The Meso-Cenozoic thermo-tectonic evolution of the Eastern Pyrenees: an ~(40)Ar/~(39)Ar fission track and (U-Th)/He thermochronological study of the Canigou and Mont-Louis massifs

摘要

Vertical displacements on the SW-NE Tet fault (Eastern Pyrenees Axial Zone, France), which separates the Variscan Canigou-Caranca and Mont-Louis massifs, were constrained using a thermochronologic multi-method approach. ~(40)Ar/~(39)Ar data from the granitic Mont-Louis massif record its Variscan cooling history and reveal no ages younger than Early Cretaceous, while the Canigou-Caranca gneiss massif records systematically younger ~(40)Ar/~(39)Ar ages. These younger ~(40)Ar/~(39)Ar ages in the Canigou-Caranca gneiss massif are the result of partial to total rejuvenation of argon isotopic systems related to a thermal flow coeval with the Cretaceous HT-BP meta-morphism in the North Pyrenean Zone. Only the deepest rocks from the Canigou-Caranca suffered this extensive Mid-Cretaceous thermal overprint probably due to differential burial around 4 km at that time. The post Mid-Cretaceous vertical displacements along the Tet fault are recorded by "low" temperature thermochronology using K-feldspar ~(40)Ar/~(39)Ar, zircon and apatite fission track and (U-Th)/He datings. The Mont-Louis granite samples experienced a long period of protracted cooling reflecting a lack of thermo-tectonic activity in this area from Late Palaeozoic to Early Cenozoic, followed by cooling from 55-60 Ma to Late Eocene at a mean rate of 15-20℃/Ma in the final stage. This cooling stage corresponds to Tet fault reactivation with a reversed component, promoting exhumation of the Mont-Louis roof zone contemporaneously with the south-vergent Pyrenean thrusting. In the Canigou-Caranga massif, the main cooling event occurred from 32 to 18 Ma at a maximum rate of 30℃/Ma during Early Oligocene followed by a more moderate rate of 3℃/Ma from Late Oligocene to Early Burdigalian, coeval with the normal reactivation of the Tet fault in brittle conditions that accommodated the final exhumation of the massif during the opening of the Gulf of Lion.