STAGNANT LID CONVECTION ON VENUS

The effect of strongly temperature-dependent viscosity on convection i n the interior of Venus is studied systematically with the help of fin ite element numerical models. For viscosity contrasts satisfying exper imental constraints on the theology of rocks, Venus is likely to be in the regime of stagnant Lid convection. This regime is characterized b y the formation of a slowly creeping, very viscous lid on top of the m antle-Venusian lithosphere and is in agreement with the tectonic style observed on Venus. Stagnant Lid convection explains large geoid to to pography ratios on Venus by the thermal thinning of a thick lithospher e. The thickness of the Lithosphere can be as large as 400-550 km for Beta Regio and 200-400 km on average. Geoid and topography data and ex perimental data on the theology of rocks provide constraints on the vi scosity of the mantle, 10(20)-10(21) Pa s; the convective stresses in the interior, 0.2-0.5 MPa; the stresses in the lid, 100-200 MPa; the v elocity in the interior, 0.5-3 cm yr(-1); and the heat flux beneath th e lithosphere, 8-16 mW m(-2). Parameterized convection calculations of thermal history of Venus are difficult to reconcile with a thick pres ent-day lithosphere. However, a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagn ant Lid convection around 0.5 b.y. ago. One of the possible explanatio ns for the cessation of Venusian plate tectonics is that during the ev olution of Venus, stresses in the lid dropped below the yield strength of the lithosphere. This model predicts a drastic drop in the heat fl ux, thickening of the lithosphere, and suppression of melting and is c onsistent with the hypothesis of cessation of resurfacing on Venus aro und 0.5 b.y. ago.