SKAGERRAK EVOLUTION DERIVED FROM TECTONIC SUBSIDENCE

Stratigraphic data gathered from 32 points in the continental Skagerrak Basin have been analyzed by basin modelling methods. Non-tectonic effects of sediment loading were removed by backstripping, giving the "observed" tectonic subsidence. This was compared to synthetic subsidence curves constructed on the basis of a combined lithospheric extension and metamorphism model, where gabbroic magma, which was emplaced during the Permian, is transformed to granulite-eclogite facies. The overall picture is that prior to 230 Ma subsidence was roughly similar north and south of the Fennoscandian Border Zone (FBZ). After this time the northern area sagged due to the still-preserved lithospheric thermal anomaly and continued metamorphism, whereas the southern area was also affected by extension. A physical explanation for this contrast in evolution is given by considering the relative lithospheric strength history of the areas, the northern area becoming progressively stronger with time relative to the southern area as crustal thicknesses are reduced. Total post-570 Ma extension is approximately 1.4 (40%) north of the FBZ and 1.5 (50%) south of it. North of the FBZ the average thickness of the metamorphic layer is between 3 and 7 km, whereas south of the zone a thickness of 1-2 km is estimated. This difference may reflect variations in the degree of Permian magmatism. The temperature history of the sediments and the degree of organic maturation has been calculated by the TTI method. The youngest sediments to have experienced temperatures high enough for significant oil generation are estimated to be of Bathonian age. The present model heat flow of about 57 mW m-2 is in agreement with observations. Lateral displacements along the FBZ are of the order of 10-20 km, and have consequently only played a minor role in the development of the Skagerrak Basin. A thermal lithosphere thickness of about 125 km is inferred.