Delayed Tectonic Evolution due to LLSVPs?

Considering the Earth's history, neither the formation of tectonic plates nor the origin of LLSVPs is well constrained. However, if LLSVPs are assumed to be compositionally dense remnants of the magma ocean period, they have likely influenced the evolution of plate tectonics. Dense material at the core-mantle boundary, comparable to LLSVPs, is known to interact with the flow of the Earth's mantle and has a stabilizing effect on mantle convection. Likewise, the development of lithospheric plates, which is strongly coupled to the convecting mantle, will experience a restoring effect by dense basal material. LLSVPs of primordial origin may therefore be of major importance for the evolution of plate tectonics.

To investigate the effect of primordial dense structures in the deep mantle, we employ thermochemical mantle convection models with self-consistent plate generation. We perform an extensive parameter study to analyze the influence of different parameters, as, for instance, the density excess of compositionally dense material in relation to the ambient mantle or the volume fraction of the dense material.

Generally we observe an evolutionary path from a stagnant lid, via episodic surface mobilizations to a permanently mobile state with continuous plate-like surface motion. Yet, this evolution, particularly the episodic stage, is altered by the presence of dense material in the deep mantle (cf. Figure). An increase of the excess density leads to a considerably longer episodic stage and thus to a delayed mobile state, i.e. to a later development of modern-style plate tectonics. Similarly, other parameters like a larger volume fraction of dense material also lead to a longer evolution. Our results consequently suggest that compositionally dense structures at the core-mantle boundary likely lead to a late development of modern-style plate tectonics. Primordial LLSVP-like structures could thus have played a crucial, delaying role for the Earth's tectonic evolution.