Mantle Plumes in the Vicinity of a Subducting Plate: The Ordinary Interaction

Abstract:

The interaction between a mantle plume and a subducting plate depends on 1) the relative position of the plume relative to the subducting plate, and 2) the relative "strength" of the slab compared to that of the plume. To study such an interaction, we are using three-dimensional laboratory models of a compositional plume within the vicinity of an entirely dynamically driven but fixed slab. We introduced a dimensionless number to calibrate the relative "strength" of the slab compared to that of the plume that is the ratio of the buoyancy flux of the slab by that of the plume. Here we describe the style of the interaction for three positions of the plume relative to the fixed slab, and for a plume buoyancy flux that was fixed and chosen to be about 20 times lower than the slab’s flux. In nature, this would be equivalent to consider a subducting plate of relatively narrow width (1000 km) and moderate buoyancy flux (20000 kg/s. e.g. Manila slab) impacting on the geometry and dynamics of a mantle plume of modest buoyancy flux (1000 kg/s. e.g. Canary plume). Under these conditions, we show that 1) rising plumes can be dragged either away or towards a slab in rollback motion by the mantle return flow, 2) the plume motion can vary from slight deflection to rotation, and 3) significant flattening of the plume can occur. Yet, no extreme impact of subducting plates on plumes is to be observed. To this end, the slab/plume buoyancy flux ratio needs to vary. For instance, in the case of plumes initiated at the apex of the slab tip and at a larger slab/plume buoyancy flux ratio, a plume head can detach from its conduit. Conversely, for the same initial slab/plume geometry but at a smaller slab/plume buoyancy flux ratio, a front plume head can be dragged down into subduction. The question that remains to be answered is at which buoyancy flux ratios occur the transitions from head detachment to "simply" head sucked towards the trench to head plume being subducted. This is currently being studied in our laboratory.