Plate tectonics, plate moving mechanisms and rifting

Abstract

Abstract Analysis of a sequence of palaeo-reconstructions of the distribution of continents in the Western Hemisphere suggests that frictional forces exerted on the base of the lithosphere by the slowly convecting sub-lithospheric upper mantle play an important role as a driving mechanism of plate movements. With such a scenario, slab-pull and roll-back, ridge-push and deviatoric tensional stresses, related to upwellings of the asthenosphere as well as to lithospheric over-thickening in orogenic belts, can be considered as secondary, albeit important, plate moving forces. The present stress state of the globe and circumstantial geological evidence suggest that major compressional stresses can be transmitted over great distances through continental and oceanic lithosphere. Assembly of major continental masses (like Pangea) probably has an insulating effect on upper-mantle convection systems, causing their decay and reorganization. Development of new asthenospheric upwelling systems under mega-continents causes their break-up by development of deviatoric tensional stresses in the lithosphere and by exerting drag forces on its base. Extension of the lithosphere, culminating in its failure, is followed by passive advection of asthenospheric material into the space opening between the diverging plates to which it is accreted as oceanic lithosphere. At the same time developing ridge-push forces contribute to plate divergence. However, activity along seafloor spreading axes can terminate abruptly if far-field compressional stresses impede further divergence of the respective plates. This may explain the nearly contemporaneous decay of seafloor spreading axes in often distant areas during periods of plate boundary reorganization. Assuming a finite globe, the generation of new oceanic lithosphere at seafloor spreading axes has to be compensated for elsewhere by the subduction of commensurate amounts of oceanic lithosphere and/or shortening of continental crust and subduction of lower crustal material and the subcrustal lithosphere. Plate interaction, driven by mantle convection systems and their changes, ridge-push and slab-pull and roll-back, plays probably an all-important role in the development of intra-continental rift systems, the opening of new oceanic basins and the inception and development of subduction zones. Mantle plumes rising from the deep mantle do not appear to play a significant role in the development of intra-continental rifts though they probably contribute to a weakening of the lithosphere. The relative importance of the various processes contributing to the movement of lithospheric plates differs probably during the assembly and break up of Pangea-type mega-continents.