Resonances of the fluid core for a tidally decelerating Earth: cause of increased plume activity and tectonothermal reworking events?

Abstract

In the geological past when the Earth's decelerating rotation passed through the critical length of day of ∼ 22.2 h, the free nutation or ‘tipping’ of the fluid core (for a core shape in hydrostatic equilibrium) would have resonated precisely with the Earth's retrograde annual forced nutation caused by solar torques. This annual resonance may have caused orders-of-magnitude amplification of core rotation and velocity of core motions, leading to temperature increase and heat release near the core-mantle boundary, instability of the D″ thermal boundary layer at the base of the mantle, and the upwelling of deep mantle plumes. Neoproterozoic palaeotidal data indicate 21.9 ± 0.4 h/day at ∼ 620 Ma, thus placing the critical length of day of ∼ 22.2 h and core resonance at 530 ± 100 Ma. A core shape departing by ±5% from its equilibrium value places resonance in the interval 530 ± 250 Ma. The one distinctive, major tectonothermal event in the interval 530 ± 250 Ma was that of ‘late Pan-African’ age, which affected much of Gondwana during the late Neoproterozoic-early Palaeozoic. This tectonothermal activity, which peaked between ∼ 570 and ∼ 500 Ma, was marked by high temperatures at mid-crustal levels and elevated heat flow, widespread granulite facies metamorphism and crustal reworking and anatexis, commonly in the absence of obvious pervasive deformation, extensive epeirogenic uplift and extrusion of flood basalts; such features indicate an intense crustal heating event that may be ascribed to mantle plumes and hotspots. Furthermore, flood volcanism and rifting in Laurentia during the late Neoproterozoic-early Palaeozoic suggest the coeval upwelling of mantle plumes that led to continental breakup including the opening of the Iapetus Ocean. The apparent global peak in plume activity between ∼ 570 and ∼ 500 Ma may reflect an increase in core-mantle heat flux and instability of D″, caused by the annual resonance of the fluid core (for a core shape in or near hydrostatic equilibrium). The core evidently underwent stronger resonances with the semi-annual and 1/3-annual forced nutations at widely spaced times during the Archaean, when the length of day was approximately 17.7 ± 0.4 h and 15.4 ± 0.4 h, respectively; these two resonances probably caused much greater increases in the velocity and amplitude of core motions and in core-mantle heat flux than occurred during the annual resonance. Proterozoic palaeorotational data suggest that the very strong semi-annual resonance may correlate with the worldwide tectonothermal event at ∼ 2700 Ma; this event is marked by widespread mantle and crustal anatexis, granitoid intrusion and the extrusion of flood basalts and komatiites, and has been ascribed to the upwelling of high-temperature plumes from the core-mantle boundary. The hypothesis that major tectonothermal reworking events may have resulted from thermal plumes generated at the core-mantle boundary in response to resonances of the fluid core for a tidally decelerating Earth may be tested by the acquisition of high quality palaeorotational data for the early Palaeoproterozoic and Archaean.