Exhumation of high-pressure metamorphic rocks: Why extension would not be the rule?

International audienceSince the discovery of ultra-high pressure (UHP) metamorphic rocks in the Alps and the Norwegian Caledonides,more than three decades ago, the exhumation of high pressure metamorphic rocks has been most often consideredto occur in the frame of continental collision. However, numerous natural examples of (U)HP rocks, from earlyPaleozoic to late Tertiary, document that exhumation occurred in extension either driven by slab rollback prior tocontinental collision (e.g. in backarc basins like in Mediterranean, Papua New Guinea, Carribean) or by eductionafter continental collision (e.g. in Norway). We first summarize the mechanisms of extensional exhumation in slabrollback and eduction modes, using numerical models and natural examples. We then present a reappraisal of thetectonic history of some famous natural UHP rocks examples (e.g. Alps and Himalayas) either showing, or stronglysuggesting, that exhumation occurred prior to continental collision likely driven by slab rollback. In addition, werecently showed that the pressure history, including a catastrophic pressure drop at the onset of exhumation, that isrecorded in most well documented (U)HP rocks worldwide fits remarkably well with a two-fold tectonic history,from compressional during subduction to extensional during exhumation. On the basis of all above argumentswe strongly suggest that the extensional exhumation of (U)HP metamorphic rocks, instead of being one mode ofexhumation among others, is more probably the rule