Convective and Tectonic Plate Velocities in a Mixed Heating Mantle

Abstract

Mantle convection and, by association, plate tectonics is driven by the transport of heat from a planetary interior. This heat comes from the internal energy of the mantle and from heat flowing into the base of the mantle from the core. Past investigations of such mixed mode heating have revealed unusual behavior that confounds our intuition based on end‐member cases. In particular, increased internal heating leads to a decrease in convective velocity. We investigate this behavior using a suite of numerical experiments and develop a scaling for velocity in the mixed heating case. We identify a planform transition, as internal heating increases, from sheet‐like to plume‐like downwellings that impacts heat flux and convective velocities. More significantly, we demonstrate that increased internal heating leads not only to a decrease in internal velocities but also a decrease in the velocity of the upper thermal boundary layer (a model analog of the Earth's lithosphere). This behavior is connected to boundary layer interactions and is independent of any particular rheological assumptions. In cases with a temperature‐dependent viscosity and weak plate margin analogs, increased internal heating does not cause an absolute decrease in surface velocity but does cause a decrease relative to purely bottom or internally heated cases as well as a transition to rigid‐lid behavior at high heating rates. The differences between a mixed system and end‐member cases have implications for understanding the connection between plate tectonics and mantle convection and for planetary thermal history modeling.

Dynamics and Evolution of Venus’ Mantle Through Time

“…In contrast, an additional argument for the promotion of stagnant lid behaviour independent of rheological arguments may come from the physics of mantle systems heated from below and within. In such systems, high levels of radiogenic heating tend to inhibit convection, and thus stress imparted via internal convective velocities, due to transition in convective planforms from predominantly sheet-like to plume-like Lenardic et al 2021).…”

Penetrative Superplumes in the Mantle of Large Super‐Earth Planets: A Possible Mechanism for Active Tectonics in the Massive Super‐Earths

“…As shown, the statistical behavior of the models (the average values) varies with temperature dependence of the viscosity and the internal heating rate. The geotherm plots show that higher values of the internal heating rates diminish the deep mantle layering (e.g., Lenardic et al., 2020; Moore, 2008; Sotin & Labrosse, 1999).…”

Plate Tectonics, Mixed Heating Convection and the Divergence of Mantle and Plume Temperatures

“…Figures 2 and 3 show results from several suites of numerical experiments. The suites follow from the study of Lenardic et al [2021] (full model equations and solution procedures can be found in that reference). Models are formulated in terms of a basal heating Rayleigh number (Ra) and a ratio of the internal to basal heating Rayleigh numbers (H ).…”

“…Moore [2008] developed theoretical scaling relationships that predict how thermal outputs should vary with H and Ra for mixed heating convection in an active lid mode. The theory has been extensively tested against numerical convection experiments in both 3-D Cartesian [Lenardic et al, 2021] and 3-D Spherical modeling domains . We can use the theory, calibrated to a spherical geometry in line with Earth mantle dimensions, to test the ability of continuous plate tectonics to account for observational constraints.…”

“…The theory of Moore [2008] did explicitly consider how thermal plumes impinging on the base of the lithosphere affected the scaling of surface heat loss. This was extended by Lenardic et al [2021] to consider the role of plume impingement on plate velocities. Both studies made predictions for H and Ra values where plume impact on the base of the lithosphere would not have an effect on scalings (see also Lenardic and Moresi [2003] for related results).…”