GRANITE TOP

Cosmogenic dating of the Brandberg Granite (Namibia)

We have recently proposed (Braun et al, 2014) a simple mechanism to explain why granites usually form topographic highs in the landform : it is a simple consequence of the isostatic rebound associated with erosional unloading where the denser the material being eroded, the larger the rebound. We have shown that for variations in surface rock density of the order of 400 kg/m3, which is typical of a granitic pluton intruding unmetamorphosed sediments, the difference in estimated rebound per unit erosion is approximately two-fold between the denser and the lighter regions. This result relies on the assumption of local isostasy and depends only on the ratio between surface rock density and rock density at the assumed depth of isostatic compensation. Taking into account the flexural strength of the crust and/or lithosphere, our argument only holds for denser bodies of relatively large size (more than a few tens of kilometers). We have also demonstrated that our proposed mechanism can easily be differentiated from the one commonly considered, i.e. that granites are topographic highs because they are more resistant to erosion, by comparing the exhumation rate between the granite and the surrounding lower elevation areas, as measured, for example, by low-T thermochronometry. We have recently collected rock samples from in and around the Brandberg Granite in western Namibia. These samples must now be prepared and analyzed to obtain erosion rates using the cosmogenic isotope method in the ISTerre Department of the University Grenoble ALpes. We propose un research internship to measure these erosion rates and interpret them using a landscape evolution model coupled to a flexural isostatic foundation. The intern will have a sold training in Earth Sciences as well as an interest in performing laboratory measurements.

Braun, J., Simon-Labric, T., Murray, K. E., and Reiners, P. W. (2014). Topographic relief driven by variations in surface rock density. Nature Geoscience.