HIGH-TEMPERATURE DEFORMATION OF DRY DIABASE WITH APPLICATION TO TECTONICS ON VENUS

We have performed an experimental study to quantify the high-temperatu re creep behavior of natural diabase rocks under dry deformation condi tions. Samples of both Maryland diabase and Columbia diabase were inve stigated to measure the effects of temperature, oxygen fugacity, and p lagioclase-to-pyroxene ratio on creep strength. Flow laws determined f or creep of these diabases were characterized by an activation energy of Q = 485+/-30 kJ/mol and a stress exponent of n = 4.7+/-0.6, indicat ive of deformation dominated by dislocation creep processes. Although n and Q are the same for the two rocks within experimental error, the Maryland diabase, which has the lower plagioclase content, is signific antly stronger than the Columbia diabase. Thus the modal abundance of the various minerals plays an important role in defining rock strength . Within the sample-to-sample variation, no clear influence of oxygen fugacity on creep strength could be discerned for either rock. The dry creep strengths of both rocks are significantly greater than values p reviously measured on diabase under ''as-received'' or wet conditions [Shelton and Tullis, 1981; Caristan, 1982]. Application of these resul ts to the present conditions in the lithosphere on Venus predicts a hi gh viscosity crust with strong dynamic coupling between mantle convect ion and crustal deformation, consistent with measurements of topograph y and gravity for that planet.