Resolving the age of the first-order topography of Africa

This project will test the hypothesis that southern Africa came to acquire its unusually high average elevation-nearly all of it is above 1000 metres-30 million years ago when Africa stopped moving relative to the underlying mantle, and did not inherit its high elevation from the previously elevated super continent of Gondwana. Plate tectonics theory successfully explains how high mountain ranges form as a result of squeezing and thickening of the crust along converging plate boundaries (eg. Andes and Alps). It is less successful in explaining why extensive high plateaus exist in some continental regions far away from plate boundaries and unrelated to where plate boundaries existed in the geological past. The southern African plateau is the most significant of these 'topographic anomalies' on Earth-and is often referred to as the African superswell. While several different models have been proposed to explain the formation of the superswell, each suggests the high topography was formed at different times and at different rates. The most contentious of these ideas is that the first-order topography is not related to the break-up of Gondwana about 150 Myr ago but is much younger, less than 30 Myr, and is related to deeper mantle processes. Recent studies of the deep mantle have identified a region beneath southern Africa of hot, upward flowing mantle which originates close to the Earth's core. Some scientists now believe that it is this active flow that is literally pushing the Earth's surface upwards from below and is the cause of the unusually high elevation of southern Africa. This project will provide a definitive test of when the major topography of southern Africa was formed thus resolving a critical sticking point in understanding how continental topography evolves. We cannot test these models by precisely measuring when the surface uplift occurred because there is no direct evidence which enables us to reconstruct changes in elevation in the geological past. However, uplift of the surface at different times in the past would have caused an acceleration of erosion at these times as river gradients would have been steepened, especially around the edges of the uplifted region. Fortunately there are techniques which tell us about the history of erosion. These techniques provide a record of the temperatures that a rock experienced in the ancient geological past (over millions of years). This is relevant because when the Earth's surface erodes, rocks cool as they are brought up from deeper, hotter levels. The methods are based on measurement of the radioactive decay of U238 which occurs in trace amounts within the mineral apatite by two different processes; fission decay and alpha decay. Fission decay causes a 238U nucleus to split in two roughly equal parts which are rapidly repelled away from each other causing a linear zone or track of damage to the crystal lattice-we call these fission tracks. By counting the number of these tracks and measuring their lengths we can reconstruct the thermal history a rock has experienced because the track lengths are very sensitive to temperatures of 110-60 deg. C typical of the shallow crust. Alpha decay results in ejection of Helium nuclei, we call these alpha particles, from the 238U nucleus. By carefully measuring the amount of Helium gas that has accumulated within a grain of apatite we can determine how a rock has cooled from temperatures of c. 70-40 deg. C to its present temperature. Combined these techniques provide a powerful tool for measuring the deep erosion of continental topography over geological time scales. In this project we will analyse samples from deep bore holes across southern Africa and once we know the rocks' past temperatures and relate it to the depth at which those temperatures occurred in the crust, we can accurately determine when, and how much of, the land surface has eroded and hence resolve when the topography was created.