Evolving dynamical regimes during secular cooling of terrestrial planets : insights and inferences from numerical models

Abstract

Although plate tectonics is the present-day mode of geodynamics on Earth, it is not so on Mars and Venus, and probably also not during the early history of the Earth. In this thesis, the conditions under which plate tectonics may operate on terrestrial planets are investigated. Numerical model studies, presented in the thesis, show show that plate tectonics on Earth may be opposed by lithosphere buoyancy at potential mantle temperatures above about 1500C. For Venus, this value is about 1450C, and for Mars, it is even lower (1300-1400C). Parametric models of plate tectonics and massive flood volcanism are used to determine the cooling characteristics of these mechanisms. Numerical mantle convection models including differentiation by partial melting are applied to investigate an alternative type of dynamics which may have operated under conditions too hot for plate tectonics. The results show a suite of processes which produce and recycle oceanic crust and cool the planetary mantle: 1) small scale delamination of the lower crust; 2) large scale resurfacing events in which sections of crust of 1000 km long may sink into the mantle in about 2 million years, being replaced by newly produced crust; 3) intrusion of hot and fertile material from the lower mantle into the upper mantle, causing diapirs which are significantly hotter (excess temperature of 250K) than those predicted from boundary layer theory for a hot mantle and which produce large volumes of basaltic melt. In order to link model results to observations, the conditions of melting of basaltic crust in the model settings described above are compared to the conditions of formation of an important consituent of Archean crations, TTG (Tonalite-Trondhjemite-Granodiorite) plutons, which have been formed by partial melting of meta-basalt under amphibolite conditions. The results show that specifically the resurfacing events create conditions favorable for the production of TTG rocks, and the rock associations produced in the models resemble those in Archean granite-greenstone terrains. Therefore, the resurfacing mechanism may have contributed to the formation of proto-continents. The long-term stability of Archean cratons has been ascribed to the presence of thick roots underneath these regions, and addition of depleted mantle rock by mantle upwellings undergoing partial melting has been shown to contribute to the formation of these roots. In model experiments presented in this thesis, the effect of different types of rheology (grainsize dependent diffusion creep, diffusion creep + dislocation creep, diffusion creep + dislocation creep + dehydration by partial melting) on the development of these diapirs is studied. The model results show a self-regulating mechanism to be operative, which causes the dynamics to be relatively insensitive to grainsize. Tentative planetary histories are constructed from the results obtained in the production of this thesis: After cooling down from the magma ocean regime, episodic resurfacing possibly further cooled the planet down to the conditions favorable for plate tectonics, which then took over. This transition was probably gradual. It is speculated that Venus is still in the stage of episodic resurfacing. Mars was probably relatively inactive from its very early history on.