An Early, Transient, Impact-driven Tectonic Regime in the Hadean?

Abstract:

The earliest preserved crust formed in a tectonic regime that different from present in several significant ways. Earth was still losing a significant fraction of its primordial heat, and heat production was declining more rapidly than in any period subsequently. Additionally, a waning impact flux provided an important thermal – and mechanical – forcing on the early Earth. These factors affect tectonics in a number of ways – internal velocities are faster, and internal viscosities lower due to hotter internal temperatures. Previous modelling of the tectonic evolution of the Earth under these conditions suggests that the early Earth may have exhibited periods of hot stagnant-lid convection – where vigorous internal convection – and volcanic activity – occurred under a tectonically quiescent lid. As the Earth cooled the planet would have transited into an episodic regime – characterised by periods of tectonic quiescence interspersed by extreme tectonic activity. Such scenarios explain a number of facets of the early Earth, including its observed slow mixing rates, inefficient cooling, and its paleomagnetic and tectonic history. Here we expand these models to incorporate the effects of major impacts. We find tectonic forcing due to impacts to be a major dynamic driver during the Hadean, with major return flow driving horizontal surface deformation, and transient subduction events. Such events may be reflected in the Hadean zircon record. Post this impact-dominated phase, models suggest an over-heated Earth may have then experienced a long phase of stagnant-lid convection, lasting till the meso-Archaean.