THE IMPLICATIONS OF BASALT IN THE FORMATION AND EVOLUTION OF MOUNTAINS ON VENUS

The highland region of Ishtar Terra on Venus has mountains that reach up to 11 km in height and are thought to be basaltic in composition. A ssuming that dynamic uplift of crust to this height is unlikely, we ex amine the topography produced by an isostatically supported thickening basaltic crust. It is found that regardless of whether the crust thic kens by crustal shortening or by volcanic construction, the high-densi ty basalt-eclogite phase transition is the limiting factor for produci ng significant elevation of the mountains. The maximum height attained by basaltic mountains depends on the nature of the basalt-eclogite ph ase transition. Without a phase transition, a basaltic crust must thic ken to greater than 100 km to reach heights over 10 km. An instantaneo us phase transition of basalt to eclogite allows a maximum topographic height of less than about 2 km. However, with a time lag of 100 Ma ow ing to slow rates of solid-state diffusion, our calculations show that the mountains can reach elevations greater than 10 km only if they ar e less than 25 Ma old. Higher temperatures within the Venusian crust m ay decrease the extent of the stability fields of high-density basalt phases and allow high topography if the thickening crust melts. This c an occur if the radioactive element concentrations measured on the sur face of Venus are uniformly distributed throughout the crust, the crus t thickens to greater than 65 km, and the thickened crust is older tha n about 400 Ma. The conflicting results of a young age predicted for h igh basaltic mountains and an almost uniform surface age of 500 Ma fro m crater populations, coupled with similarities in bulk physical prope rties of Venus and Earth, suggest that the basaltic surface compositio n found at several landing sites on the planet may not be representati ve of the entire crust. We suggest that Ishtar Terra formed from the c ollision of continent-like highly silicic cratons over a region of man tle downwelling. Lakshmi Planum resulted from the thickening of a basa ltic crust and the peripheral mountain belts formed from the collision of granitic cratons that were pulled toward a downwelling region of m antle.