The Physics of Changing Tectonic Regimes: Implications for the Temporal Evolution of Mantle Convection and the Thermal History of Venus

Given similar sizes and compositions, Venus invites comparisons with Earth. While Earth convects in the plate tectonic regime, Venus' current and past tectonic states remain uncertain. Venus' impact crater distribution has led to competing models for its tectonic state, steady (e.g., stagnant lid) or catastrophic (e.g., episodic lid). Terrestrial geochemical evidence and geodynamic models suggest that planets may transition between tectonic states over time. Transitions can be triggered by increasing yield stress due to loss of water or by increasing surface temperature. Here, we revisit the classic endmember models of Venusian tectonics by modeling the transition from stable mobile lid convection into a stable stagnant lid in 3D spherical geometry. Transitions between states are governed by both regional and global scale instabilities, resulting in oscillations in heat flux, velocity, and magma production over Gyr time scales. The thermal and tectonic evolution of a convectively transitioning planet is neither catastrophic nor steady. Volcanism and tectonic yielding may be non-global in nature. At any given time, portions of the planetary surface may reflect apparently different styles of convection, with some regions being highly active and other regions being sluggish and inactive. For Venus, the implications are that global melt production and resurfacing are a natural consequence of lid-state evolution, without needing ad hoc resurfacing mechanisms which rely on a single governing lid-state. Geologic evidence suggests that the transition from mobile lid to stagnant lid is still on-going. Venus may have lost liquid surface water in the last third of its history. Plain Language Summary Given broad similarities, Venus naturally invites comparisons with the Earth, yet Venus' surface reveals a world strikingly different. Mantle convection on Earth is linked to plate tectonics, but the current tectonic state of Venus is hotly debated. The distribution of impact craters on Venus' vast volcanic plans have been used to generate two endmember tectonic models: a steady single plate with waning melt production over time, or a catastrophic overturn model with strong, short-lived melting events. Here, we explore a change in tectonic regimes for Venus triggered by either loss of water or increasing surface temperature. Transitions in regimes are highly disruptive with significant changes in surface motions, mantle temperatures, melt production, and heat flow. These disruptive events can be restricted to hemispheric or sub-hemispheric regions, indicating that different portions of the planet may record apparently contrasting tectonic regimes. Such a transition between tectonic regimes is neither steady nor catastrophic, and it naturally results in punctuated resurfacing and melting events. This can explain many puzzling aspects of Venus' geologic evolution within the framework of a single evolutionary model.