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BP Azerbaijan, Chertsey Road, Sunbury on Thames TW16 7LN, UK (e-mail: stewarsal{at}bp.com)
The North Sea Basin contains a widespread Permian salt layer that reached a depositional thickness of c. 1 km in the basin centre. This layer profoundly affected structural style of the post-salt succession and the basin can be divided into structural domains on this basis. In combination with regional 3D seismic data and several thousand wells this makes the North Sea a natural laboratory for salt tectonics. Four principal structural domains are illustrated here. (1) Minibasin subsidence and salt wall growth on the West Central Shelf in the Late Permian to Triassic. This area was exhumed and differentially eroded prior to Jurassic rifting, creating palaeogeomorphology analogous to the present-day Paradox Basin, Utah. (2) Regional tilt during the Mesozoic and Cenozoic led to basin-scale gravity sliding with updip detached extensional faults and downdip compressional structures, similar to gravity sliding in the circum-Atlantic salt basins. (3) Jurassic rifting propagated across the salt basin, displaying spatial variation in extensional fault style, partly as a function of salt layer thickness. (4) North Sea salt thickness was not sufficient for salt canopy development but there are two suites of minor intrusions: cylindrical, passive diapirs with associated fault and fracture patterns in the central North Sea, and sills where Permian salt from reactive diapirs intruded along thin Triassic salt layers in the southern North Sea. Cretaceous to Palaeogene regional shortening affected all these domains, resulting in a variety of reactivation styles that do not fit within commonly used definitions of inversion tectonics. The North Sea salt tectonic domains form the basis of a matrix approach to salt structure initiating and driving mechanisms, and a mechanostratigraphic scheme for tectonic structure classification.
