Role of crustal anisotropy in modifying the structural and sedimentological evolution of extensional basins: the Gamtoos Basin, South Africa
D.A. Paton and J.R. Underhill
Journal name: Basin Research
Issue: Vol 16, No 3, September 2004 pp. 339 - 359
Info: Article, PDF ( 6.52Mb )
Through the investigation of crustal heterogeneities, sedimentary basin architecture and seismic stratigraphy, we demonstrate how a crust- scale anisotropy controls the initiation of rifting and the subsequent structural and sedimentological evolution of the Mesozoic Gamtoos Basin, southern South Africa. The results demonstrate that the >90-km-longGamtoos Fault established its length very early in its syn-rift phase (within ~5Ma of rift initiation) before accruing over 6 s (two-waytravel time (TWT)), or >12 km, of displacement without any significant subsequent increase in length. In addition, there is no evidence at the resolution of the data of fault segmentation, isolated depocentres nor of intra-basin faults progressively coalescing during the syn-rift interval. The early establishment of length resulted in a rapid transition from a terrestrial depositional environment to anoxic, deepmarine conditions. The Gamtoos Fault has a 90º bend in the fault trace that we propose is inherited from the underlying structure. Immediately adjacent to the bend the basin- fill is significantly deformed and a high-amplitude (>1.7s TWT) monoclinal fold is observed. Previous workers proposed that the fold was a consequence of a complex interplay between compression and extension. Through a restoration of the basin- fill deformationwe produce a model that suggests that the fold is a consequence of the accommodation of extension by the unusual plan-view trace of the fault. The evolution of the basin does not conform to current fault growth models and it is proposed that its unusual and complex development can be attributed to the underlying crustal- scale anisotropy, a fact that is likely to be important in other areas in which crustal stretching is superimposed on heterogeneous continental crust.