The theory of plate tectonics has proven highly successful at explaining the distribution of mountain belts, oceanic basins, volcanoes, and earthquakes. These features generally form linear chains along the boundaries of the plates, with significantly less geological activity occurring within their stable interiors. Nevertheless, there are a number of mountainous areas located in these intraplate settings that appear to be actively uplifting by 1000s of meters over timescales of 10s of million years. They often form approximately circular topographic domes with radial drainage systems, and are in some cases associated with young mafic volcanism.
In this study, we investigate two of the best known onshore examples — the Borborema dome in northeast Brazil and the Angolan dome in southwest Africa — to try and unravel their geological histories and understand the driving mechanism behind their uplift. To do this, we exploit the elevations of uplifted marine rocks, the shapes of river profiles, and the history of erosion to constrain uplift rates through time. We then use seismology to map out the present-day temperature structure of the plate, and the geochemistry of recent volcanic rocks collected in the field to place a maximum bound on the temperature of the underlying convecting mantle. We find evidence of asthenospheric excess temperatures that are in both cases less than 100°C above the ambient background.
In Angola, we perform thermobarometry on mantle xenocrysts carried to the surface by kimberlites to investigate the thickness of the lithosphere in Creatceous times (~120 Ma). The paleo and present-day lithospheric structure is unchanged in southern Congo and northeast Angola, but appears to have thinned by ~30 km beneath the centre of the Angolan dome. The mechanism of uplift of these intraplate topographic domes therefore appears to be the arrival of a modest thermal anomaly in the upper part of the convecting mantle, which subsequently causes thermomechanical erosion and thinning of the overlying lithosphere. We intend to continue investigating the process of lithospheric thinning in future studies.
