1887
Volume 16, Issue 3
  • E-ISSN: 1365-2117

Abstract

Abstract

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‐long Gamtoos Fault established its length very early in its syn‐rift phase (within ∼5 Ma of rift initiation) before accruing over 6 s (two‐way‐travel 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, deep marine conditions. The Gamtoos Fault has a 90° 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 deformation we 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.

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2004-08-23
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