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Project Progress

Plate tectonics plays an important role in the evolution of global planetary climate and life,however there is very little consensus on when and how plate tectonics began on Earth. Various scenarios have been proposed ranging from initiation of plate tectonics as early as after crystallization of magma ocean to a relatively late start of plate tectonics, in the Late Archean. The most difficult aspect of plate tectonics is initiation of subduction. Although initiation of subduction has been extensively studied, the focus has been on the present-day Earth where plate tectonics is already occurring. The major difference between initiation of subduction on the present-day Earth and the early Earth is that, in the absence of plate tectonics, forces associated with plate tectonics are absent and cannot participate in subduction initiation. We have been working on the hypothesis that sublithospheric small-scale convection might be the mechanism of initiating plate tectonics. Sublithospheric small-scale convection was proposed as an explanation for the bathimetry, the heat flow, and the geoid in the old oceanic region and for the heat flow and seismic velocity anomalies in the continental regions. Sublithospheric small-scale convection was proposed to be the major convective mode on other terrestrial planets. We developed constraints for this mechanism with the help of systematic finite element simulations in two dimensions (paper published) and began a systematic study in three dimensions (paper submitted). Two-dimensional studies showed that small-scale convection generates stresses comparable with the stresses required for initiation of subduction on the present-day Earth and can indeed be a trigger mechanism for plate tectonics. This conclusion seems to be even stronger in spherical geometry. The derived scaling relationships provide a theoretical basis for predicting plate tectonics on a silicate planet with arbitrary parameters. This will be the focus of our next paper.