The kinematics of inversion in the Danish Central Graben

Abstract

Summary The Danish Central Graben is a complex mosaic of half-grabens and structural highs that formed during Permo-Triassic and Upper Jurassic rift phases at the confluence of the NW-SE and N-S trending elements of the Central North Sea Rift. The extensional structures forming the margins to the rift were reactivated in a reverse sense in the Late Cretaceous as part of the regional inversion of basins in the southern North Sea. Deposition proceeded throughout the deformation, and thickness variation in the deep marine Chalk succession is directly attributable to compressional deformation of the basin floor. The total shortening across the rift amounts to only a few percent, and the structural expression of the reactivated margins is typically as low amplitude folds with minor associated reverse faulting. Compressional reactivation was restricted to basement faults, and the shallow inclination of the margin faults may have been an important factor in their subsequent reactivation. Zechstein evaporites and overpressured Jurassic shales influenced the development of some of the compressional structures. The orientation of the growth fold axes is consistent with a net convergence in an approximately NE-SW direction. The fold axes are arranged in a segmented pattern which can be related to the underlying basement fabric. This segmentation not only resulted in variations in the amounts and style of shortening along the length of the rift, but seems to have influenced the timing of the deformation. Seismic-stratigraphic analysis of the Upper Cretaceous sequence indicates that the compressional deformation was markedly diachronous. Abrupt changes in the deformational chronology are found to occur across the transverse segment boundaries.