Crustal structure of the northern part of the Vøring Basin, mid-Norway margin, from wide-angle seismic and gravity data

Abstract

Three regional Ocean Bottom Seismograph (OBS) profiles with a total of fifteen recovered OBSs were acquired in the northern part of the Vøring Basin, mid-Norway margin, in 1996. The data have been modelled by inversion and forward modelling of the OBS vertical components (P-waves). The velocity is found to increase downwards within the sedimentary layers, due to increasing depth of burial (confining pressure). Within the two deepest sedimentary layers there seems to be an increase of 0.5–1 km/s in velocity northwestwards. This increase is most likely caused by high-velocity sill-intrusions in the sedimentary rocks, emplaced during the rift episode leading to Early Eocene opening of the NE Atlantic. The presence of significant amounts of sills seems to terminate close to the Nyk High along profile 5 (dip-profile), and close to a dome northwest of the shelf edge along profile 6 (dip-profile). The velocity in the top of this dome is anomalously high (3.4 km/s at only 1 km depth beneath the seafloor), suggesting that the high consists of volcanic rocks, or heavily intruded sedimentary rocks. The relatively low velocities derived from within the upper crystalline crust (5.9–6.2 km/s) confirms that the crust is of continental origin. Within the lower crust there seems to be a clear increase in velocity northwestwards (from about 7.0 km/s to about 7.4 km/s), suggesting that the amount of high-velocity intrusions (underplating) in the lower crust decreases landwards. The modelling of the strike-profile (profile 7) suggests that the transition from a lower crust dominated by magmatic intrusions to a more undisturbed continental lower crust is located approximately beneath this profile. The model for profile 7 ties closely southwestwards to the models derived from OBS data acquired in 1992, and northeastwards to Lofoten (OBS data acquired in 1988). The models indicate that the thick magmatic intrusions in the sedimentary rocks and lower crust extend further landwards in the area of the 1992 survey, and that the transition zone is related to the Surt Lineament. The distribution of magmatic rocks thus seems to correlate strongly with pre-breakup structures. In addition to the P-wave modelling, gravity modelling has been performed along all profiles. The gravity modelling confirmed all main aspects of the OBS models, and provided constraints on the crustal structures towards the ends of the profiles.