Structural evolution of the Gert-Mjølner area

Abstract

The Gert-Mjølner area is located in the northern part of the Danish Central Trough at the border between Denmark and Norway. A three-dimensional survey covering the Gert-Mjølner area and selected two-dimensional seismic lines which reveal the regional geology are interpreted here. The structural evolution of the Danish Central Trough from the Late Jurassic to the Early Cretaceous was characterized by a tensional/transtensional tectonic regime. In general, this resulted in the subsidence of major fault blocks dipping towards the north-east, forming a series of half-graben. Two of these present day half-graben are known as the Feda Graben and the Gertrud Graben. The two graben are separated by a NW-SE anastomosing fault zone named the Gert Fault. The structural development of the area was characterized by three main fault phases. During the Late Kimmeridgian a series of active N-S striking faults controlled the depositional conditions in the Gert-Mjølner area. The N-S trending faults became inactive in the latest part of the Kimmeridgian and were replaced by subsidence along NW-SE trending faults that were especially important in the formation of the Volgian Feda Graben. It is suggested that during the Late Volgian and Early Cretaceous, minor clockwise rotation of the Feda Graben relative to the Gertrud Graben created a local compressional tectonic regime along the so-called Gert Fault zone, most pronounced in the northern part of the investigated area. During the Cretaceous the rifting phase of the Central Graben ceased and subsidence was driven by thermal cooling. However, additional subsidence related to salt movements is observed in both the Feda and Gertrud Grabens. In the Late Cretaceous, former depocentres experienced inversion within the western part of the Feda Graben and the eastern part of the Gertrud Graben.