Plume driven plate motion changes: New insights from the South Atlantic realm

Plate motion changes are increasingly well documented in the geologic record. Directional plate motion changes, in particular, provide powerful constrains on plate tectonic torques, because they require a change in plate boundary torques, basal shear stresses or both that can be spatially located. Over the past years a pressure driven, so-called Poiseuille, flow model for upper mantle flux in the asthenosphere has gained increasing geodynamic attention-for a number of fluid dynamic arguments. This conceptually simple model makes a powerful testable prediction: Poiseuille flow induced plate motion changes should coincide with interregional scale mantle convection induced dynamic topography around the origin of the Poiseuille source. Here we focus on the South American plate, which undergoes two distinct directional changes in motion in the Paleogene. During this time period there is evidence for Paleocene and Eocene/Oligocene high dynamic topography in South America and Africa, which we infer by mapping geological hiatus (i.e., gaps in the stratigraphic record). This suggests a high-pressure source in the upper mantle in the South Atlantic. We apply Poiseuille/Couette flow models to show that the Sierra Leone and Tristan plumes are suitably located to provide the torque to initiate these plate motion changes. Our modeling results are entirely consistent with changes in proxy elevation in the African and South American continents at those times. Moreover, they imply that it is possible to locate torques from sources of active upper mantle flow that can explain directional plate motion changes, and identify them for the South American plate with geological features such as the Sierra Leone and Tristan plumes.