust. These are the first high-precision
dates using modern techniques from an Alpine ophiolite and are in
excellent agreement with the following:
1) The few, somewhat younger, reliable ages on ophiolites from the
probable continuation of the Piemont basin into the Apennines and
Corsica;
2) Recent data on the age of the first supra-ophiolitic sediments (Late
Bathonian to Early Callovian radiolarites);
3) The structural and stratigraphic evolution of the Brianconnais (s.s.)
domain, the future NW margin of the Piemont ocean. We note a remarkable
coincidence, in Late Bajocian time, between: (A) the end of tensile
fracturing in the Brianconnais continental crust; (B) the beginning of
its subsidence; (C) the age of the Gets ophiolites. This coincidence is
consistent with an ocean opening mechanism based on a combination of
subhorizontal extension and thermally driven vertical movements of the
lithosphere.