History of Stress at Vaalputs, Namaqualand, South Africa: Evidence for a Mid-Cretaceous “Wegener-type Orogeny” in Western Southern Africa

The paper reviews more than 20 years of structural, stratigraphic and seismic monitoring studies focused on the Vaalputs radioactive waste disposal site, 100 km SSE of Springbok, in Namaqualand. Our finds, supported by the recordings of two 3-components seismometers, show that the frequency of seismic events in this region may be slowly increasing over time, that the predicted Mmax is ~5.8, and that deformation is governed by a NNW-SSE oriented horizontal σ1, typical of an Andersonian strike-slip regime (σ1 > σv > σ3). The history and dynamics of this large scale (≥ 2x 106 Km2) stress field, known as the Wegener stress anomaly, appears to be complex. The palaeostress record suggests that a stress field comparable to the current one became established after the opening of the Atlantic, perhaps at ~102 Ma and waned at about ~72 Ma, when it was replaced by a markedly different Andersonian thrust regime (σ1 > σ2 > σ v) oriented NNE-SSW. It is uncertain when the current Wegener stress field was re-established, but some evidence points to a pre-Quaternary event. Our finds at Vaalputs are consistent with published accounts of mid-Cretaceous, NW-SE oriented crustal shortening through reverse faulting, thrusting, and folding in Namibia and also in the offshore Bredasdorp Basin. This tectonic activity locally resulted in mountain building such as the Groot and Klein Karas Mts. of southern Namibia. As such, this tectonic style is difficult to reconcile with the extensional regime of a classic (passive) “Atlantic-type” continental margin, and calls for a new approach to the way the Kalahari epeirogeny of southern Africa is perceived.