THE ROLE OF MANTLE THERMAL STATE IN THE INITIATION OF PLATE TECTONICS

The evolution of terrestrial planets is marked by cooling of the mantle over time, and possibly changes in the relative contributions of internal and basal heating to the mantle. Such changes in thermal state can potentially have an important impact on whether plate tectonics can operate on a planet. In particular it has been hypothesized that Earth transitioned from an early stagnant lid state into a plate tectonic state as the mantle cooled. Previous work on early Earth geodynamic evolution, and the influence of mantle thermal state on tectonic mode in general, has typically used simple plastic rheologies to describe plate boundary formation. However, shear localization in the lithosphere is likely more complicated, involving processes such as grain-size reduction. Here, new models of mantle convection that include grain-size reduction are used to determine how mantle temperature, internal heating rate, and heating mode (i.e. bottom heating versus internal heating) affect the mode of surface tectonics. The results show that the mode of surface tectonics is insensitive to mantle thermal state, contrary to previous results from convection models with plastic rheologies. Increasing mantle temperature and/or transitioning from dominantly bottom heated to internally heated convection causes stresses in the lithosphere to decrease, explaining the results of previous studies. However, the rate of viscous dissipation, which drives grain-size reduction, increases with increasing mantle temperature, and is largely insensitive to heating mode. Therefore shifts between stagnant lid and plate tectonic regimes as a result of mantle cooling are unlikely to occur when grain-size evolution is taken into account. Instead, the style of surface tectonics may transition gradually from a sluggish lid to a plate tectonic mode as mantle temperatures decrease over time, as lower mantle temperatures suppress grain-growth and allow more localized plate boundaries to form. Early sluggish lid convection would be characterized by widespread upper mantle melting as a result of inefficient convective heat loss. Such melting may explain why the Archean geologic record is dominated by rocks interpreted as forming in ocean plateau settings rather than subduction zone settings.