EFFECT OF THE BRITTLE-DUCTILE TRANSITION ON THE TOPOGRAPHY OF COMPRESSIVE MOUNTAIN BELTS ON EARTH AND VENUS

The Coulomb critical taper model has been very successful in explainin g the large-scale topography of a number of terrestrial accretionary w edges; however, this model is limited to cases of purely brittle-frict ional deformation. In this paper we extend the range of applicability of the critical taper model by explicitly including the effects of tem perature-dependent ductile deformation. The new model includes tempera ture-dependent power law flow, an assumed velocity field, and linear t hermal gradients in the atmosphere and within the crust. Flexural isos tasy is also incorporated so that the decollement geometry is computed as a response to the applied load of the wedge material. We assume th at ductile deformation within the decollement zone is controlled prima rily by diffusion flow, whereas ductile deformation within the wedge i tself is controlled by dislocation creep. The topographic profiles pre dicted by the model are very similar to those of a number of fold-and- thrust belts on both Earth and Venus. A typical wedge profile includes three distinctive topographic regions: a narrow taper toe, where both the wedge and the decollement zone deform in a brittle-frictional man ner; a region of relatively steep slope, where the wedge base deforms ductilely and the decollement zone is still frictional; and a flat pla teau region, where both the wedge base and the decollement zone are de forming by ductile flow. We have applied the model to two fold-and-thr ust belts on Venus (Maxwell Montes and Uorsar Rupes) and to the Andes on Earth, and we find good agreement between observed and predicted to pography using reasonable parameter values. The model accounts for the observed positive correlation between relief and elevation of Venusia n fold-and-thrust belts on the basis of different thermal environments at different elevations. It is also able to explain the first-order d ifferences between terrestrial and Venusian fold-and-thrust belts; fun damentally, this difference is due to a combination of the lower tempe ratures and the presence of water on Earth.