This study focuses on the deep structure of the Viking Graben and adjacent areas of the northern North Sea (60-62°N), and its implications for the amount, timing and nature of lithospheric extension. Two regional transects have been constructed based on an integrated analysis of deep seismic reflection and refraction data, gravity and magnetic data, and correlations between offshore and onshore geology. The shallow interpretation is based on high-quality conventional seismic reflection data calibrated against a large number of exploration wells. The new and partly reprocessed seismic data, combined with the other geophysical data, make possible a better documentation of the crustal configuration, such as the pre-Jurassic sediment distribution, basement and Moho relief, and deep fault geometries. A lower-crustal body characterized by an 8+ km s 1 velocity and an average bulk density of 2.95 g cm 3 is present beneath the Horda Platform. This body probably represents a deep crustal root of partially eclogitized rocks that formed during the Caledonian orogeny. Heterogeneities within this body give rise to the non-typical velocity density relation. The crust mantle boundary is located at the base of this body at a depth of 30 35 km and does not coincide with the seismically defined Moho. The geometry of crustal thinning reflects the cumulative effect of several post-Caledonian rift phases. Results show that Permian rifting affected a wide area, from the Oygarden Fault Complex to the Hutton Fault.Deep seismic reflection data have formed the basis for numerous papers through the last decade, focusing on the crustal structure and basin evolution in the northern North Sea rift system. However, poor data quality, especially the low S/N ratio at depth, has led to many model-driven interpretations. The NSDP84-1ines were first described by Gibbs (1987a, b) and Klemperer (1988). Additional interpretations have been presented by Harrison

Two semi-regional wide-angle Ocean Bottom Seismograph (OBS) profiles, acquired east of the Faroe Islands, have been analysed by use of forward and inverse modelling to map the crustal structures. In the present wide-angle data, the Tertiary basalt shows a maximum thickness of 3 km under the Faroe Islands, decreasing towards the Faroe–Shetland Channel where it terminates. Sedimentary rocks are present below the basalts and vary in thickness from 2 km to a maximum of 8 km towards the Faroe–Shetland Channel. These sedimentary rocks appear mainly as a low-velocity zone, and the presence of high-velocity intrusions in these layers generate several step-back features in the wide-angle refraction data. Pre-Cretaceous sedimentary rocks are only inferred north of the Clair fracture zone, while Cretaceous rocks dominate southeast of the Westray fracture zone. The crystalline basement is divided into an upper granitic and a lower granodioritic part. P-wave velocities around 7.0 km s −1 are modelled in the lowermost part of the crust, indicating that magmatic underplating is not present below the Faroe Islands. The depth to the Moho is modelled with a maximum depth of 29–30 km below the northern part of the Faroe Islands, decreasing both southeastwards and southwestwards to 25 km and 17 km, respectively.

The enormous quantity of commercial reflection seismic lines across the North Sea Basin have made the area one of the most thoroughly studied continental settings in the world. Further insight in the deep architecture of the crust is provided by c. 10 000 km deep reflection seismic data. Unfortunately, these unique databases have rarely been combined systematically to constrain possible tectonic models for the area. This paper is built on a full integration of high-quality commercial lines (7stwt) and the deep (15stwt) NSDP84-1 and -2 lines. The deep lines have been post-stack reprocessed and depth-converted. A number of deep wells have provided stratigraphic control along the lines. The overall reflective pattern in the lines divides the crust in three, with a reflective upper and lower crust separated by a less reflective middle crust. The lateral changes in reflectivity matches the observed variation in crustal thickness, where the thinnest crust coincides with the Viking Graben area with a total crustal thickness of 21-24km, increasing to 30-36km in the platform areas. The lower crust is seen as an undulating 4-10 km thick band with shallow dipping reflections, with a Moho that consists of reflections with variable lateral thickness and amplitude, rather than one single strong reflection. The structural analysis shows that the crust is cut by a number of large normal faults with varying geometries. It is assumed that some of these major faults are long-lived features rooted in old basement grains. The most spectacular normal faults developed during the Permo-early Triassic extensional phase, but were often reactivated during the Jurassic extensional phase, and with continued minor fault movement into the Cretaceous thermal cooling period. Integration of commercial and deep reflection seismic sections shows that three detachment levels are present within the crust. These levels, which control changes in fault geometries, are believed to represent lateral rheological interfaces combined with or intersected by long-lived zones of weaknesses. The uppermost level is represented by supra-basement low-angle normal faults controlled by gravity and/or lithological changes during extension. An intra-basement (middle crust) level between 5 and 7 s (twt) coincides with decreasing dip of the larger basin bounding faults. The lower crust is the deepest detachment level, which probably exerts control on the geometric changes of the upper-mantle shear zones and the largest crustal normal faults.

An upper Jurassic, wedge-shaped syn-rift succession, comprising the Heather and Draupne Formations, is present in the hangingwall trough of the Snorre Fault Block. The succession is bounded to the west by the Statfjord East Fault, whereas it onlaps the snorre Fault Block to the east. It consists of a two-fold coarsening-upward sequence from shale to sandstone of shallow marine/shoreline origin.Active fault block rotation and subsidence in the Snorre-H area commenced in the Mid-Bathonian and lasted through the Ryazanian. The Heather Formation was deposited during the early rift stage (Mid-Bathonian-Early Oxfordian; 3° cumulative tilt), whereas the Draupne Formation (Late Oxfordian-Ryazanian; 9° cumulative tilt) accumulated during the main and late rift stages. The lower part of the Heather Formation was likely deposited across a submerged tilted fault block terrain, with a predominant extra-basinal sediment supply. Deposition of the upper Heather Formation, however, was governed by gradually emerging footwall islands, albeit yet without significant local erosion. As a result of increased fault/block rotation during deposition of the Draupne Formation Shale, Sequences I–II (late Early Oxfordian-early Mid-Volgian) footwall islands became firmly established, providing a predominant local sediment source to the Snorre-H sub-basin. Clay and silt were supplied from erosion of the Heather Formation on the Snorre-H hangingwall, with subordinate input of sand from the Statfjord East footwall. Subsequent deposition of the Draupne Formation, Sequences III–V (Late Mid-Volgian-Ryazanian), was governed by significant relief on the footwall islands, causing deep erosion into the Brent Group on the Snorre-H hangingwall dip-slope and leading to progradation of the Upper Draupne Sandstone shoreline complex across the Snorre-H area. Deposition of the Draupne Formation and temporal shoreline position were likely partly controlled by northwards fault-tip propagation of the Statfjord East Fault.Various syn-rift play models and depositional reservoir facies are present within the Snorre-H hangingwall basin. They include dip-slope shallow marine/shoreline sands, basin floor gravity transported sands and likely footwall talus sands enveloped in organic rich shales of the Draupne Formation. The distribution of reservoir facies is intimately linked to exposure and erosion of the middle Jurassic Brent Group below the syn-rift unconformity.