Effect of Fluid Circulation on Intermediate Depths Subduction Dynamics: From Field Observations to Numerical Modelling

Large amounts of water, released in subduction zones during the burial and dehydration of hydrothermally altered oceanic lithosphere, are expected and observed in the mantle wedge (20-30% serpentinization), just below the continental Moho. Mechanical effects of fluid circulation and dehydration on subduction dynamics remain largely unknown and require further investigation. We herein present a new fluid migration algorithm based on thermodynamic modelling where fluids are free to migrate, driven by rock fluid concentration, non-lithostatic pressure gradients and deformation. Oceanic subduction is modelled using a forward visco-elasto-plastic thermomechanical code (FLAMAR algorithm). After 15 Ma of convergence between the two plates, we show that deformation is accommodated along a low-strength shear zone in the wall of the subduction thrust interface, characterized by a weak (10-25% serp.) and relatively narrow (~6km) serpentinized front/channel. Dehydration associated with eclogitization of oceanic crust (60-75km depths) and serpentinite breakdown (110-130km depths) significantly decreases mantle mechanical strength at these depths through serpentinization, thereby favoring the detachment of large slices of oceanic crust along the plate interface. In our experiments, the resulting morphologies are in good agreement with reconstructions derived from structural field observations from the Alpine eclogite-facies ophiolitic belt (corresponding to coherent fragment of oceanic crust detached at c.80km depth in the Alpine subduction zone and exhumed along the subduction interface). Through a parametric study, we therefore investigate the role of various parameters (such as fluid circulation, oceanic crust structure and rheology) on the formation of such large tectonic slices. We conclude that detachment of oceanic crust slices is largely promoted by fluid circulation acting within the subduction interface and by the implication of strong and originally discontinuous mafic crust in the subduction zone.