Structure and tectonics of the northern North Sea: new insights from deep penetration regional seismic data

Abstract

Abstract Recently acquired, deep penetration regional seismic data provide important new insights into the structure and tectonic evolution of the northern North Sea. Improved data quality permits better understanding of fault geometries and salt tectonics in the graben areas and reveals the pre-Tertiary geology of the surrounding platforms for the first time. This paper presents seismic data from five structurally distinct areas. (1) South Viking Graben . This relatively narrow graben formed the principal sedimentary basin during the Mesozoic period and contains a thick Triassic-Cretaceous succession. Low-angle, eastward-dipping faults developed at the western basin margin control a relatively simple half-graben geometry. The Beryl Embayment is notable for the local rotation of Triassic-Jurassic strata independent of the underlying basal Permian reflectors, indicating local Mesozoic salt movement in this area. Faulted and anticlinal highs east of the graben axis may also record limited salt tectonics. (2) Horda Platform . This area comprises a series of eastward-dipping rotated Mesozoic fault blocks. Mesozoic extensional faults developed as splays in the hanging wall of a major westward-dipping deep detachment surface. (3) East Shetland Platform . This area formed a structural high throughout the Mesozoic era. Regional seismic data provide a clear image of the geometry of a Palaeozoic basin, thought to represent the northward continuation of the Orcadian Basin recognized onshore in mainland Scotland, Orkney and Shetland. The sedimentary sequence is probably of Devonian (-Carboniferous?) age and contains major unconformities, which may permit subdivision of this succession into tectono-stratigraphic units. The contact with the underlying basement is characterized by a distinctive change in seismic character, traceable over the entire area. The deep structure of the East Shetland Platform is dominated by westward- or northwestward-dipping low-angle basement faults, interpreted as thrust surfaces. This structural grain is associated with the local development of eastward-vergent folds interpreted as hanging wall anticlines developed in the Upper Palaeozoic cover sequence. These structures provide evidence of a Late Palaeozoic deformation phase. (4) Unst Basin . Lying to the west of the East Shetland Basin and bounded by a faulted horst on its eastern margin, the Unst Basin is underlain by a spectacular low angle, westward-dipping detachment surface. Roll-over into this fault is interpreted as the result of extensional roll-back of an older (?Caledonian) thrust plane. The fault geometry may indicate structural analogies with the West of Shetlands Province. (5) East Shetland Basin . The structure of this area is characterized by the development of extensive, westward-dipping fault blocks underlain by low-angle, eastward-dipping extensional faults traceable to great depth. The seismic data indicate thickening of the sedimentary succession in the footwalls of these faults, consistent with Jurassic faulting of a pre-existing Triassic basin.