Multiagent simulation of evolutive plate tectonics applied to the thermal evolution of the Earth

Abstract

[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.

The Sensitivity of Core Heat Flux to the Modeling of Plate‐Like Surface Motion

“…Working within the limitations of mantle convection modeling, the latter physical requirement imposes constraints on core‐mantle boundary (CMB) heat flux. By influencing surface heat flux and focusing the flux of cold material, plate tectonics principally controls the temperature of the mantle by the process of subduction (e.g., Coltice et al, ; Combes et al, ; Monnereau & Quéré, ). Globally, the rate of subduction, spatial distribution of subduction zones, and longevity of subduction (once initiated) act together to influence mantle temperatures (Bercovici et al, ).…”

Thermal History of the Earth: On the Importance of Surface Processes and the Size of Tectonic Plates

“…We use the prefactor  2.1 c yielding an oceanic lithospheric thickness of 10 km for  τ 1 Myr and 95 km for  τ 80 Myr. A small scale convection mechanism can be used as an option in the MACMA model, in order to limit seafloor thickness after a critical age (for details, see Combes et al [2012]).…”

“…The resulting shear force F , which also depends on mantle viscosity, has to overcome a fixed yield strength in order for the continent to break up (see Combes et al. (2012) and Appendix for calculation details). This simplified oceanization parameterization does not reflect complex interactions between continents and mantle flow (e.g., O'Neill et al., 2009) but yields a physically reasonable behavior as a function of T : (1) a plateau in the shear force F is reached after a transient period lasting a few hundreds of Myr (see Figure 1c), (2) wider continents induce larger shear forces F and are therefore breaking more easily than narrow continents, (3) there is a minimum width for a continent to break, which depends on the chosen value of and on the thermal state of the mantle, and (4) because of a lower viscosity in a hotter mantle, the shear force F after a given duration and for a given continental width was lower in the past, implying that the minimum size for a continent to break was larger in the past.…”

“…With this approach, the geometry of plate tectonics is not fixed but free to adapt as the temperature and plate speeds evolve. Combes et al (2012) conducted several experiments that yielded moderate cooling rates comprised between 55 and 110 K/Gyr over the past 3 Gyr, but the initial conditions and the main parameters of the model were not varied on a wide range of possibilities. Here, we present a full analysis of Earth's thermal evolutions obtained with this approach and compare them with numerous available geochemical and geological data.…”

“…In the present study, we look into Earth's thermal evolution with aiming at explaining not only past moderate cooling rates, but also past plate speeds and rates of tectonic events such as continental collisions. We use a two‐dimensional (2D) model of plate tectonics (Multi‐Agent Convective Mantle [MACMA]) (Combes et al., 2012) based on a force balance for each plate and on empirical parameterizations to treat local plate reorganizations. The model includes mobile plate boundaries that can collide, vanish or be created, asymmetric subduction zones, and their consequent overriding plates, together with breakable insulating continents, bordered by active or passive margins.…”