Temperature Effect of Magmatic Underplating in the Gjallar Ridge, Mid-Norway

W. Fjeldskaar, H. Johansen, T. Dodd, and M. Thompson

Previous investigations have suggested significant magmatic underplating along the continental margin off mid-Norway. The magmatic underplating, connected with the breakup and formation of the Norwegian-Greenland Sea in Late Paleocene time, is interesting because of the potential affect on the thermal regime of the sedimentary basin. The most obvious effect is increased heatflow caused by the hot mass intruded into the earth's crust. The underplating will, however, also affect the basin subsidence and thereby the reconstructed palaeo heatflow of the basin.

The reconstruction of the basin evolution is modelled by steady state porosity-depth trends, and listric fault modelling. The result (observed total subsidence) is fitted by the sum of the isostatic and tectonic subsidence.

Four tectonic events were specified: one at the opening of the basin in the Permian, a second one of Early Cretaceous age, a third one of Late Cretaceous age and a final event in the Early Tertiary. The four tectonic events generally gave a good fit between observed and calculated total subsidence of the area. With the introduction of a 5 km magmatic body at 15-20 km depth, the Early Tertiary stretching factors, however, increase significantly.

The increased thinning in Early Tertiary time leads to an increased heatflow of 15-20 mW/m². The heating effect of the underplated body is of similar importance, but shortlived, it has almost died out after 5 million years. The modelled total temperature effect of the magmatic underplating is ca. 25 to 30°C at 5 km depth in the sedimentary basin.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California