Recognising the Different Roles and Expressions of Dynamic Mantle Flow and Plate Kinematics in the Evolution of Africa's Topography

Abstract:

Continental elevation can be partitioned into contributions from intrinsic isostatic buoyancy forces arising from thermal, compositional and thickness variations within the crust or lithosphere and external geodynamic forces. External forces include horizontal in-plane stress and vertical forces arising from convection within the deep mantle giving rise to dynamic uplift of Earth’s surface. In essence both of these are ultimately expressions of the energy driving and/or contained within the convecting lithosphere-mantle system, and so are not really separate, but their expression in the topography at the surface is often quite different. The term dynamic uplift refers to topography that is supported by the vertical stresses arising from the upward viscous flow of mantle impinging on the base of the lithosphere, and it is typically regional in extent. In recent years the importance of dynamic uplift as a major control on continental topography has been emphasised by the advances in seismology and progress in understanding the structure of the shallow as well as deep mantle. However, the difficulty of quantifying surface uplift within non-orogenic regions has hampered progress in understanding how continental topography reacts to the competing interaction between mantle process creating the topography and surface processes that destroy it. Any geomorphic response, and the resulting erosion or sedimentary signal related to these uplift events, is strongly filtered through the response times of surface processes that are responding to relatively small changes in surface gradient. But with care, the erosional history, measured over large areas, can monitor the evolution of continental scale topography and this can be very effectively measured at appropriate temporal and spatial scales using low temperature thermochronometry. This approach, combined with analysis of the offshore sedimentary record around southern Africa, has proved to be very effective at documenting the topographic evolution of the sub-continent and has identified the expression of ‘conventional’ plate tectonic processes of rifting and rift flank uplift as well as discrete responses to significant plate kinematic changes and the more regional expression of dynamic uplift likely arising from flow within the deep mantle.