Surface deflections due to transient subduction in a convecting mantle

Transient surface deflections associated with non-continuous subduction are studied through the use of a sequence of idealized numerical models of time dependent mantle convection. The major concerns of this study are the magnitude and duration of dynamically supported topographic fluctuations resulting from either the initiation or termination of a subduction zone at the Earth's surface. The former is modelled by prescribing an initial temperature structure which mimics the presence of a cold slab of lithosphere immersed in a warm uniform-temperature mantle, while the latter is induced by thermally detaching this slab from the upper surface of the convection cell. The model “slabs” are thus defined thermally, rather than mechanically, and their negative buoyancy induces convective flow in the neighbouring mantle. The full hydrodynamic equations governing natural convection are solved numerically in order to follow the evolution of the temperature and velocity fields with time. The resulting model flows are, therefore, dynamically self-consistent, and differ from previous kinematic models in that the flow velocities are determined at each instant from the evolving thermal field, rather than prescribed as boundary conditions. As the lithospheric slab sinks into the mantle, the induced flow produces normal stresses, which in turn result in a broad topographic depression. Subsequently, as the slab-surface distance increases, the topography rebounds at a rate which is strongly dependent upon the imposed temperature contrast between the slab and the surrounding mantle material. Assuming that these depressions are filled with sediments, if a subsequent episode of subduction were to begin before the initial depression was eliminated, new sediments would be superimposed upon the old. Repeated episodes of subduction at a continental margin may, therefore, be an important factor contributing to repeated cycles of platform sedimentation.