Rapid Plate Motion Variations and Continental Uplift as Surface Expressions of Asthenospheric Flow

It is well known that the South Atlantic Ocean experienced two phases of fast spreading in Late Cretaceous and Oligocene-Miocene, separated by a period of slow spreading around the K-T boundary. These two phases of fast spreading are correlated with two main periods of widespread uplift in the African continent. The present-day situation is characterized by a strong topographic gradient across the oceanic basin, with Africa being elevated and South America being depressed by non-isostatic forcing. These observations are not easily explained through shallow tectonics, but can be linked dynamically under the assumption of a thin and low-viscosity asthenosphere. In particular, they can be easily understood as the result of unsteady pressure-driven flow in such a low-viscosity sublithospheric layer.