Recent tectonic and lithospheric thermal evolution of Venus

Venus, a planet similar in size, mass, and substance to Earth, has clearly undergone a vastly different thermal evolution because it does not currentl y lose heat by means of plate tectonics. Other manifestations of this diffe rence are the thickness of the mechanical lithosphere and the geothermal gr adient, which are intrinsic to the outer thermal boundary layer of the mant le convective system and are regulated by the interior heat loss. Observed tectonic styles and the results of geodynamic modeling indicate that the me an lithospheric thickness on Venus has increased with time. Pervasive defor mation of the plateau highlands tessera, the oldest preserved terrain, requ ires a weak, thin lithosphere. Later features, such as ridge belts, coronae , chasmata, and shield volcanoes, reflect a lithosphere of intermediate thi ckness that causes deformation to be distributed. A broad are of focused, c oherent underthrusting at Artemis Chasma and the inferred partial regional support of volcanic highlands suggest that at present the average lithosphe re 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 f eatures representative of the thermally stable plains are suggestive of a s imilar to 200-km thermal lithosphere. Heat loss on Venus today cannot be in steady state with mantle radiogenic heat production. Collectively, the def ormation styles and modeled lithospheric thicknesses imply a passive, monot onic cooling of the thermal boundary layer over the interval of Venus's his tory recorded on the surface, approximately the past billion years. (C) 199 9 Academic Press.