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

This study reports the results of a 2-D numerical mantle convection model, featuring plate kinematics and non-vertical subduction. The main question w e address is the combined effects of plate velocities, convection intensity , and subduction dip angle on the Earth's upper-mantle dynamics in the vici nity of a subduction zone. Through a systematic investigation using Rayleig h numbers in the range 1.2 x 10(4)-6 x 10(5), a motionless overriding plate , subduction dip angles of 70 degrees, 45 degrees and 30 degrees, and subdu ction velocities in the range 0.3-10 cm yr(-1), we aim to study the interac tions of the subducting and overriding plates with the underlying mantle. W e focus on the possible transitions of flow structures, from a multicellula r to a monocellular circulation, and from an unsteady to a steady global st ructure. Unsteady states can be either periodical or non-periodical. The mo del predicts that unsteady structures are characterized by instabilities th at affect both horizontal thermal boundary layers under the moving plate, b ut only the upper thermal boundary layer under the motionless overriding pl ate. The movement of the subducting plate may be sufficient to drive the lo wer part of the circulation under the motionless overriding plate, characte rized by an elongation of cells. This latter influence is favoured by a dec reasing subduction dip angle. Thus different transition relations for the f low structure are obtained according to whether we consider the moving plat e or the motionless overriding one, and whether we consider a shallow- or a steep-dipping subduction. By focusing first on the dynamics under the movi ng plate, we are better able to quantify the relationship between the buoya ncy and boundary forces associated with the subducting lithosphere. For ste ep 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 mantl e-plate coupling. This is not, however, characteristic of a shallow-dipping subduction, since we show that the down-going plate has a stabilizing infl uence, suggesting the existence of a critical subduction dip angle for the mantle-plate coupling. Thus the investigation of internal dynamics under th e motionless overriding plate reveals a surprising single-cell circulation under this plate for a sufficiently high convection intensity. This single- cell circulation is obtained for smaller subducting plate velocities since the value of the subduction dip angle decreases. The different lateral conv ection styles under this plate at moderate subduction velocities, namely mu lticellular and monocellular for dip angles of 70 degrees and 30 degrees, r espectively, are consistent with stress states in the overriding lithospher e. Indeed, steep- and shallow-dipping subductions are likely to be associat ed with back-are extension and compression, respectively. For a dip angle o f 70 degrees under the overriding plate, the model predicts an upwelling ne xt to the trench, the location of which is consistent with observed back-ar e basins.