Recent Tectonic and Lithospheric Thermal Evolution of Venus

Abstract

Venus, a planet similar in size, mass, and substance to Earth, has clearly undergone a vastly different thermal evolution because it does not currently lose heat by means of plate tectonics. Other manifestations of this difference are the thickness of the mechanical lithosphere and the geothermal gradient, which are intrinsic to the outer thermal boundary layer of the mantle convective system and are regulated by the interior heat loss. Observed tectonic styles and the results of geodynamic modeling indicate that the mean lithospheric thickness on Venus has increased with time. Pervasive deformation of the plateau highlands tessera, the oldest preserved terrain, requires a weak, thin lithosphere. Later features, such as ridge belts, coronae, chasmata, and shield volcanoes, reflect a lithosphere of intermediate thickness that causes deformation to be distributed. A broad arc of focused, coherent underthrusting at Artemis Chasma and the inferred partial regional support of volcanic highlands suggest that at present the average lithosphere is considerably stronger and thicker, and the heat flow much lower, than expected by comparison to Earth. The relatively thin lithosphere at young volcanic structures is consistent with localized reheating, while the few features representative of the thermally stable plains are suggestive of a ∼200-km thermal lithosphere. Heat loss on Venus today cannot be in steady state with mantle radiogenic heat production. Collectively, the deformation styles and modeled lithospheric thicknesses imply a passive, monotonic cooling of the thermal boundary layer over the interval of Venus's history recorded on the surface, approximately the past billion years.