Signatures of downgoing plate-buoyancy driven subduction in Cenozoic plate motions

The dynamics of plate tectonics are strongly related to those of subduction. To obtain a better understanding of the driving forces of subduction, we compare relations between Cenozoic subduction motions at major trenches with the trends expected for the simplest form of subduction. i.e., free subduction, driven solely by the buoyancy of the downgoing plate. In models with an Earth-like plate stiffness (corresponding to a plate-mantle viscosity contrast of 2-3 orders of magnitude), free plates subduct by a combination of downgoing plate motion and trench retreat, while the slab is draped and folded on top of the upper-lower mantle viscosity transition. In these models, the slabs sink according to their Stokes' velocities. Observed downgoing-plate motion-plate-age trends are compatible with >80% of the Cenozoic slabs sinking according to their upper-mantle Stokes' velocity, i.e., subducting-plate motion is largely driven by upper-mantle slab pull. Only in a few cases, do young plates move at velocities that require a higher driving force (possibly supplied by lower-mantle-slab induced flow). About 80% of the Cenozoic trenches retreat, with retreat accounting for about 10% of the total convergence. The few advancing trench sections are likely affected by regional factors. The low trench motions are likely encouraged by low asthenospheric drag (equivalent to that for effective asthenospheric viscosity 2-3 orders below the upper-mantle average), and low lithospheric strength (effective bending viscosity similar to 2 orders of magnitude above the upper-mantle average). Total Cenozoic trench motions are often very oblique to the direction of downgoing-plate motion (mean angle of 73 degrees). This indicates that other forces than slab buoyancy exert the main control on upper-plate/trench motion. However, the component of trench retreat in the direction of downgoing plate motion (approximate to slab pull) correlates with downgoing-plate motion, and this component of retreat generally does not exceed the amount expected for free buoyancy-driven subduction. High present-day slab dips (on average about 70 degrees) are compatible with largely upper-mantle slab-pull driven subduction of relatively weak plates, where motion partitioning and slab geometry adjust to external constraints/forces on trench motion. (C) 2010 Elsevier B.V. All rights reserved.