Some effects of a dry crustal flow law on numerical simulations of coupled crustal deformation and mantle convection on Venus

Abstract

Models of coupled crustal deformation and mantle convection on Venus are reevaluated in light of recent experimental evidence suggesting that the strength of dry basalt is comparable to that of olivine. A previous model which assumed a relatively weak basalt flow law, more appropriate for hydrated crust, is compared to a similar model that assumes a strong basalt flow law, more appropriate for dry crust. A principal difference is the timescale for significant crustal thickening, of the order of 108 years for the weak basalt model and 109 years for the strong basalt model. These results can be understood in the context of theoretical scaling relations for convecting temperature dependent media which imply that in the absence of concentrated zones of near‐surface weakness, of the type associated with plate margins on Earth, the lithospheric overturn time, which sets the timescale required for convective thickening of crust, scales dominantIy with the Rayleigh number defined by the average viscosity of the lithosphere. Theoretical scalings, as well as numerical models, suggest crustal thickening timescales of the order of 108 years remain possible for high bulk mantle Rayleigh numbers and effective viscosity contrasts from lithosphere to interior mantle of the order of 103. Such timescales imply a relatively thin lithosphere and heat loss comparable to that of the present‐day Earth. The large values inferred for the thickness of the mechanical lithosphere on Venus, if correct, would thus tend to favor timescales of the order of 109 years, if one assumes relatively low convective mantle heat loss over an equivalent time. This, in turn, would argue against the likelihood of highland plateaus being manifestations of crustal thickening above mantle downflows. If, instead, one assumes that convective heat removal and implied lithospheric overturn rates were significantly higher over the last several hundred million years, then highland formation due to crustal thickening cannot be ruled out based solely on the strength of dry crust.