Continents, supercontinents, mantle thermal mixing, and mantle thermal isolation: Theory, numerical simulations, and laboratory experiments

Abstract

[1] Super-continental insulation refers to an increase in mantle temperature below a supercontinent due to the heat transfer inefficiency of thick, stagnant continental lithosphere relative to thinner, subducting oceanic lithosphere. We use thermal network theory, numerical simulations, and laboratory experiments to provide tighter physical insight into this process. We isolate two end-member dynamic regimes. In the thermally well mixed regime the insulating effect of continental lithosphere can not cause a localized increase in mantle temperature due to the efficiency of lateral mixing in the mantle. In this regime the potential temperature of the entire mantle is higher than it would be without continents, the magnitude depending on the relative thickness of continental and oceanic lithosphere (i.e., the insulating effects of continental lithosphere are communicated to the entire mantle). Thermal mixing can be short circuited if subduction zones surround a supercontinent or if the convective flow pattern of the mantle becomes spatially fixed relative to a stationary supercontinent. This causes a transition to the thermal isolation regime: The potential temperature increases below a supercontinent whereas the potential temperature below oceanic domains drops such that the average temperature of the whole mantle remains constant. Transition into this regime would thus involve an increase in the suboceanic viscosity, due to local cooling, and consequently a decrease in the rate of oceanic lithosphere overturn. Transition out of this regime can involve the unleashing of flow driven by a large lateral temperature gradient, which will enhance global convective motions. Our analysis highlights that transitions between the two states, in either direction, will effect not only the mantle below a supercontinent but also the mantle below oceanic regions. This provides a larger set of predictions that can be compared to the geologic record to help determine if a hypothesized super-continental thermal effect did or did not occur on our planet.

The impact of Rayleigh number on assessing the significance of supercontinent insulation

“…In a series of 2‒D models presented here, this thermal mixing theory was addressed by modeling insulating continents prior to the formation of a supercontinent. The calculations described in section 3.5 reproduce some of the findings of Lenardic et al [], indicating that if continental insulation has a strong effect prior to supercontinent formation, then the oceanic and continental mantle would cool and warm (respectively) after continental aggregation (with the overall mantle temperature staying the same). However, we find that subcontinental temperatures still do not exceed suboceanic temperatures on the timescale of a supercontinent episode, despite taking into consideration an increased background temperature due to continental insulation.…”

“…The i n s 2, c m b 2, i n s 4, and c m b 4 models were used in order to analyze the effect of insulation prior to supercontinent formation on suboceanic and subsupercontinental mantle temperatures. Figure shows the suboceanic, subcontinental, and average mantle temperatures for the models featuring well‒mixed initial conditions with temperatures determined by a history of continental insulation (in accord with the study by Lenardic et al []). For all models, the average mantle temperature does not appear to change significantly, while the isolated subcontinental material warms as the suboceanic material cools.…”

Interaction between the supercontinent cycle and the evolution of intrinsically dense provinces in the deep mantle

“…A well‐documented corollary of continental drift is the supercontinent cycle, characterized by the alternating assembly and dispersal of the majority of the Earth's continental cratons. Modeling studies show that supercontinents influence the mantle's thermal evolution due to their influence on subduction zone positioning and suggest feedback between continental drift and the deep mantle [e.g., Gurnis , ; Lowman and Gable , ; Li and Zhong , ; O'Neill et al , ; Heron and Lowman , ; Lenardic et al , ; Rolf et al , ; Heron et al , ].…”

Major and trace element composition of the high 3He/4He mantle: Implications for the composition of a nonchonditic Earth

“…Lenardic et al [2011a], however, show that this approach misses the most important effect of continents, which is to insulate the mantle. Building on earlier work [Lenardic et al, 2005;Jellinek and Lenardic, 2009;Lenardic et al 2011b], Lenardic et al [2011a] show that continental insulation governs the interior mantle temperature, which, in turn, determines the mantle viscosity and the rate of subduction and mantle overturning. The greater is the rate of continental radiogenic heat production, the higher will be the mantle internal temperature.…”