HP-UHP exhumation during slow continental subduction : Self-consistent thermodynamically and thermomechanically coupled model with application to the Western Alps
Résumé
The exhumation of ultra-high pressure (UHP) crustal material often occurs at the rear of sedimentary accretion wedges (e.g., Alps, Himalaya, Norway). However, the mechanisms of deep (> 100 km) burial in slow (< 1-2 cm yr− 1) continental collision zones, where thermal diffusion competes with advection, resulting in weak slabs and Rayleigh-Taylor instabilities, may be different from those inferred from common kinematic models that are more applicable to fast convergence. In this study, we provide a thermodynamically and thermomechanically consistent numerical model explaining the mechanisms of exhumation of continental material in slow convergence zones such as the well-studied Western Alps. The results of the experiments are compatible with topographic and structural observations, and with pressure and temperature estimates from metamorphic petrology studies. The reported bimodal exhumation rate of HP rocks, fast at the initial (> 10 mm yr− 1) and slow at the later stage (< 4 mm yr− 1), is also well reproduced. The presence of a double-layered continental crust in the subducting plate leads to self-localization of non-predefined crustal splitting zones at the level of the brittle-ductile transition, from which the low-density continental material is exhumed. We conclude that syn-convergent exhumation at the rear of the accretionary wedge is a transient process (~ 10 Myr) largely controlled by buoyancy forces in the depth interval of 100-35 km, and by erosion at shallower depths, without significant impact from slab break-off.