The Gjallar Ridge is an area of complex geology situated in the west of the Vring Basin. Regional structural work in the area suggests multiple phases of rifting during basin development. Seismic refraction data indicate the existence of a "low-velocity layer," which can be interpreted as magmatic underplating.

The aim of this study was to evaluate the effects of a possible magmatic underplating over the Gjallar Ridge. More precisely, we have estimated the temperature effects and the isostatic effects associated with the emplacement of a lower-density 5-km-thick magmatic body at a depth of 15–20 km.

To model the basin evolution, a series of stretching events was assumed: the opening of the basin during the Permian-Triassic, an event in the Middle Jurassic to Early Cretaceous, an event during the Middle to Late Cretaceous, and an event in the Paleocene. Models were run with and without underplating by a magmatic body emplaced during the early Tertiary. The effect of underplating has dramatic consequences on the interpretation of the early Tertiary stretching event. In the model with no underplating, low stretching ( = 1.2) is required to match subsidence. If underplating is considered, increased stretching is required, with factors up to 1.5.

The magmatic underplating has two main effects. One is a short-lived (less then 5 m.y.) increase in heat flow by 100 mW/m² related to dissipation of heat. The other is a longer-term effect associated with increased early Tertiary stretching. The combined effects result in an increased temperature of 40C for the Upper Jurassic source rock interval over the Gjallar Ridge. Maturity effects of the magmatic underplating are significant, particularly because the heat pulse gives a kick in the generation 5–10 m.y. earlier, which could be important for oil and gas available to traps formed in Late Cretaceous–Paleocene.

Models were calibrated to the observed present-day crustal thicknesses derived from seismic refraction profiles. The model with underplating calibrates best to these data.