Regimes of subduction and lithospheric dynamics in the Precambrian: 3D thermomechanical modelling

Abstract

Comparing the early Earth to the present day, geological–geochemical evidence points towards higher mantle potential temperature and a different type of tectonics. In order to investigate possible changes in Precambrian tectonic styles, we conduct 3D high-resolution petrological–thermomechanical numerical modelling experiments for oceanic plate subduction under an active continental margin at a wide range of mantle potential temperature TP (∆TP=0−250K, compared to present day conditions). At present day mantle temperatures (∆TP=0K), results of numerical experiments correspond to modern-style subduction, whereas at higher temperature conditions important systematic changes in the styles of both lithospheric deformation and mantle convection occur. For ∆TP=50−100K a regime of dripping subduction emerges which is still very similar to present day subduction but is characterised by frequent dripping from the slab tip and a loss of coherence of the slab, which suggests a close relationship between dripping subduction and episodic subduction. At further increasing ∆TP=150−200K dripping subduction is observed together with unstable dripping lithosphere, which corresponds to a transitional regime. For ∆TP=250K, presumably equivalent to early Archean, the dominating tectonic style is characterised by small-scale mantle convection, unstable dripping lithosphere, thick basaltic crust and small plates. Even though the initial setup is still defined by present day subduction, this final regime shows many characteristics of plume-lid tectonics. Transition between the two end-members, plume-lid tectonics and plate tectonics, happens gradually and at intermediate temperatures elements of both tectonic regimes are present. We conclude, therefore, that most likely no abrupt geodynamic regime transition point can be specified in the Earth's history and its global geodynamic regime gradually evolved over time from plume-lid tectonics into modern style plate tectonics.