Synconvergent and coherent (ultra)high-pressure crustal rockexhumation
Résumé
Ultrahigh-pressure (UHP) continental crustal rocks were first discovered in the Western Alps in1984 and have since then been observed at many convergent plate boundaries worldwide.Unveiling the processes leading to the formation and exhumation of (U)HP metamorphic crustalrocks is key to understand the geodynamic evolution of orogens such as the Alps.Previous numerical studies investigating (U)HP rock exhumation in the Alps predicted deep (>80km) subduction of crustal rocks and rapid buoyancy-driven exhumation of mainly incoherent(U)HP units, involving significant tectonic mixing forming so-called mélanges. Furthermore, thesepredictions often rely on excessive erosion or periods of divergent plate motion as importantexhumation mechanism. Inconsistent with field observations and natural data, application ofthese models to the Western Alps was recently criticised.Here, we present models with continuous plate convergence, which exhibit local tectonic-drivenupper plate extension enabling compressive- and buoyancy-driven exhumation of coherent (U)HPunits along the subduction interface, involving feasible erosion.The two-dimensional petrological-thermo-mechanical numerical models presented here predictboth subduction initiation and serpentinite channel formation without any a priori prescription ofthese two features. The (U)HP units are exhumed coherently, without significant internaldeformation. Modelled pressure and temperature trajectories and exhumation velocities ofselected crustal units agree with estimates for the Western Alps. The presented models supportprevious hypotheses of synconvergent exhumation, but do not rely on excessive erosion ordivergent plate motion. Thus, our predictions provide new insights into processes leading to theexhumation of coherent (U)HP crustal units consistent with observations and natural data fromthe Western Alps.
