High to ultra-high pressure metamorphism in the Alpine orogeny: Challenges and controversies

International audienceDepending on observed parageneses, the pressure-temperature (P-T) conditions experienced by rocks can beevaluated and quantified by using thermodynamic calculations. Time (t) is obtained by dating mineralsinvolved in symptomatic parageneses, considered at equilibrium. The resulting P-T-t paths are then used toconstrain the tectonic history, most of the time, by assuming that the pressure recorded by metamorphicrocks directly depends on burial depth. However, in regions where deformation is large and heterogeneous,like in the Alps, the above assumption is questionable. I will show that the pressure history, including acatastrophic pressure drop at the onset of exhumation, as recorded in most well documented high-pressuremetamorphic rocks worldwide (including those from the Alps) fit remarkably well with a two-fold tectonichistory, from compressional during subduction to extensional during exhumation.This contradicts the common assumption that (ultra)high-pressure rocks exhume during continentalcollision. However, numerous natural examples of (ultra)high-pressure rocks, from early Paleozoic to lateTertiary, document that exhumation occurred in extension either driven by slab rollback prior to continentalcollision (e.g. in numerous backarc basins) or by eduction, after continental collision (e.g. NorwegianCaledonides). The so-called “Alpine collision” appears to correspond to two successive events of slabrollback leading to the extensional exhumation of (ultra)high-pressure rocks, prior to continental collision