LLSVPs of primordial origin: Implications for the evolution of plate tectonics

Today's large low shear velocity provinces (LLSVPs) are assumed to be compositionally dense leftovers from the magma-ocean phase that are formed to dense accumulations (piles) by mantle flow. Due to the stabilising effect of dense material on mantle convection, LLSVPs could be of major importance for the evolution of plate tectonics. Here, we examine the influence of dense primordial material using thermochemical mantle convection models with self-consistent plate generation. Our results show that the general evolutionary path from a stagnant lid to an episodic to a permanently mobile stage is almost unaffected by dense LLSVP-like structures. The stabilising effect of a dense basal layer, however, leads to delayed surface mobilisation events. In particular, we find a prolonged episodic stage and a later emergence of continuously moving surface plates. Mobilisation episodes occur when the rigid surface breaks in either a top-down (by downwellings), a bottom-up (by upwellings) or a mixed, push-pull manner. As dense material is constantly entrained by rising currents, the stabilising effect decreases with time. Consequently, we observe a change of the mechanism which triggers a surface mobilisation from top-down to late pushpull, then bottom-up and finally push-pull if the stabilising effect is decreased. Our results suggest that, for LLSVP-like volumes and density contrasts, plate mobilisation is triggered by mantle upwellings, either directly in a bottom-up manner or delayed after a long phase of stagnant lid convection (late push-pull). Generally, a primordial dense layer has a profound impact: continuous plate motion as known today is strongly delayed and early Earth plate tectonics most likely acted in an episodic manner. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

Today's research reveals the significant role of large low shear velocity provinces (LLSVPs) in the evolution of plate tectonics. The presence of dense primordial material delays surface mobilization events due to its stabilizing effect on mantle convection. Plate motion is primarily triggered by mantle upwellings and may have occurred in an episodic manner during the early Earth.