A modelling study of vertical surface displacements at convergent plate margins

During the evolution of a subduction zone system, variations are likely to occur in, for example, surface plate velocities and buoyancy of the subduct ing lithosphere. We quantify vertical surface displacements at convergent p late margins resulting from such imposed variations. For this purpose we us e a 2-D numerical model in which the lithospheric plates have an effective elastic thickness. We first define a model in which the subducting plate is driven by its negative buoyancy and a velocity at its surface side boundar y. Its equilibrium topography (after around 2 Myr) is the reference level f or examination of surface displacements resulting from variations in buoyan cy, velocity of the surface plates, friction along the interplate contact a nd subduction zone roll-back. We find that a decrease (increase) in buoyanc y of the subducting material leads to a deepening (uplift) of the plate mar gins. An increase in friction along the subduction fault deepens the overri ding plate margin. Subduction zone roll-back due to sinking of the negative ly buoyant subducting plate induces subsidence of the overriding plate marg in. This subsidence is reduced when roll-back takes place in a land-locked basin setting. Trench retreat forced by the motion of the overriding plate is characterized by higher topography of the overriding plate margin than t he case of retreat due to the sinking of the negatively buoyant slab. In th e first case in-plane stress in the back-arc region is compressive while it is tensional for roll-back due to the sinking of the slab. We conclude tha t vertical surface displacements during ongoing subduction may reach a magn itude of a few kilometres on the overriding and subducting plate margins.