Heat Pipes and Vertical Tectonics in Terrestrial Planets

Terrestrial planet mantles cannot transport the very high heat production in their early stages through subsolidus convection and instead produce voluminous melt that makes its way to the surface to transport the heat. This heat-pipe mode of heat transport implies a very different tectonics than either the rigid or mobile-lid tectonics driven by subsolidus convection. Although  similar to rigid-lid convection in that there is relatively little horizontal motion, heat-pipe lithospheres are by no means stagnant. Vertical transport through the continuous eruption of new material on the surface reaches rates of several mm/year (with significant spatial and temporal variations). This strongly impacts the shape of the geotherm, producing a cold and strong lid (despite the high heat flow). In addition, this vertical transport produces global compressional stresses as old surfaces are buried and forced downward to smaller radii. The horizontal variations in burial rates will lead to stress concentrations and ultimately plastic failure and thrusting (see Io’s numerous tectonic uplifts as an example). The transition from the advectively dominated heat-pipe lithosphere to a thin conductive lithosphere reverses this process, resulting in a period of global extension (again with large horizontal variations) as global volcanism wanes. An additional aspect of vertical transport in the heat-pipe lithosphere is the cycling of water and other volatiles into the lithosphere and mantle as surface materials are buried. This material is available for metamorphic reactions and will interact with rocks at the wet solidus, producing evolved rock compositions and volatile by-products (e.g. methane) that will contribute to the early atmospheres of these planets. Evidence of vertical transport in ancient Earth rocks has generally been attributed to subduction but heat-pipe advection provides a more global opportunity for such cycling.