The influence of overriding-plate velocity on surface topography in subduction zones

Topography at subduction zones is the result of multiple processes operating at various temporal and spatial scales. At intermediate wavelengths (~100 km), the models predict the formation of dynamically-induced flexural topography that affects the overriding plate (OP) from the trench to the back-arc [Davies 1981, Crameri et al., 2017]. In our study, we assess how the velocity of the OP affects such a non-isostatic topography by using numerical mechanical models of subduction. We particularly investigate the effects of changes in OP velocity on the evolution of topography.

Our models consist of two converging visco-elastic plates with free surfaces. Friction is imposed along the planar subduction interface. We consider an isoviscous upper mantle with an impermeable barrier at a 660-km depth. We consider cases where the subducting plate (SP) has reached the bottom of the upper mantle and has a stationary motion. The models are performed with the code ADELIM [Cerpa et al., 2014].

We first characterize the main topographic features at a constant OP velocity, using spatial definitions that are based on estimations of the volcanic arc position. The models exhibit the formation of a bulge in the forearc area followed landwards by a depression and a smaller second bulge, the latter two of which are predicted to bracket the arc region. The steady-state distance to the trench of these three flexural features increase with OP velocity. Their amplitude is more sensitive to kinematics when the interplate friction is high and less when the SP viscosity is low.

We next test the effect of sudden changes in OP velocity. An OP acceleration yields a transient topographic tilt, during which the outer forearc quickly subsides whereas the arc region uplifts. The tilt is followed by reverse slower motions. An OP slowdown induces opposite motions. The rates of elevation during the tilt are approximately proportional to velocity variations and mainly sensitive to the SP strength. They are higher than 0.1 mm/yr for velocity changes higher than 1 cm/yr. We suggest that topographic accommodations of OP velocity changes should be considered when quantifying non-isostatic topography.