The influence of plate kinematics, convection intensity, and subduction geometry on the Earth’s upper-mantle dynamics in the vicinity of a subduction zone

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SUMMARY This study reports the results of a 2-D numerical mantle convection model, featuring plate kinematics and non-vertical subduction. The main question we address is the combined eVects of plate velocities, convection intensity, and subduction dip angle on the Earth’s upper-mantle dynamics in the vicinity of a subduction zone. Through a systematic investigation using Rayleigh numbers in the range 1.2◊104‐6◊105 ,a motionless overriding plate, subduction dip angles of 70° ,4 5° and 30°, and subduction velocities in the range 0.3‐10 cm yr’1, we aim to study the interactions of the subducting and overriding plates with the underlying mantle. We focus on the possible transitions of flow structures, from a multicellular to a monocellular circulation, and from an unsteady to a steady global structure. Unsteady states can be either periodical or nonperiodical. The model predicts that unsteady structures are characterized by instabilities that aVect both horizontal thermal boundary layers under the moving plate, but only the upper thermal boundary layer under the motionless overriding plate. The movement of the subducting plate may be suYcient to drive the lower part of the circulation under the motionless overriding plate, characterized by an elongation of cells. This latter influence is favoured by a decreasing subduction dip angle. Thus diVerent transition relations for the flow structure are obtained according to whether we consider the moving plate or the motionless overriding one, and whether we consider a shallowor a steep-dipping subduction. By focusing first on the dynamics under the moving plate, we are better able to quantify the relationship between the buoyancy and boundary forces associated with the subducting lithosphere. For steep to intermediate subduction angles, we find, as in previous studies with no down-going slab boundary, that the greater the convection intensity, the greater the plate velocity that is required to achieve the dynamical mantle‐plate coupling. This is not, however, characteristic of a shallowdipping subduction, since we show that the down-going plate has a stabilizing influence, suggesting the existence of a critical subduction dip angle for the mantle‐plate coupling. Thus the investigation of internal dynamics under the motionless overriding plate reveals a surprising single-cell circulation under this plate for a suYciently high convection intensity. This single-cell circulation is obtained for smaller subducting plate velocities since the value of the subduction dip angle decreases. The diVerent lateral convection styles under this plate at moderate subduction velocities, namely multicellular and monocellular for dip angles of 70° and 30°, respectively, are consistent with stress states in the overriding lithosphere. Indeed, steep- and shallow-dipping subductions are likely to be associated with back-arc extension and compression, respectively. For a dip angle of 70° under the overriding plate, the model predicts an upwelling next to the trench, the location of which is consistent with observed back-arc basins.