SOME EFFECTS OF A DRY CRUSTAL FLOW LAW ON NUMERICAL SIMULATIONS OF COUPLED CRUSTAL DEFORMATION AND MANTLE CONVECTION ON VENUS

Models of coupled crustal deformation and mantle convection on Venus a re reevaluated in light of recent experimental evidence suggesting tha t the strength of dry basalt is comparable to that of olivine. A previ ous model which assumed a relatively weak basalt flow law, more approp riate 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 10(8) years for the weak basalt model and 10(9) years for th e 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-su rface weakness, of the type associated with plate margins on Earth, th e lithospheric overturn time, which sets the timescale required for co nvective thickening of crust, scales dominantly with the Rayleigh numb er defined by the average viscosity of the lithosphere. Theoretical sc alings, as well as numerical models, suggest crustal thickening timesc ales of the older of 10(8) years remain possible for high bulk mantle Rayleigh numbers and effective viscosity contrasts from lithosphere to interior mantle of the order of 10(3). Such timescales imply a relati vely thin lithosphere and heat loss comparable to that of the present- day Earth. The large values inferred for the thickness of the mechanic al lithosphere on Venus, if correct, would thus tend to favor timescal es of the order of 10(9) years, if one assumes relatively low convecti ve mantle beat loss over an equivalent time. This, in turn, would argu e against the likelihood of highland plateaus being manifestations of crustal thickening above mantle downflows. If, instead, one assumes th at convective heat removal and implied lithospheric overturn rates wer e significantly higher over the last several hundred million years, th en highland formation due to crustal thickening cannot be ruled out ba sed solely on the strength of dry crust.