Thick- and thin-skinned basin inversion in the Danish Central Graben, North Sea – the role of deep evaporites and basement kinematics

Abstract

Using borehole-constrained 3D reflection seismic data, we analyse the importance of sub-salt, salt, and supra-salt deformation in controlling the geometries and the kinematics of inverted structures in the Danish Central Graben. The Danish Central Graben is part of the failed Late Jurassic North Sea rift. Later tectonic shortening caused mild basin inversion during the Late Cretaceous and Paleogene. Where mobile Zechstein evaporites are present, they have played a significant role in the structural evolution of the Danish Central Graben since the Triassic. Within the study area, Jurassic rifting generated two major W- to SW-dipping basement faults (the Coffee Soil Fault and the Gorm–Tyra Fault) with several kilometres of normal offset and associated block rotation. The Coffee Soil Fault system delineates the eastern boundary of the rift basins, and within its hanging wall a broad zone is characterized by late Mesozoic to early Paleogene shortening and relative uplift. Buttressed growth folds in the immediate hanging wall of the Coffee Soil Fault indicate thick-skinned inversion, i.e. coupled deformation between the basement and cover units. The western boundary of the inverted zone follows the westward pinch-out of the Zechstein salt. Here, thin-skinned folds and faults sole out into Zechstein units dipping into the half-graben. The most pronounced inversion structures occur directly above and in prolongation of salt anticlines and rollers that localized shortening in the cover above. With no physical links to underlying basement faults (if present), we balance thin-skinned shortening to the sub-salt basement via a triangle zone concept. This implies that thin Zechstein units on the dipping half-graben floor formed thrust detachments during inversion while basement shortening was mainly accommodated by reactivation of the major rift faults further east. Disseminated deformation (i.e. “ductile” at seismic scales) accounts for thin-skinned shortening of the cover units where such a detachment did not develop. The observed structural styles are discussed in relation to those found in other inverted basins in the North Sea Basin and to those produced from physical model experiments. Our results indicate that Zechstein units imposed a strong control on structural styles and kinematics not only during rift-related extension but also during basin inversion in large parts of the Danish Central Graben. Reactivated thin-skinned faults soling out into thin Triassic evaporite units within the carapace above Zechstein salt structures illustrate that even thin evaporite units may contribute to defining structures during tectonic extension and shortening. We thus provide an updated and dedicated case study of post-rift basin inversion, which takes into account the mechanical heterogeneity of sub-salt basement, salt, and supra-salt cover, including multiple evaporite units of which the Zechstein is the most important.