EVOLVING PLATE TECTONIC REGIMES ON PLANET EARTH

The role of plate tectonics during the early part of Earth history can be monitored using a combination of results from geodynamic modeling, seismic reflection profiles, the time/space distribution of distinctive plate tectonic rock assemblages, metamorphic P-T paths as preserved in convergent margin assemblages, deformational analysis, and the geochemistry of oceanic basalts from greenstone belts. Combined results from these studies suggest an evolving and changing plate tectonic regime tied directly to the cooling of the mantle with the most prominent changes occurring at convergent plate margins. Modeling and lava lake tectonics are consistent with symmetric subduction at convergent margins during Earth's magma ocean stage (? 4.55 Ga). This is may be followed by flat subduction coupled with eclogite-driven delamination until at least 3 Ga. As reflected by large igneous province rock assemblages and basalt geochemistry, mantle plumes were also widespread during this period. Steep, modern-like, subduction zones appeared around 3 Ga as recorded by arc rock assemblages and basalt geochemistry, only becoming widespread, however, at about 2.7-2.5 Ga. Not until about 2 Ga, when mantle cooling resulted in production of oceanic crust < 10 km thick, were ophiolites preserved, and not until 1 Ga, when subduction geotherms fell into low-T, high-P space did blueschists appear in subduction assemblages.

Thus, although plate regimes changed and evolved, especially during the first 2 Gy of Earth history, unlike other terrestrial planets, it would appear that plate tectonics was the dominant mechanism by which Earth cooled throughout its history.