Crustal structure in the northern North Sea: an integrated geophysical study

Abstract

Abstract This study focuses on the deep structure of the Viking Graben and adjacent areas of the northern North Sea (60–62°N), and its implications for the amount, timing and nature of lithospheric extension. Two regional transects have been constructed based on an integrated analysis of deep seismic reflection and refraction data, gravity and magnetic data, and correlations between offshore and onshore geology. The shallow interpretation is based on high-quality conventional seismic reflection data calibrated against a large number of exploration wells. The new and partly reprocessed seismic data, combined with the other geophysical data, make possible a better documentation of the crustal configuration, such as the pre-Jurassic sediment distribution, basement and Moho relief, and deep fault geometries. A lower-crustal body characterized by an 8+ km s −1 velocity and an average bulk density of 2.95 g cm −3 is present beneath the Horda Platform. This body probably represents a deep crustal root of partially eclogitized rocks that formed during the Caledonian orogeny. Heterogeneities within this body give rise to the non-typical velocity-density relation. The crust-mantle boundary is located at the base of this body at a depth of 30–35 km and does not coincide with the seismically defined Moho. The geometry of crustal thinning reflects the cumulative effect of several post-Caledonian rift phases. Results show that Permian rifting affected a wide area, from the Øygarden Fault Complex to the Hutton Fault.