How Rigid is a Rigid Plate? Geodetic Constraint from the TrigNet CGPS Network, South Africa

Rigidity and continuity of the Nubia plate is a fundamental assumption for the kinematic description, the dynamic implications of its interaction with surrounding plates and ultimately an important constraint to the geodynamics processes involved in continental lithospheric rupture. Geophysical, neotectonic and geodynamics considerations suggest the possibility that the Nubia plate is not completely rigid but could be undergoing internal deformation due to the southward propagation of the East African Rift. Here, we utilize the South African TrigNet geodetic network to evaluate the amount of internal deformation within the South African region and the possibility of motion between South Africa and the rest of the African continent. Our results show that the South African region behaves rigidly, with deformation of the order of 1 nanostrain yr−1 or less. The analysis shows some higher strain rates in the eastern region, and the presence of spatially correlated residuals in the Cape Town region and the region east of Johannesburg. Although not statistically significant, the spatial coherence of those residuals could indicate tectonic activity. A comparison of the Euler vector for the South African region with previously published Euler poles for the Nubia plate as well as the analysis of the residuals of Nubia sites with respect to a ‘rigid’ TrigNet are compatible with clockwise rotation of the South African block with respect to the African continent, consistent with a propagation of the East Africa Rift along the Okavango region