The influence of backstop dip and convergence velocity in the growth of viscous doubly-vergent orogenic wedges: insights from thermomechanical laboratory experiments

We investigate the effects of convergence velocity and backstop geometry on the evolution of experimental viscous doubly-vergent orogenic wedges. Experiments are performed in a thermomechanical shortening box, using the Newtonian properties of 52/54 EN type paraffin as analogue to the natural strength distribution in the continental crust. A velocity discontinuity is imposed at the subduction slot, where the base plate is pulled below a rigid backstop. The mobile base plate dips 5°. Two different backstop dips (60° and 30°) are used in combination with constant convergence velocities of 2 and 10 cm h−1. Results point out that backstop geometry exerts a first-order control on the evolution of doubly-vergent experimental wedges. Steeply-dipping backstops cause the stationary uplift of the wedge axial region above the subduction slot, whereas shallow-dipping backstops allow its retroward translation, in concomitance with a more pronounced deformation in the retrowedge. Convergence velocity controls the uplift in the wedge axial region and the amount of the outward migration of the deformation front in both the prowedge and retrowedge regions. For fast convergence velocity and shallow-dipping backstop, deformation in the retrowedge is favoured. An increase in the backstop dip and/or a decrease in the convergence velocity favour deformation in the prowedge. Both prowedge and retrowedge tend to grow in self-similar conditions, with an aspect ratio depending upon the backstop dip and convergence velocity. These results may have important consequences for the evolution and deformation style of ductile-dominated orogenic wedges, suggesting that location of the axial region and deformation progression in natural orogens can be strongly controlled by boundary and kinematic conditions.