SMITH,WILLIAM LECTURE 1992 - PLATE-MOVING MECHANISMS - THEIR RELATIVEIMPORTANCE

The lithosphere forms the upper boundary layer of the Earth's mantle c onvection system which facilitates escape of thermal energy from the E arth's interior. Motion and interaction of lithospheric plates is prob ably governed by the combination of drag-forces excerted on the base o f the lithosphere by the convecting mantle and by plate boundary force s. Phanerozoic movements and interaction of major continental blocks a re difficult to explain in terms of conventional plate moving mechanis ms, such as slab-forces, ridge-push and deviatoric tensional stresses developing over upwellings of the asthenosphere and in response to lit hospheric over-thickening in orogenic belts. Circumstantial evidence s uggests that shear-traction exerted by the convecting asthenosphere on the base of the lithosphere plays an important, and at times even a d ominant role as a plate moving mechanism. The relative importance of t he different processes contributing to the motion of lithospheric plat es probably varies during the assembly and break-up of Pangaea-type me gacontinents. Continents assemble in areas of downwelling branches of the asthenospheric convection system. Development of major orogens alo ng the trailing edges of drifting continents and subduction-progradati on 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 s uturing, causes their decay and a reorganization of the global upper-m antle convection system. Development of new upwelling and outflowing a sthenospheric cells under mega-continents gives rise to tensional stre sses in the lithosphere, causing its extension. Following crustal sepa ration, the asthenosphere advects passively into the space opening bet ween diverging plates. Progressive opening of oceanic basins is couple d with the development of ridge-push forces, contributing to plate div ergence. Activity along sea-floor spreading axes can terminate abruptl y if far-field stresses, resulting from plate interaction, impede furt her plate divergence. The nearly contemporaneous decay of sea-floor sp reading axes in often distant areas reflects changes in plate interact ion. Assuming a finite globe, generation of new lithosphere at sea-flo or spreading axes has to be compensated for elsewhere by subduction of commensurate amounts of oceanic lithosphere and/or shortening and sub duction 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 th e development of intra-continental rift systems, the opening of new oc eanic basins and the inception of and activity along subduction zones. A two-layered mantle Convection system is envisaged, that may be coup led to a greater degree during the break-up of mega-continents than du ring periods of dispersed continents.