Topography evolution in convergent flows of two liquid layers

We have recently developed a two-layer model that considers the convergent motion of two initially uniform liquid layer with different densities and viscosities and assumes that the flow is due to the basal traction that acts at the bottom of the lower layer. We have used this model to describe successfully the evolution of mountains belts (Perazzo & Gratton, Phys. Fluids 22, 056603, 2010). In this work we discuss how to modify our model to also describe the formation of plateaus. To this end we assume that below of a given level the viscosity of the upper layer drops abruptly, and in consequence the flow of this layer becomes decoupled of the motion of the lower region of the system.