Plate-moving mechanisms: their relative importance

Abstract

The lithosphere forms the upper boundary layer of the Earth's mantle convection system which facilitates escape of thermal energy from the Earth's interior. Motion and interaction of lithospheric plates is probably governed by the combination of drag-forces excerted on the base of the lithosphere by the convecting mantle and by plate boundary forces. Phanerozoic movements and interaction of major continental blocks are difficult to explain in terms of conventional plate moving mechanisms, such as slab-forces, ridge-push and deviatoric tensional stresses developing over upwellings of the asthenosphere and in response to lithospheric over-thickening in orogenic belts. Circumstantial evidence suggests that shear-traction exerted by the convecting asthenosphere on the base of the lithosphere plays an important, and at times even a dominant role as a plate moving mechanism. The relative importance of the different processes contributing to the motion of lithospheric plates probably varies during the assembly and break-up of Pangaea-type megacontinents. Continents assemble in areas of downwelling branches of the asthenospheric convection system. Development of major orogens along the trailing edges of drifting continents and subduction-progradation from the megasuture of colliding continents to their distal margins suggests that plate convergence is controlled, apart from slab-forces and ridge-push, also by shear-traction. Assembly of a Pangaea has an insulating effect on the downwelling convection cells, governing its suturing, causes their decay and a reorganization of the global upper-mantle convection system. Development of new upwelling and outflowing asthenospheric cells under mega-continents gives rise to tensional stresses in the lithosphere, causing its extension. Following crustal separation, the asthenosphere advects passively into the space opening between diverging plates. Progressive opening of oceanic basins is coupled with the development of ridge-push forces, contributing to plate divergence. Activity along sea-floor spreading axes can terminate abruptly if far-field stresses, resulting from plate interaction, impede further plate divergence. The nearly contemporaneous decay of sea-floor spreading axes in often distant areas reflects changes in plate interaction. Assuming a finite globe, generation of new lithosphere at sea-floor spreading axes has to be compensated for elsewhere by subduction of commensurate amounts of oceanic lithosphere and/or shortening and subduction of continental lithosphere. Plate interaction, driven largely by shear-traction of the mantle convection systems and their changes, ridge-push and slab forces, plays probably an all-important role in the development of intra-continental rift systems, the opening of new oceanic basins and the inception of and activity along subduction zones. A two-layered mantle convection system is envisaged, that may be coupled to a greater degree during the break-up of mega-continents than during periods of dispersed continents.