Numerical models of ductile rebound of crustal roots beneath mountain belts

Crustal roots formed beneath mountain belts are gravitationally unstable st ructures, which rebound when the lateral forces that created them cease or decrease significantly relative to gravity. Crustal roots do not rebound as a rigid body, but undergo intensive internal deformation during their rebo und and cause intensive deformation within the ductile lower crust. 2-D num erical models are used to investigate the style and intensity of this defor mation and the role that the viscosities of the upper crust and mantle lith osphere play in the process of root rebound. Numerical models of root rebou nd show three main features which may be of general application: first, wit h a low-viscosity lower crust, the rheology of the mantle lithosphere gover ns the rate of root rebound; second, the amount of dynamic uplift caused by root rebound depends strongly on the rheologies of both the upper crust an d mantle lithosphere; and third, redistribution of the rebounding root mass causes pure and simple shear within the lower crust and produces subhorizo ntal planar fabrics which may give the lower crust its reflective character on many seismic images.