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

Philippe Yamato (1)

1 GR - Géosciences Rennes

Depending on observed parageneses, the pressure-temperature (P-T) conditions experienced by rocks can be evaluated and quantified by using thermodynamic calculations. Time (t) is obtained by dating minerals involved in symptomatic parageneses, considered at equilibrium. The resulting P-T-t paths are then used to constrain the tectonic history, most of the time, by assuming that the pressure recorded by metamorphic rocks 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 a catastrophic pressure drop at the onset of exhumation, as recorded in most well documented high-pressure metamorphic rocks worldwide (including those from the Alps) fit remarkably well with a two-fold tectonic history, from compressional during subduction to extensional during exhumation. This contradicts the common assumption that (ultra)high-pressure rocks exhume during continental collision. However, numerous natural examples of (ultra)high-pressure rocks, from early Paleozoic to late Tertiary, document that exhumation occurred in extension either driven by slab rollback prior to continental collision (e.g. in numerous backarc basins) or by eduction, after continental collision (e.g. Norwegian Caledonides). The so-called “Alpine collision” appears to correspond to two successive events of slab rollback leading to the extensional exhumation of (ultra)high-pressure rocks, prior to continental collision.

Domains: Tectonics; Petrography

Philippe Yamato. High to ultra-high pressure metamorphism in the Alpine orogeny: Challenges and controversies. Emile Argand Conference on Alpine Geological Studies 2019, Sep 2019, Sion, Switzerland. pp.92. ⟨insu-02392455⟩