Geological hiatus surfaces across Africa in the Cenozoic: implications for the timescales of convectively-maintained topography

Geological maps contain a large amount of information that can be used to constrain geodynamic models, but which has been often overlooked by the geodynamic community. Particularly significant are unconformable geologic contacts at continental scales: what is usually perceived as a lack of data (material eroded or not deposited) becomes instead part of the signal of dynamic topography.

We were able to use geological maps to constrain the dynamic processes in the mantle beneath Africa by understanding its Cenozoic elevation history, and by using it to distinguish between different uplift and subsidence scenarios. This was accomplished by mapping the spatio-temporal patterns of geological contacts at the series level using continental-scale geological maps, under the assumption that continental-scale unconformable contacts are proxies for vertical motions and for paleotopography. We also mapped the present-day elevation of marine sediments for each series.

We found that significant differences exist in interregional hiatus surfaces. For example, the total unconformable area at the base of the Miocene expands significantly compared to the base of the Oligocene, strongly suggesting that most of Africa underwent uplift in the Oligocene. In southern Africa there are no marine Oligocene or Pleistocene sediments, suggesting that this region reached a high in the Oligocene, subsided in the Miocene and Pliocene, and has been high again since late Pliocene to Pleistocene. More generally, to reproduce the pattern of marine sedimentation in Africa that we mapped, sea level increases between 300 and at least 500 m above present level would be required. These are well in excess of the maximum 150 m eustatic sea level rise that has been postulated by several authors for the Cenozoic. Our results therefore support a dynamic origin for the topography of Africa. Specifically, the time-scale of geologic series (at most a few tens of millions of years) is comparable to the spreading-rate variations in the south Atlantic, which have been linked to African elevation changes through pressure-driven upper mantle flow.