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Slab-plate coupling via downbending and GPE
  • Dr. Dan Sandiford
Dr. Dan Sandiford

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The coupling between subducted slabs and trailing plates is often conceptualised in terms of a net in-plane force. If a significant fraction of upper-mantle slab buoyancy (e.g. ~ 25%) were transferred in this manner, a net in-plane force on the order of 5-10 TN/m would be typical of the trailing plates. Results from a numerical subduction model are presented here which question both the magnitude and-perhaps more profoundly-the mode of force transmission. In this model the subducting plate (SP) driving force is predominantly supplied by differences in gravitational potential energy (GPE). The GPE associated with plate downbending (flexural topography) provides about half the total driving force. The magnitude of the trench GPE is related to the amplitude of topography, but is mediated by the internal stress distributions associated with bending. Above the elastic core, the stress is Andersonian and vertical normal stresses are lithostatic. This implies horizontal gradients in the vertical normal stress, across columns of different elevation in the outer slope. The bulk of the trench GPE arises from this upper, extensional section the lithosphere. Vertical shear stress (and horizontal gradients thereof) are concentrated in the elastic core of the slab, where principal stresses rotate through 90 degrees. In this region, horizontal gradients in vertical normal stress rapidly diminish; they fully equilibrate at about twice the neutral plane depth. For the deepest trenches on Earth, these relationships imply trench GPE of up to about 5 TN/m. The model demonstrates that mantle slabs can drive plate tectonics simply through downbending, where the predominant mode of slab-plate coupling is via the vertical shear force and bending moment.