Ca. Williams et al., EFFECT OF THE BRITTLE-DUCTILE TRANSITION ON THE TOPOGRAPHY OF COMPRESSIVE MOUNTAIN BELTS ON EARTH AND VENUS, J GEO R-SOL, 99(B10), 1994, pp. 19947-19974
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.