Reassessing the structural and geomorphic evolution of a 'classic' Atlantic type passive margin: an integrated study of the Namaqualand sector of the South African continental margin

The origin of high elevation topography at so-called “passive” continental margins and their interior hinterlands has been an outstanding question in geoscience for decades. An important step towards answering this question is to improve our understanding of the response of the landscape to deformation of the lithosphere over different length scales. During continental rifting, elevated rift flanks may develop as a result of lateral extension of the lithosphere combined with vertical movements of the lithosphere driven by isostasy or convection of buoyant mantle flow. However, mechanisms capable of maintaining rift-related topography over geological timescales or driving post-rift rejuvenation of margin topography remain largely speculative and are strongly dependent on theoretical models. By constraining the timing and magnitude of major erosional events that have occurred across a particular margin using suitable empirical data we can begin to unravel the geomorphic development of the margin and identify the forces driving surface uplift. Apatite fission track (AFT) and apatite (U-Th-Sm)/He (AHe) thermochronometry has the unique ability to deliver these constraints by providing information on the cooling of rocks through temperatures of c. 120 – 40°C as they are exhumed from depth (c. 4 – 6 km) by erosion of overlying rock. Along the western continental margin of South Africa recent insights from thermochronology, structural geology and geomorphology has revealed that the margin may have experienced a more complex post-rift tectonic history than is to be expected for a “passive” margin. In this study, AFT and AHe analysis was performed on samples collected across the high relief escarpment zone along the continental margin (Namaqualand Highlands) and across the continental interior plateau (Bushmanland Plateau) to determine the post-break up cooling history of the continental margin. Sampling was undertaken from a structural perspective by sampling individual fault blocks within the heavily faulted Namaqualand Highlands and by collecting a profile of samples, from the interior plateau, that crosses major structural features at the boundary of the Kaapvaal craton. The approach for AHe analysis was to obtain multiple single grain age measurements (up to 20 grains per sample) for selected samples in order to investigate and exploit the primary causes of natural dispersion of AHe single grain ages and the influence of this dispersion on thermal history modelling. AFT and AHe data from 56 outcrop samples are jointly inverted using a Bayesian transdimensional approach incorporating the compositional influence on fission track annealing and radiation damage enhanced He retention. Two major discrete cooling episodes are recorded in thermal history models at c. 150 – 130 Ma and 110 – 90 Ma, respectively. These cooling episodes are broadly coeval with periods of enhanced deposition in the offshore Orange Basin and are therefore linked to discrete periods of enhanced continental erosion. The first phase of erosion is believed to involve the progressive destruction of syn-rift topography which prevailed across the developing continental margin and inland to the SW boundary of the Kaapvaal craton. The second phase of erosion is proposed to have been induced by regional uplift of southern Africa coupled with localised reactivation of basement structures at the continental margin and craton boundary. A vertical thickness of at least c. 3 – 5 km of material was eroded across the continental margin during the Cretaceous with only minor erosion (typically < 0.5km) occurring during the Cenozoic. There is now considerable support from the low temperature thermochronology record that km-scale denudation has occurred regionally across South Africa during the Mid-Late Cretaceous, long after the end of continental rifting in the South Atlantic. Data from this study reveals a more localised structural component to this regional event and more complexity in the spatial and temporal distribution of denudation during this period. The mechanisms driving this denudation are still uncertain but it is proposed here that regional dynamic uplift of South Africa has occurred due to the presence of an underlying upwelling of buoyant mantle, while in-plane horizontal stresses have triggered reactivation of basement structures. It now seems appropriate to revise the classification of the southwest African continental margin as being “passive” in a tectonic sense and consider the implications this has for our understanding of global plate tectonics