[1] The feedback between plate tectonics and mantle convection controls the Earth's thermal evolution via the seafloor age distribution. We therefore designed the MACMA model to simulate time-dependent plate tectonics in a 2D cylindrical geometry with evolutive plate boundaries, based on multiagent systems that express thermal and mechanical interactions. We compute plate velocities using a local force balance and use explicit parameterizations to treat tectonic processes such as trench migration, subduction initiation, continental breakup and plate suturing. These implementations allow the model to update its geometry and thermal state at all times. Our approach has two goals: (1) to test how empirically-and analyticallydetermined rules for surface processes affect mantle and plate dynamics, and (2) to investigate how plate tectonics impact the thermal regime. Our predictions for driving forces, plate velocities and heat flux are in agreement with independent observations. Two time scales arise for the evolution of the heat flux: a linear long-term decrease and high-amplitude short-term fluctuations due to surface tectonics. We also obtain a plausible thermal history, with mantle temperature decreasing by less than 200 K over the last 3 Gyr. In addition, we show that on the long term, mantle viscosity is less thermally influential than tectonic processes such as continental breakup or subduction initiation, because Earth's cooling rate depends mainly on its ability to replace old insulating seafloor by young thin oceanic lithosphere. We infer that simple Copyright 2012 by the American Geophysical Union 1 of 24 convective considerations alone cannot account for the nature of mantle heat loss and that tectonic processes dictate the thermal evolution of the Earth.

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The Earth's interior has been cooling over the past 3 billion years. Chemical analyses of ancient rocks show that the mantle was then about 250 degrees hotter than today, corresponding to a fairly low amount of heat released until now. However, when the mantle was hotter in the past, it was less viscous, and the dragging forces on tectonic plates were weak: plate motion was probably much faster. Heat from the Earth's interior is released mainly at oceanic ridges, where new seafloor is created. The faster the plates move, the more heat is released. How so little heat has been released with significantly faster plates in the past? Several authors proposed complex phenomena playing on plate tectonics to "slow down" plates in the past. Here, we propose a new model including surface processes controlling plates creation and disappearance, leading to evolving plate sizes. We show that the geometry of plates is a key factor: for a hotter Earth in the past, plates were larger, so that ridges were less numerous and a moderate amount of heat was released, even if plates were faster. This new paradigm (larger plates in the past) is compatible with observations in geological data.

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