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

Abstract

AbstractGiven 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.