GRAVITY-DRIVEN THIN-SKINNED EXTENSION ABOVE ZECHSTEIN-GROUP EVAPORITES IN THE WESTERN CENTRAL NORTH-SEA - AN APPLICATION OF COMPUTER-AIDED SECTION RESTORATION TECHNIQUES

Normal faulting and halokinesis have been important controls on the de position and subsequent deformation of Mesozoic and Tertiary strata in the North Sea. In addition to the previously documented mechanisms of salt withdrawal, dissolution and differential sedimentary loading, it is recognized that gravity-driven thin-skinned extension above inclin ed salt layers has played an important part in North Sea basin develop ment. Commercial section restoration software has been used to facilit ate depth conversion, restoration and decompaction of seismic sections selected from an interpreted regional database in the western central North Sea, allowing validation of the interpretations, and a graphica l and highly quantitative description of salt-assisted extension. Resu lts of this work show that Zechstein Group evaporites were deposited i n shallow sag basins during the Permian. Triassic sedimentary pods wer e generated by localized deposition in synclinal basins and grabens ab ove the evaporites. Bedded salt became folded, while mobile salt flowe d to fill anticline cores. Since the early Jurassic, regional tectonic tilting related to post-rift subsidence and increasing sedimentary ov erburden have caused allochthonous Mesozoic and Tertiary strata to ext end by gravity spreading above the mobile salt layer, which detaches t he allochthon from the underlying autochthonous Late Palaeozoic rocks. Concave-up listric normal faults sole out in the salt layer, propagat e into the overlying cover sequence, and have been active at different geological times causing stratal thickening and folding within the al lochthon. Antithetic and synthetic normal faults have developed, produ cing complex upward branching fault systems. In map view, the listric faults form curvilinear en echelon arrays, the faults linked by relay ramps. Fault blocks are typically 3-7 km wide, 2-3 km thick and 7-10 k m long. Salt movement during the Jurassic-Tertiary has been driven by active extension of the cover, causing salt to fill potential voids cr eated by fault block rotation. Thus salt highs occur beneath sites of extension. The listric faults generally dip in the same direction as t he sub-salt surface, although there are also some major counter-region al faults. During extension, regional dips have increased up to about 5 degrees, which is sufficient for gravity-driven extension above a sa lt layer. A total extension of about 6% has occurred. The gravity-driv en thin-skinned extension documented in the western central North Sea is a phenomenon which can be recognized elsewhere in the North Sea bas in, and can be readily compared with similar phenomena already documen ted in offshore Angola, Brazil, Nova Scotia and the Gulf of Mexico.