Subduction: the role of downgoing plate buoyancy in driving motions

***** CE SEMINAIRE EST ANNULE EN RAISON DE LA GREVE ***** The negative buoyancy of subducting plates, slab pull, is generally thought to be the main driving force for mantle convection and plate motions. Although plates with attached subducting slabs move faster than plates without, regional subduction motions and downgoing slab morphology do not correlate well with the age-controlled density of the downgoing plate. To characterize the effects of downgoing plate density on subduction motions and slab morphology, we ran a set of free subduction models, driven only by buoyancy of the downgoing plate and resisted passively by the mantle and in some cases an overriding plate. Our models are fully dynamic, unlike the majority of subduction models where plate motion, trench motion and/or slab geometry are prescribed. We find that while the slab is confined to the upper mantle, motions should have a clear correlation with plate buoyancy, while slab dip is strongly controlled by rheology, and the balance of forces near the bend. When compared with global subduction motions and dips, we find that present-day absolute motions of the downgoing plate exhibit the expected signature of plate density, but trench motions appear hampered by other forces. Dips indeed do not correlate with plate age, and are generally so high that plates must be close to yielding in the bend. Present-day downgoing plate motions are close to the slab's upper mantle Stokes sinking velocities, requiring low traction at the base of the plate (as for an asthenosphere that is 2-3 orders of magnitude lower in viscosity than the average upper mantle). Past downgoing plate motions exhibit no age correlation. In fact, often the youngest plates move the fastest. We attribute the episodes of fast motion to the preferential lower mantle penetration of young plates. -------------------------------------------------------------------------------- Séminaires généraux de l'Institut de Physique du Globe de Paris --------------------------------------------------------------------------------