GEORGE P. WOOLLARD TECHNICAL LECTURE: PLATE TECTONICS AND SUBDUCTION; FROM REGIONAL TO GLOBAL?

The movement of the tectonic plates is controlled by same processes that drive convection deeper within the Earth’s interior. For the plates, these density differences are largely generated by the temperature structure of the plates and subducting slabs, which are themselves composed of a rheologically complex crust and mantle. For regional tectonic systems where the convergence rates of the bounding plates are small (e.g. Hellenides, Apennines, Banda Arc, Scotia Sea) theoretical and analog models and observations agree in showing that the buoyancy of the subduction plate exerts a first-order control on the evolution of the tectonic system and my explain most or all of the geologic and geophysical features. Analysis of the stress distribution along these subducting slabs indicates that the “engine” driving subduction originates from the negative buoyancy of the deeper slab while the dominant forces retarding subduction are related to viscous flow in the mantle wedge at depths shallower than about 100 km. Changes in plate motion and subduction rate in response to changes in slab buoyancy occur on short time scales, commonly a few million years or less. This raises the question of the extent to which the larger-scale organization of multi-plate systems can be understood largely as the result of negative buoyancy subducting slabs and the density and thickness of the oceanic lithosphere, etc. For example, when do ocean basins become asymmetrical, such as between South America and Australia? What produces westward (or eastward) drift of lithosphere relative to the top of the upper mantle? What role does the continental play in organizing the scale and pattern of plate tectonics?