IS PLATE TECTONICS A PHASE IN THE EVOLUTION OF EARTH-LIKE PLANETS?

The tectonic regime of a planet depends critically on the contributions of basal and internal heating to the planetary mantle, and how these evolve through time. We use viscoplastic mantle convection simulations, with evolving core-mantle boundary temperatures, and radiogenic heat decay, to explore how these factors affect tectonic regime over the lifetime of a planet. The simulations demonstrate i) hot, initial starting conditions can produce a "hot" stagnant-lid regime, whilst simulations starting cool may begin in a plate tectonic regime; ii) Planets may evolve from an initial hot stagnant-lid condition, through an episodic regime lasting 1-3Gyr, into a plate-tectonic regime, and finally into a cold, senescent stagnant lid regime after ~10Gyr of evolution, as heat production and basal temperatures wane; and iii) initial conditions are one of the most sensitive parameters affecting planetary evolution - systems with exactly the same physical parameters may exhibit completely different tectonics depending on initial conditions employed. Estimates of Earth's initial temperatures suggest Earth may have begun in a hot stagnant lid mode, evolving into an episodic regime throughout most of the Archaean, before finally passing into a plate tectonic regime. The implication of these results is that, for many cases, plate tectonics may be a phase in planetary evolution between hot and cold stagnant states, rather than an end-member.