SELF-CONSISTENT GENERATION OF TECTONIC PLATES IN 3-DIMENSIONAL MANTLECONVECTION

Despite the fundamental importance of plates in the Earth's mantle con vection, plates have not generally been included in numerical convecti on models or analog laboratory experiments, mainly because the physica l properties which lead to plate tectonic behavior are not well unders tood. Strongly temperature-dependent viscosity results in an immobile rigid lid, so that plates, where included at all in 3-D models, have a lways been imposed by hand. An important challenge is thus to develop a physically reasonable material description which allows plates to de velop self-consistently; this paper focuses on the role of ductile she ar localization. Tn two-dimensional geometry, it is well-established t hat strain-rate softening, non-Newtonian rheologies (e.g. power-law, v isco-plastic) cause weak zones and strain-rate localization above up- and down-wellings, resulting in a rudimentary approximation of plates. Three-dimensional geometry, however, is fundamentally different due t o the presence of transform plate boundaries with associated toroidal motion. Since power-law and visco-plastic rheologies do not have the p roperty of producing shear localization, it is not surprising that the y do not produce good plate-like behavior in three-dimensional calcula tions. Here, it is argued that a strain-rate-weakening rheology, previ ously shown to produce plate-like behavior in a two-dimensional sheet representing the lithosphere, is a reasonable generic description of v arious weakening processes observed in nature. One-and two-dimensional models are used to show how this leads to shear localization and the formation of 'faults'. This rheology is then applied to the high-visco sity lithosphere of 3-D mantle convection calculations, and the veloci ty-pressure/viscosity solution for the entire 3-D domain (lid and unde rlying mantle) is solved self-consistently, It is found that the litho sphere divides into a number of very high-viscosity plates, separated by narrow, sharply defined weak zones with a viscosity many orders of magnitude less than the plate interiors. Broad weak zones with dominan t convergent/divergent motion above up- and down-wellings are intercon nected by a network of narrow weak zones with dominant strike-slip mot ion. Passive spreading centers are formed in internally heated cases. While the resulting plates are not fully realistic, these results show that self-consistent plate generation is a realizable goal in three-d imensional mantle convection, and provide a promising avenue for futur e research. (C) 1998 Elsevier Science B.V. All rights reserved.