Effect of fluid circulation on subduction interface tectonic processes: Insights from thermo-mechanical numerical modelling

Both geophysical and petrological data suggest that large amounts of water are released in subduction zones during the burial of oceanic lithosphere through metamorphic dehydration reactions. These fluids are generally considered to be responsible for mantle wedge hydration, mechanical weakening of the plate interface and to affect slab-interface seismicity. In order to bridge the gap between subduction dynamics and the wealth of field, petrological and experimental data documenting small-scale fluid circulation at mantle depths, we designed a bi-phase model, in which fluid migration is driven by rock fluid concentrations, non-lithostatic pressure gradients and deformation. Oceanic subduction is modelled using a forward visco-elasto-plastic thermo-mechanically and thermodynamically coupled code (FLAMAR) following the previous work by Yamato et al. (2007). After 16.5 Myr of convergence, deformation is accommodated along the subduction interface by a low-strength shear zone characterised by a weak (10–25% of serpentinite) and relatively narrow (5–10 km) serpentinized front in the reference experiment. Dehydration associated with eclogitization of the oceanic crust (60–75 km depth) and serpentinite breakdown (110–130 km depth) significantly decreases the mechanical strength of the mantle at these depths, thereby favouring the detachment of large slices of oceanic crust along the plate interface. The geometries obtained are in good agreement with reconstructions derived from field evidence from the Alpine eclogite-facies ophiolitic belt (i.e., coherent fragments of oceanic crust detached at ca.80 km depth in the Alpine subduction zone and exhumed along the subduction interface). Through a parametric study, we further investigate the role of various parameters, such as fluid circulation, oceanic crustal structure and rheology, on the formation of such large tectonic slices. We conclude that the detachment of oceanic crust slices is largely promoted by fluid circulation along the subduction interface and by the subduction of a strong and originally discontinuous mafic crust.