INVERSION OF THE BROAD FOURTEENS BASIN OR GRABEN-DE-LA-HAYE (SOUTHERNNORTH-SEA) - CONTRIBUTION OF 3D SEISMICS

3D seismics are now widely used in areas of intensive oil exploration, being a relatively recent technique that provides a powerful tool for detailed investigation of the often complex geometry oi sedimentary b asins. The present study is concerned with the use of this technique t o investigate a graben inversion structure in the southern North Sea. The North Sea is a geological province with a wide variety oi sediment ary basins, ranging in age from the Paleozoic to the Quaternary, compr ising an area largely covered by geological and geophysical surveys ca rried out during intensive oil industry exploration. Grabens were deve loped during the Mesozoic in a crustal segment previously formed by th e Caledonian and Hercynian orogenies, then tectonically inverted durin g the Late Cretaceous and Tertiary. In the southern North Sea, the Bro ad Fourteens Basin (or ''Graben de La Haye'') is a spectacular example of a very inverted graben. The Zechstein Salt, which is intercalated between basement and cover, provides a decollement layer observed to t he north oi this graben but not in the south. This layer is responsibl e for the various types of inversion structure found out in the graben A study of the inversion oi the Broad Fourteens Basin was carried out using the data from 2D as well as 3D seismic techniques. The data fro m 3D seismics are used to analyse the details of fault kinematics duri ng inversion, as well as the influence of the decollement layer on the development oi major structures, particularly the thrusts. The invers ion of the Broad Fourteens Basin took place through dextral strike-sli p faulting. Because the salt layer leads to a detachment between basem ent and cover, it is possible in places to observe the presence of two contrasting tectonic regimes - compressive in the basement and disten sive in the cover. According to their shape, salt diapirs have contrib uted to the different styles of deformation; cylindrical diapirs are o nly slightly deformed, whereas walls of salt extending parallel to the graben boundaries lead to the setting and development of thrusts.