Abstract

The main structural elements resulting from Late Permian-Early Triassic extension have been resolved across the total width of the northern North Sea. The northern segment of the basin (north of 61°N), bounded to the west by the Hutton Fault and to the east by the Norwegian mainland, was occupied by four westerly tilted half grabens, 25–35 km in width, prior to Jurassic extension. The central segment of the basin (60°20′–61°N) exhibits a full graben beneath the Jurassic Horda Platform, flanked by half grabens tilted towards the basin margins. The change in Permo-Triassic fault-block configuration, defining structural domains, is largely constrained by a basement discontinuity which runs diagonally (NE-SW) across the basin.

Maximum Jurassic extension is represented by several centres of subsidence, aligned NNE-SSW across a N-S-oriented and broader Permo-Triassic basin. Jurassic extension was concentrated within the Viking Graben proper south of 60°N, which in this area is represented by single westerly-tilted, en échelon half graben segments 20–25 km wide. The area affected by Jurassic extension becomes progressively wider to the north, again governed by the basement discontinuity, eventually encompassing the total width of the basin (c. 200 km) north of 61°N.

The central segment of the northern North Sea displays the greatest contrast in Permo-Triassic and Jurassic structural styles and basin geometry. Jurassic stretching and faulting was negligible to the east (Horda Platform), precisely the area which had been the site of maximum Permo-Triassic stretching and fault activity. It has generally been assumed that Jurassic fault configuration along the eastern margin of the central segment of the Viking Graben, to a large extent reflects the reactivation of major Permo-Triassic faults. This study demonstrates the interference of Permo-Triassic and Jurassic basin geometries in this area, due to major faults of the two generations dipping in opposite directions. As a consequence, the E-tilted Oseberg Fault Block of Jurassic origin and c. 20km in width, is underlain by a wider (c. 35 km) and W-tilted Permo-Triassic fault-block.

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