Application of extensional models to the northern Viking Graben, North Sea

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1987-05

Authors

Giltner, John Patrick, 1962-

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Abstract

Several previous authors have concentrated on the Central Graben in an attempt to model the North Sea as an extensional basin. Theoretical stretching models predict a certain amount of subsidence, crustal thinning, and fault displacement for a given amount of lithospheric extension. The validity of the models has been questioned since the extension suggested by subsidence data is at least twice the measured elongation on the observed faulting. In the Central Graben, the observations mentioned above are hampered by salt diapirism, extensive erosion, structural inversion, and strike-slip faulting. The Viking Graben, 300 km to the north, is a better location to test the stretching models since it appears to display extensional tectonics in a more simplified form. Subsidence curves corrected for compaction, estimated water depths of deposition, and changes in sea level from the Northern Viking Graben are matched to a uniform time-dependent extensional model. The accumulation of Lower Triassic to Recent sediments may be explained within a framework of two extensional phases that took place first during the Triassic and then again during the Late Jurassic/Early Cretaceous. Present day subsidence along the graben axis suggests a total extension of approximately 1.8, consisting of a Triassic extension of 1.5 and an Late Jurassic extension of 1.2. On the Horda Platform, Triassic rifting is even more dominant, with little expression of an Late Jurassic event. Within the graben, the model predicts subsidence very well, but the flanks are problematic. A more complicated model involving depth dependent stretching, mantle convection, or isostatic uplift of individual fault blocks may be needed to explain the uplift/non-subsidence of the graben flanks during rifting. Geometry of rotated fault blocks in the area accounts for extension of between 1.1 and 1.3, which is consistent with the Late Jurassic event. Although Triassic faulting is clearly seen on the eastern margin of the basin, early phase faulting in the graben axis has probably been rotated by the later tectonic activity and is not clearly imaged due to its depth and orientation. Crustal thinning determined from recent deep reflection and older refraction profiles suggest an extension of approximately 2.0, which is compatible with the observations of total subsidence. My modeling suggests that most of this thinning occurred during the Triassic.

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