Repeated lithosphere extension in the northern Viking Graben: A coupled or a decoupled rheology?

The Late Permian-Early Triassic and Jurassic-Cretaceous rift events that influenced the structure of the northern Viking Graben are examined. Here, a 2D forward numerical model including faults is applied on seismic line NVGT88-04, which crosses the basin in the E-W direction. We concentrate on (1) the amount of Jurassic basin subsidence that can be explained by thermal contracting resulting from the Permo-Triassic event, (2) the spatial connection between the two rift events and (3) the apparent mode of flexure (coupled v. decoupled) of the lithosphere. Modelling results indicate that post-rift subsidence as a result of Permo-Triassic rifting has practically ceased at the onset of Jurassic rifting. The position of the rift axis is shown to migrate with time, from the Horda Platform in Permo-Triassic time to the present Viking Graben centre in Jurassic-Cretaceous time. The major difference between the consequences of a coupled versus a decoupled mode of flexure is that the latter causes a larger amount of fault-block rotation. On this basis, we suggest a decoupled mode of flexure for the Permo-Triassic rift phase, a coupled mode of flexure for the Early Jurassic rift phase, and a decoupled mode of flexure again for the Late Jurassic period.