Thesis defense of Luc de Hoÿm de Marien

Pressure-temperature-time evolution of the Variscan high-pressure units of the Massif-Central: Geodynamic implications


The Variscan belt in France is interpreted as a collisional orogen where crustal thickening and subsequent thermal relaxation account for most of the metamorphic and structural evolution. The role of the earlier subduction event on the tectonometamorphic record is not well understood yet.
This study combines a petrological analysis using numerical modelling of phase equilibria and petrologically constrained LA–ICP–MS, U/Pb dating on eclogites from the Massif Central (Haut-Allier). Dating of the zircons from the eclogites indicates a Devonian (c. 360 Ma) age of subduction of an Ordovician (c. 470 Ma) oceanic crust. Burial of the sample is indicated by the prograde P–T path followed by partial melting during isobaric heating that testifies to heat and fluid influx to the subducted crust at high-pressure conditions. Subsequent near isothermal decompression points to a fast exhumation, consistent with the U/Pb closure age of apatite indicating a ~10 Myr duration of the exhumation and subsequent cooling.
It is argued that the early high-temperature event is inconsistent with a thermal relaxation during a continental collision. It is proposed that a slab steepening at the onset of subduction of continental ribbons and consecutive thinning of the lithospheric mantle of the upper plate allowed an asthenospheric flow against the subducted crust accounting for heating at high-pressure. Induced partial melting of the subducted crust facilitated slab detachment and allowed the roll back of the subducting slab resulting in the exhumation the high-pressure rocks during a period of extension.