Thick- and thin-skinned basin inversion in the Danish Central Graben, North Sea – the role of deep evaporites and basement kinematics
Authors: Hansen Torsten Hundebøl , Clausen Ole Rønø , Andresen Katrine Juul 
Published: August 2021

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.