The role of plate geometry and boundary type in global plate ductility and mantle convection

Abstract

This paper tracks the role of plates in plate tectonics and mantle convection. Geometric and morphologic plate analyses are based on paleoreconstructions, beginning at 200 Ma, large-scale volcanic events, APWPs, and geoid data. Plates organize themselves to form a space-fitting geometry with a minimum perimeter/area (P/A) ratio (i.e., SA/V in three-dimensions) and a coordination number close to 5. Plates are either broken and subducted in order to remove plates with high P/A ratios or combined with other plates to reduce the total perimeter in the lithospheric plate mosaic. Various permutations of breaking and/or combining of high P/A plates are applied throughout earth's history to reduce the global P/A ratio. During this process, plates change their shape and/or size, which aids plate mobility to ultimately reach a stable geometry. Plates can only move with a suitable geometry and the global pattern keeps adjusting to reach a stable geometry. The lithospheric plates follow a pattern of ‘punctuated steady state’, where the global geometry cycles between ‘ductile’ periods, when the plates are moving to reach a new steady state geometry, to more static, or stable, periods, when the plates are relatively immobile and connected by a stable network of compressional stresses. Here, the Bénard-Marangoni theory is used to explain plate patterns in terms of surface tension because it allows for convection to be driven by temperature dependent surface tension. Mantle flow drives the temperature gradient at the surface, which results in a surface tension gradient. So, temperature dependent buoyancy and surface tension both play a role in convection. This suggests that there is feedback between the plates and the mantle, so neither is entirely passive.