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Morra Gabriele, ETH Zürich (Switzerland)
Saskia Goes, Imperial College (United Kingdom)
Fabio Antonio Capitanio, Monash University, Clayton (Australia)
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The dynamics of plate tectonics are strongly related to those of subduction. We gain new insights in the thus far elusive dominant forces in subduction, by comparing relations between subduction motions and dips as predicted by a fully dynamic model for free subduction (i.e., driven solely by downgoing plate buoyancy while resisted passively by the mantle and upper plate), with data for the major subduction zones from the recent compilation by Sdrolias and Muller (2006). We find that at the present day, subduction and sinking velocities are as for slabs driven by their own upper-mantle buoyancy, with Stokes velocities that require effective average upper mantle viscosity variations of less than 20\% and effective slab widths of 2000-3000 km. Dips and subduction velocities imply that effective plate resistance to bending is low, as for viscosities two orders of magnitude above that of the upper mantle. Steep dips and high plate advance velocities require a low-drag asthenosphere (as when its viscosity is 2 to 3 orders of magnitude lower than the upper mantle average). Back through the Cenozoic, old plates in the Pacific subducted at rates expected for upper-mantle slab pull, while young plates often subducted at velocities up to two or three times those expected from their upper mantle buoyancy, accompanied by very low trench retreat rates. Such high rates require an additional driving force, which is most likely supplied by lower mantle slab penetration. Young lithosphere may be more prone to penetrate the lower mantle, as its lower density and strength limit trench retreat and facilitate slab thickening in the transition zone which increases Stokes' sinking velocity.
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