THE GABBRO ECLOGITE PHASE-TRANSITION AND THE ELEVATION OF MOUNTAIN BELTS ON VENUS

Maxwell Montes, standing up to 7 km above the adjacent highland platea us, constitute the highest mountain belt on Venus. Because the thickne ss of the crust is likely to be limited by the gabbro-garnet granulite -eclogite phase transitions, this relief is difficult to reconcile wit h the assumption of thermodynamic equilibrium and a standard Airy isos tatic model. We explore the hypothesis that the crust-mantle boundary is not in phase equilibrium, but rather is rate limited by the tempera ture-dependent volume diffusion of the slowest ionic species. Under th e simplifying assumption that the mountains formed by uniform horizont al shortening of the crust and lithospheric mantle at a constant rate, we solve the one-dimensional thermal evolution problem. The time-depe ndent density structure and surface elevation are calculated by assumi ng a temperature-dependent reaction rate and local Airy isostatic comp ensation. For a rate of horizontal strain of 10(-15) s-1 or greater, t he rise in temperature at the base of the crust during mountain format ion is modest to negligible, the deepening lower crust is metastable, and surface elevation increases as the crust is thickened. For strain rates less than 10(-16) s-1, in contrast, crustal temperature increase s with time because of internal heat production, and the lower crust i s more readily transformed to the dense eclogite assemblage. For such models a maximum elevation is reached during crustal shortening. While this maximum relief is 7 km or more for some models, a smaller densit y contrast between crust and mantle than assumed here (500 kg m-3) and incorporation of horizontal heat transport would lessen this value. W e therefore favor formation of the mountain belt at a strain rate at l east of order 10(-15) s-1. By this reasoning, Maxwell Montes must be c omparatively young, of order 50 Ma.