Rheological dependence of extension in wedge models of convergent orogens

Although several mechanical models of extension in contractional orogens ha ve been proposed, in many cases these models have been based on viscous con stitutive laws. Lack of more complicated rheological models has primarily b een due to the technical difficulty of solving for deformation with complex rheological behavior and boundary conditions. A finite-element model is pr esented that allows for solution of the wedge mechanical problem with visco us, non-linear viscous and Coulomb plastic theologies. This model is used t o investigate extension in a doubly-vergent wedge model in which deformatio n is primarily contractional, driven by a reversal of basal traction. Propo sed models showing extension in the crust at shallow crustal levels, in spi te of overall contraction and contraction at depth, are confirmed, but only for high Argand number (low crustal viscosity). At low Argand number littl e or no extension is observed. Non-linear stress dependence (power-law flow ) also suppresses extension, but not entirely; some extension is observed a t high effective Argand number However with a Coulomb plastic theology, no extension is observed for a wide range of parameter values. No extension is observed for models with viscous deformation at depth and plastic deformat ion near the surface. This suggests that the extension is a function of the theologic model, not the effective strength of the crust. Coeval extension and contraction in convergent orogens with steady external forcing therefo re requires viscous deformation to very shallow depths. (C) 1999 Elsevier S cience B.V. All rights reserved.