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

Abstract. Using 3D reflection-seismic data constrained by wells, we address the kinematic connections between Permian Zechstein evaporites, basin-inversion structures in the sedimentary units above, and reactivated structures in the sub-salt basement in the Danish Central Graben. The Danish Central Graben is part of the failed North Sea rift system. Where present, mobile Zechstein evaporites have played a significant role in its structural development since the Triassic, while tectonic shortening caused mild inversion in the Late Cretaceous and Paleogene. Shortening was accommodated mainly by reverse reactivation of older normal faults (i.e. fault inversion) along with folding and uplift of their hangingwalls. Within the study area, rifting generated two major W-SW-dipping basement faults with several kilometres of normal offset. The larger Coffee Soil Fault delineate the eastern boundary of the rift basins. Within its hangingwall, a broad zone is characterised by inversion-related uplift and deformation. Along the fault, buttressed growth folds in the immediate hangingwall indicate thick-skinned inversion, i.e. coupled deformation between the basement and cover units. The opposite margin of the inverted zones follows the westwards pinch-out of the Zechstein salt. Here, thin-skinned folds and faults sole out into Zechstein units on the half-graben dip slopes. The most pronounced inversion occurred directly above and in extension of salt ridges and –rollers that localized shortening in the cover above. With no apparent links to underlying basement faults, we balance thin-skinned shortening to the sub-salt basement via a triangle-zone concept. This implies that thin Zechstein units on the half-graben dip slopes formed thrust detachments during inversion, and that basement shortening was mainly accommodated by reactivation of the major rift faults further east. Ductile deformation at seismic scales accounts for thin-skinned shortening of the cover units where such a detachment did not develop. We discuss the related mechanisms. The documented structural styles are similar to those found in other inverted basins in the region, and to those produced from physical-model experiments. Our results indicate that Zechstein units imposed a strong control on structural style and kinematics during basin inversion in large parts of the Danish Central Graben. We emphasize that even thin evaporite units may act as detachments during tectonic extension and contraction if favourably orientated. Salt ridges and diapiric structures can localise shortening and generate thin-skinned faults and folds in the cover above. In mildly inverted rifts, extensive mobile salt may mask the effects of basin inversion if shortening is accommodated by salt structures without the formation of clearly defined inversion structures at the surface or significant uplift.