Résumé
The subduction interplate domain (considered either as a plane or a channel, depending on the setting), is an interface of seismogenic coupling at the scale of one seismic cycle but also of kinematic decoupling on long-term time scales. The properties of this very particular interface is likely to affect not only the seismogenic potential of the subduction area but also the overall subduction process, as it influences its viability. However, the different mechanisms governing the subduction interplate dynamics remain poorly known. For instance, we observe a great variability of the downdip limit of the seismogenic zone, between 30 and 70 km. This depth might be controled by the brittle-ductile transition occurring along the subduction channel, and could thus depend on many parameters, such as temperature, pressure, compositional variations, strain rate. This suggests a self-consistent equilibrium state of the subduction interplate, whose characteristics should depend on the subduction setting. Numerical simulations are performed to model the long-term equilibrium state of the subduction interplate when the diving lithosphere interacts with both the overriding plate and the surrounding convective mantle. The thermomechanical model combines a non-Newtonian viscous rheology and a pseudo-brittle rheology. Rock strength here depends on depth, temperature and stress, for both oceanic crust and mantle rocks. We study the evolution through time of, on one hand, the kinematic decoupling depth, and, on the other hand, of the brittle-ductile transition (BDT) depth simulated along the subduction interplate. The latter is used as a proxy for an upper bound of the downdip limit of the seismogenic zone. We previously showed that a convergence rate increase induced a shallowing of both the kinematic decoupling depth and the BDT, while it is generally thought that fast subductions promote interplate cooling and deepening. This comes from complex plate interactions with the convecting mantle wedge. We simulate the time evolution of the interplate thermal state, and study how it interacts with the global force budget of the subduction system. In some cases, a shallowing of the subduction interface, by decreasing frictional stresses resistant to subduction, is able to trigger backarc spreading, even if the tectonic regime was initially compressive. We will also present how the subducting lithosphere age affects the brittle-ductile transition depth and the kinematic decoupling depth in our model. These results will be compared to observed downdip limits of oceanic seismogenic zones, and to their statistical dependence on physical characteristics of subduction zones (convergence rate, lithosphere ages, etc).