No abstract

The structural evolution of the Southern North Sea region is controlled by a number of important tectonic phases. The dominant NW-SE structural grain first became evident during dextral wrenching in the Stephanian/Autunian. A second fault direction trends NNE-SSW, and probably also originated in pre-Permian times. Extensional faulting in the Triassic preceded major Middle Jurassic to Early Cretaceous rifting. Subsequent cooling subsidence was interrupted by inversion in the Late Cretaceous, the mid-Paleocene, and again in the mid-Tertiary. During these periods Jurassic basins were uplifted and, locally, subjected to deep erosion.Despite the multitude of tectonic events, a high degree of fault parallelism can be observed in the Southern North Sea. Fault patterns and directions are strikingly similar, whether viewed on a regional scale or at prospect level. The general structural model is, therefore, one of repeated reactivation of basement faults, which continue to control the structural grain despite changes in tectonic regime. Only rarely and by coincidence do the main structural features have orientations that conform to the regional stress. Consequently the predominant mechanism is considered to be one of oblique-slip.This phenomenon is particularly well observed on structural highs, such as the Cleaver Bank High, straddling the median line between the Netherlands and the UK. Here, the fault trend at Base Zechstein level runs essentially NW-SE. Repeated fault reactivation has led to anomalously high length-to-throw ratios. Locally short N-S and E-W link-ups accommodate oblique-slip. At higher stratigraphic levels en-echelon faults illustrate the strike-slip nature of the latest movements along the pre-existing NW-SE structural grain.The presence of salt in the overburden causes faulting at higher levels to be decoupled from basement faulting. Sandbox experiments show that reverse faults and extensional grabens can develop in the postsalt sequence in response to oblique-slip along the pre-salt structural grain. The location of such grabens is invariably offset towards the footwall of the basement fault, the amount of offset dependent on the thickness of the salt.These sandbox experiments provide valuable analogues for the structural mechanisms that are believed to have operated in the Southern North Sea. Applied to both 2D and 3D seismic interpretation, they allow fault mapping with a greater degree of confidence. Recognition of fault reactivation and oblique-slip will lead to a better understanding of the tectonic history, timing of structuration and trap development.