Influence of crustal heterogeneity on normal fault dimensions and evolution: southern South Africa extensional system

The tectonic configuration of southern South Africa is dominated by a continental scale, Mesozoic extensional system superimposed upon a significant, and well constrained, Palaeozoic lithospheric scale heterogeneity. Through integrating onshore structural analysis with offshore subsurface studies it is possible to evaluate the applicability of established normal fault growth models in a heterogeneous crustal setting at a basin scale for the first time. The >480-km-long Mesozoic extensional system comprises a number of fault arrays that vary in length from 78 to 230 km. Coupled with displacements of up to 16 km, the fault arrays are amongst the longest and largest displacement of high angle normal faults (dips of 45–60°) documented in continental lithosphere, although comprised of co-linear segments rather than the en-échelon segment configuration typical of many other extensional systems. This atypical geometry is considered to be a consequence of the high degree of structural inheritance between the underlying Paleozoic Cape Fold Belt and the subsequent extensional system. Furthermore, it is proposed that the overall displacement–length dimensions of the extensional faults were inherited from the underlying compressional faults. The establishment of a seismic–stratigraphic framework for the Pletmos and Gamtoos offshore basins reveals that the faults established their long lengths (160 and 90 km, respectively) within ∼6 Myr of rift initiation prior to accruing their substantial displacement (16 km). Furthermore, there is no evidence for the development of intra-basin faults.The southern South Africa extensional system presents an end member case of structural inheritance where extensional structures are parallel to and reactivate underlying compressional structures. In this study structural inheritance is considered to have a significant effect on mechanisms of fault growth. The pre-existing structures not only result in the rapid establishment of fault lengths of up to 160 km, but, additionally, to strain being localised onto the pre-existing fabric to such an extent that no intra-basin faults evolve and strain is accommodated entirely on the bounding faults.