Common Observables of Trench Migration and Plate Motion in Different Global Reference Frames

Plate velocities and trench migration velocities are commonly described in some sort of global "absolute" reference frame. From calculating such motions for all plates and subduction zones on Earth, one might obtain insight into the importance of various driving and resistive forces of plate tectonics and plate boundary migration. Trench migration velocities and plate velocities have been calculated for all subduction zones on Earth in eight global reference frames. The calculations show that such velocities can differ substantially between different global reference frames (up to 4 cm/yr), in particular between one Pacific hotspot reference frame (HS3- NUVEL1A) and all the others. In addition, this reference frame shows a bimodal distribution of trench velocities, while all the others show a Gaussian distribution. Nevertheless, some common features are observed irrespective of the reference frame. First, trench retreat always dominates over trench advance, with 62-78% of the trench segments retreating, while the mean and median trench velocities are always positive (retreating). Second, trench retreat is always slow in the middle of wide subduction zones, i.e. far (>2000 km) from lateral slab edges (<2 cm/yr in seven reference frames). Third, fast trench retreat (>6 cm/yr) is only found close (<1500 km) to lateral slab edges. Fourth, plates with a substantial percentage of their circumference attached to a subducting slab (Pacific, Nazca, Cocos, Philippine, Australia) move trenchward. These calculations are predicted by three-dimensional geodynamic models of free subduction with a variable slab width (300-7000 km), in which the slab to upper mantle viscosity ratio is low (100-200). This suggests that trench velocities and plate velocities are indeed primarily controlled by the negative buoyancy and width of subducting slabs. It further suggests that slab/upper mantle viscosity ratios in nature are 100-200, as the models show trench motion dominated by retreat, and forward dipping "slab-draping" geometries (e.g. Tonga) or steep slab geometries (e.g. Kermadec) as observed in nature. Overturned "roll-over" slab geometries are not observed.