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Subduction of the lithosphere at convergent-plate boundaries takes place asymmetrically--the subducted slab sinks downward, while the overriding plate moves horizontally (one-sided subduction). In contrast, global mantle convection models generally predict downwelling of both plates at convergent margins (two-sided subduction). We carried out two-dimensional (2-D) numerical experiments with a mineralogical-thermomechanical viscoelastic-plastic model to elucidate the cause of one-sided subduction. Our experiments show that the stability, intensity, and mode of subduction depend mainly on slab strength and the amount of weak hydrated rocks present above the slab. Two-sided subduction occurs at low slab strength (sin[[phi]] < 0.15, where [phi] is effective internal friction angle), regardless of the extent of hydration. In contrast, steady-state one-sided subduction requires a weak hydrated slab interface and high slab strength (sin[[phi]] > 0.15). The weak interface is maintained by the release of fluids from the subducted oceanic crust as a consequence of metamorphism. The resulting weak interplate zone localizes deformation at the interface and decouples the strong plates, facilitating asymmetric plate movement. Our work suggests that high plate strength and the presence of water are major factors controlling the style of plate tectonics driven by self-sustaining one-sided subduction processes. Keywords: subduction zones, water, numerical modeling, slab dehydration, fluid flow.

Recent work suggests that hydrated partially molten thermal-chemical plumes that originate from subducted slab as a consequence of Rayleigh-Taylor instability are responsible for the heterogeneous composition of the mantle wedge. We use a two-dimensional ultrahigh-resolution numerical simulation involving 10 x [10.sup.9] active markers to anticipate the detailed evolution of the internal structure of natural plumes beneath volcanic arcs in intraoceanic subduction settings. The plumes consist of partially molten hydrated peridotite, dry solid mantle, and subducted oceanic crust, which may compose as much as 12% of the plume. As plumes grow and mature these materials mix chaotically, resulting in attenuation and duplication of the original layering on scales of 1-1000 m. Comparison of numerical results with geological observations from the Horoman ultramafic complex in Japan suggests that mixing and differentiation processes related to development of partially molten plumes above slabs may be responsible for the strongly layered lithologically mixed (marble cake) structure of asthenospheric mantle wedges. Keywords: numerical modeling, subduction, volcanic arcs, mantle wedge structure, subducted crust melting.