Structural restoration of Mesozoic rifting phases in the northern North Sea
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Structural restoration has been carried out on the northern North Sea (60-62ºN), based on the reprocessed, interpreted and depth converted seismic lines NSDP84-1 and 2. Two major rifting events have previously been recognized in the area during the Mesozoic: the Permo-Triassic and Jurassic extension phases. Different structures were formed or, in some cases, the same structures were reactivated during the Permo-Triassic and Jurassic rifting phases. Permo-Triassic rifting affected a 125 km wide area from the Oygarden Fault Zone in the east to the Hutton Fault alignment in the west.. By measuring the length of the profiles before and after faulting, the restorations show that the stretching factors for upper crustal stretching during the Permo-Triassic rifting are 1.11 (11%) for NSDP84-1 and 1.10 (10%) for NSDP84-2 respectively. The Jurassic rifting was confined to a narrower zone mainly in the Viking Graben with the major faults formed on the western side of the graben. Low angle faults are identified in the western flank of Viking Graben in the Tampen Spur area. Low angle supra-basement detachments formed in the late Jurassic are found in Gullfaks area, beneath the Gullfaks Sør block and SE of the Visund fault block. Intra-basement detachments are also found in Tampen Spur area. These detachments are formed by normal faults which flatten in the basement. From the restorations, the stretching factor for the Jurassic rifting is calculated to be 1.12 (12%) for NSDP84-1 and 1.19 (19%) for NSDP84-2. The total extensions for the two rifting phases combined are 1.24 (24%) and for NSDP84-1 and 1.30 (30%) for NSDP84-2. Stretching factors (β) can also be measured by crustal thickness changes, stretching is measured before and after rifting for different area (Horda Platform, Shetland Platform, Viking Graben, and Tampen Spur), and βmean calculate for the Permo-Triassicxrifting phase are calculated 1.25 and 1.16 for NSDP84-1 and 2 respectively. For the Jurassic rifting βmean is calculated as 1.16 for NSDP84-1 and 1.17 for NSDP84-2. These values are similar to previous published results using the same methods in the Northern North Sea and represent the minimum amounts of upper crustal extension on large seismically resolved faults.