Analysis of the Importance of Extension in Accounting for the Post-Carboniferous Subsidence of the North Sea Basin
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Post Carboniferous sedimentary deposition in the Central North Sea basins can be separated into three major periods: Permian, Triassic and mid-Jurassic through present. Most efforts to explain the basin within an extensional framework have concentrated on the post mid-Jurassic subsidence. These efforts have ignored the large amount of prior extension required to account for the observed crustal thinning and the substantial Permian and Triassic sediment fill. In addition the models predict a mid-Jurassic through early Cretaceous extension that significantly exceeds estimates of the horizontal displacement observed on high angle faults on multichannel seismic lines. We show in areas of minimal pre-Permian subsidence that adding two earlier phase extensions, one in the late Carboniferous through early Permian and the other in the Triassic produces a nearly horizontal late Carboniferous crustal thickness. The time-dependent extensional model required to account for the three periods of sediment deposition gives an excellent match to the observed subsidence history of the basement. We present an analysis of a recent seismic reflection line nm across the Central Graben in the vicinity of published refraction and well data. We show that the extension required in the third phase of the three phase model is compatible with the observed displacement on the high angle mid-Jurassic through early Cretaceous faults. However, we find no evidence for major extension either in the Triassic or late Carboniferous through early Permian.