Thermo chronology by the fission track method of a passive marge (Ponta Grossa dome in south-eastern Brazil) and within a collision chain (external zone of the alpine arch in France)

The dating method by counting fission tracks on apatite (this rock is a geo-thermometer sensitive to weak temperature changes below 150 Celsius degrees) is an efficient tool for the thermal history of rocks. We have used this method in 2 different geological contexts: the Ponta Grossa dome in south-eastern Brazil and the alpine mountain in France. This dating method is based on the fact that some rocks like mica keep fossil remains of the passage of the fission products emitted during the simultaneous fission of uranium 238 present in the rock. This method requires the irradiation in a slow neutron flux of the sample because the initial quantity of uranium is unknown. The age t of the sample is given by the formula: t=(1/l1)*ln[1+(rs/ri)*(l2/l1)*F*σ*I] where: l1 is the alpha decay constant of U238; l2 is the simultaneous fission decay constant of U238, rs is the number of fission tracks in the sample before the irradiation; ri is the number of fission tracks induced by the irradiation; F is the thermal neutron flux; σ is the thermal fission cross-section of U235; and I is the isotopic rate U235/U238. This document is divided into 4 chapters. The first chapter presents the general principle of the method, the mechanisms capable of producing fission tracks and the techniques used to make these tracks visible with an optical microscope. The second chapter deals with the conditions of the irradiation and the calibration of the method. The 2 last chapters are dedicated to the applications to the 2 geological contexts. (A.C.)