Early Earth tectonics: A high-resolution 3D numerical modelling approach

Abstract:

Early Earth had a higher amount of remaining radiogenic elements as well as a higher amount of leftover primordial heat. Both contributed to the increased temperature in the Earth’s interior and it is mainly this increased mantle potential temperature ΔT_p that controls the dynamics of the crust and upper mantle and the style of Early Earth tectonics.

For a minor increase in temperature ΔT_p < 175 K a subduction-collision style ensues which is largely similar to present day plate tectonics. For a moderate increase in ΔT_p = 175-250 K subduction can still occur, however plates are strongly weakened and buckling, delamination and Rayleigh-Taylor style dripping of the plate is observed in addition. For higher temperatures ΔT_p > 250 K no subduction can be observed anymore and tectonics is dominated by delamination and Rayleigh-Taylor instabilities.

We conduct 3D petrological-thermomechanical numerical modelling experiments of the crust and upper mantle under Early Earth conditions and a plume tectonics model setup. For varying crustal structures and an increased mantle potential temperature ΔT_p, a thermal anomaly in the bottom temperature boundary introduces a plume. The model is able to self-sufficiently form depleted mantle lithosphere after repeated melt removal. New crust can be produced in the form of volcanics or plutonics. To simulate differentiation the newly formed crust can have a range in composition from basaltic over dacitic to granitic depending on its source rock.

Models show large amounts of subcrustal decompression melting and consequently large amounts of new formed crust which in turn influences the dynamics. Mantle and crust are convecting separately. Dome-shaped plutons of mafic or felsic composition can be observed in the crust. Between these domes elongated belts of upper crust, volcanics and sediments are formed. These structures look similar to, for example, the Kaapvaal craton in South Africa where the elongated shape of the Barberton Greenstone Belt is surrounded by multiple plutons.