Experimental deformation of dry Columbia diabase: Implications for tectonics on Venus

High-temperature creep experiments have been performed on samples of Columbia diabase to investigate the rheological behavior of rocks of basaltic composition under dry conditions. The effects of temperature, strain rate and oxygen fugacity on the creep strength were investigated. The activation energy for creep is Q = 500 ± 30 kJ/mol, and the stress exponent is n = 4.5 ± 0.3, indicating deformation controlled by dislocation creep. This rock was slightly weaker when deformed at oxygen fugacities set by an iron-wüstite buffer than by a nickel-nickel oxide buffer. The creep strength of our samples predried at 1000°C is significantly greater than values reported previously for samples deformed under as-received conditions or samples deformed after predrying at much lower temperatures (Caristan 1982, Shelton and Tullis 1981). Application of these results to the tectonics of the lithosphere on Venus, assuming a temperature gradient of 10-20 K/km and a strain rate of 10−15 s−1, predicts a relatively deep brittle-plastic transition (~5-9 km depth) and a strong lithosphere, dominated by the strength of rocks in the middle and upper crust, with no weak lower crustal zone. As the contrast in rheology between these diabase rocks and dry dunite is small at depths greater than 15 km, strong coupling between the crust and mantle seems likely.