Thermomechanical lithosphere differentiation: the key to tectonics in the early Earth and Venus?
Invited talk by
From the School of Earth, Atmosphere and Environment, Faculty of Science, Monash University, Clayton, Australia
Life is unique to our planet. Or is it? Key to this question is the emergence of plate tectonics and the outgassing of the Earth to form the atmosphere and the hydrosphere, where life thrived. These processes took place in the “early Earth” (~4.5-3.8 billion years ago), marking the transition from a rigid outer lid regime to global plate tectonics. The geological, petrological and geochemical records provide fundamental constraints, yet these are sparse indications of a very long transition occurred sometime between the Hadean and the Phanerozoic. Geodynamic models successfully reproduce single regimes, but cannot address if these regimes coexisted, nor how and why the transition occurred on Earth. In this talk, I will show how thermochemical differentiation of the lithosphere in the early Earth, through large melting, extraction and dehydration, led to the emergence of proto-plates, driving stable deformation similar to present-day plate tectonics, yet, in a non-plate regime. Numerical models embedding melting and extraction, show how broad blocks of residual stiffer lithosphere are critical for the migration of stress over large distances, leaving plate-size areas undeformed and localising strain in narrow belts. The models reproduce temperature-pressure constraints from the TTG and metamorphic records and illustrate viable mechanisms for orogeny, crust and craton formation since the Hadean. Preliminary modelling of Venus shows how similar conditions lead to the occurrence of tectonics, yet larger internal heat prevent any planetary transition, through thermochemical differentiation, and tectonic shut-down is the consequence. Other planets might operate in the same regime, where surface deformation is documented, yet no plate tectonics, therefore no life, is present.