Dynamics of interplate domain in subduction zones: influence of rheological parameters and subducting plate age

Abstract

Abstract. The properties of the subduction interplate domain are likely to affect not only the seismogenic potential of the subduction area but also the overall subduction process, as it influences its viability. 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 nonNewtonian viscous rheology and a pseudo-brittle rheology. Rock strength here depends on depth, temperature and stress, for both oceanic crust and mantle rocks. I study the evolution through time of, on one hand, the brittle-ductile transition (BDT) depth, z BDT , and, on the other hand, of the kinematic decoupling depth, z dec , simulated along the subduction interplate. The results show that both a high friction and a low ductile strength at the asthenospheric wedge tip shallow z BDT . The influence of the weak material activation energy is of second order but not negligible. z BDT becomes dependent on the ductile strength increase with depth (activation volume) if the BDT occurs at the interplate decoupling depth. Regarding the interplate decoupling depth, it is shallowed (1) significantly if mantle viscosity at asthenospheric wedge tip is low, (2) if the difference in mantle and interplate activation energy is weak, and (3) if the activation volume is increased. Very low friction coefficients and/or low asthenospheric viscosities promote z BDT = z dec . I then present how the subducting lithosphere age affects the brittle-ductile transition depth and the kinematic decoupling depth in this model. Simulations show that a rheological model in which the respective activation energies of mantle and interplate material are too close hinders the mechanical decoupling at the down-dip extent of the interplate, and eventually jams the subduction process during incipient subduction of a young (20-Myr-old) and soft lithosphere under a thick upper plate. Finally, both the BDT depth and the decoupling depth are a function of the subducting plate age, but are not influenced in the same fashion: cool and old subducting plates deepen the BDT but shallow the interplate decoupling depth. Even if BDT and kinematic decoupling are intrinsically related to different mechanisms of deformation, this work shows that they are able to interact closely. Comparison between modelling results and observations suggests a minimum friction coefficient of 0.045 for the interplate plane, even 0.069 in some cases, to model realistic BDT depths. The modelled z dec is a bit deeper than suggested by geophysical observations. Eventually, the better way to improve the adjustment to observations may rely on a moderate to strong asthenosphere viscosity reduction in the metasomatised mantle wedge.

Can subduction initiation at a transform fault be spontaneous?

“…We test the effect of the box bottom condition, either closed and free-slip or open to mantle in-and outflows. When the box bottom is open, a vertical resistance against flow is imposed along the box base, mimicking a viscosity jump 10 times higher than above (Ribe and Christensen, 1994;Arcay, 2017). The results show that the bottom mechanical condition does not modify the future evolution of the fracture zone.…”

“…The tracer density is uniform over the simulation box (∼ 3.2 per km 2 ), verifying that at least nine tracers fill the smallest meshes. This numerical discretization has been tested and validated in a previous study (Arcay, 2017). Note that because the total pressure is not directly solved by the code in Christensen (1992), the lithostatic pressure is used instead in Eq.…”

Forearc structure in the Lesser Antilles inferred from depth to the Curie temperature and thermo-mechanical simulations

“…100; Fig. 8), possibly too extreme [Arcay, 2012]. Indeed, this f  value of 100 may apply to a strongly hydrated mantle (when serpentine is predicted to be stable) but might not correspond to an amphibole-bearing mantle (i.e., modeled here for water content as low as 0.5%wt).…”

“…The second one corresponds to a stiffer mantle (rheology labeled St; [Arcay, 2012;Arcay, 2017, in revision] and is used to discuss the influence of the anhydrous viscosity law on the time scale of arc thermal thinning. To take into account the influence of water content ([OH -]) on rheology, we apply a strength reduction depending exponentially on water content (Fig.…”

Tectonic Stacking of HP/LT Metamorphic Rocks in Accretionary Wedges and the Role of Shallowing Slab‐Mantle Decoupling

“…During subduction, the sinking slab and the overlying plate form a kinetically and mechanically decoupled plate interface or subduction channel controlled by the thermal structure of subduction (Arcay, , ; Arcay et al, ; Furukawa, ; Wada & Wang, ; Wada et al, ). A common maximum depth of slab‐mantle decoupling (MDD) of approximately 80 km is inferred for all subduction zones (Syracuse et al, ; Wada & Wang, ).…”