The Norwegian Margin formed in response to early Cenozoic continental breakup and subsequent opening of the Norwegian-Greenland Sea. There is a welldefined margin segmentation and the various segments are characterized by distinct crustal properties, structural and magmatic styles, and post-opening history of vertical motions. The sedimentary basins at the conjugate continental margins off Norway and Greenland and in the western Barents Sea developed as a result of a series of post-Caledonian rift episodes until early Cenozoic time, when complete continental separation took place.

Most models of melt generation beneath mid-ocean ridges predict significant reduction of melt production at ultraslow spreading rates (full spreading rates &<20 mm x yr(-1)) and consequently they predict thinned oceanic crust. The 1,800-km-long Arctic Gakkel mid-ocean ridge is an ideal location to test such models, as it is by far the slowest portion of the global mid-ocean-ridge spreading system, with a full spreading rate ranging from 6 to 13 mm x yr(-1) (refs 4, 5). Furthermore, in contrast to some other ridge systems, the spreading direction on the Gakkel ridge is not oblique and the rift valley is not offset by major transform faults. Here we present seismic evidence for the presence of exceptionally thin crust along the Gakkel ridge rift valley with crustal thicknesses varying between 1.9 and 3.3 km (compared to the more usual value of 7 km found on medium- to fast-spreading mid-ocean ridges). Almost 8,300 km of closely spaced aeromagnetic profiles across the rift valley show the presence of discrete volcanic centres along the ridge, which we interpret as evidence for strongly focused, three-dimensional magma supply. The traces of these eruptive centres can be followed to crustal ages of approximately 25 Myr off-axis, implying that these magma production and transport systems have been stable over this timescale.

S U M M A R YBARENTS50, a new 3-D geophysical model of the crust in the Barents Sea Region has been developed by the University of Oslo, NORSAR and the U.S. Geological Survey. The target region comprises northern Norway and Finland, parts of the Kola Peninsula and the East European lowlands. Novaya Zemlya, the Kara Sea and Franz-Josef Land terminate the region to the east, while the Norwegian-Greenland Sea marks the western boundary. In total, 680 1-D seismic velocity profiles were compiled, mostly by sampling 2-D seismic velocity transects, from seismic refraction profiles. Seismic reflection data in the western Barents Sea were further used for density modelling and subsequent density-to-velocity conversion. Velocities from these profiles were binned into two sedimentary and three crystalline crustal layers. The first step of the compilation comprised the layer-wise interpolation of the velocities and thicknesses. Within the different geological provinces of the study region, linear relationships between the thickness of the sedimentary rocks and the thickness of the remaining crystalline crust are observed. We therefore, used the separately compiled (area-wide) sediment thickness data to adjust the total crystalline crustal thickness according to the total sedimentary thickness where no constraints from 1-D velocity profiles existed. The BARENTS50 model is based on an equidistant hexagonal grid with a node spacing of 50 km. The P-wave velocity model was used for gravity modelling to obtain 3-D density structure. A better fit to the observed gravity was achieved using a grid search algorithm which focussed on the density contrast of the sediment-basement interface. An improvement compared to older geophysical models is the high resolution of 50 km. Velocity transects through the 3-D model illustrate geological features of the European Arctic. The possible petrology of the crystalline basement in western and eastern Barents Sea is discussed on the basis of the observed seismic velocity structure. The BARENTS50 model is available at http://www.norsar.no/seismology/barents3d/.

Major crustal units in the western Barents Sea have been defined on the basis of depth‐converted seismic reflection data, gravity and magnetic anomaly data. The seismic data revealed three types of crustal reflectivity below the sedimentary rock units, i.e., transparency, strong lower crustal reflectivity, and thoroughly reflective. The crustal structures obtained from the seismic reflection data are supported by 2D density and magnetic modeling. Three NE‐SW‐striking crustal units have been observed south of Bjørnøya. Changes in reflectivity in southwestern Barents Sea are accompanied by gravity and/or magnetic anomalies. North of Bjørnøya, two basement provinces could be distinguished. The observed outlines of the crustal blocks in the southwestern Barents Sea match with Caledonian main thrusts onshore northern Norway. We were therefore able to outline the continuation of Scandian nappe complexes in the southwestern Barents Sea. The crustal unit to the west of the Loppa High region is supposed to be a fragment of Laurentia, since the Loppa High region aligns with the Uppermost Allochthon nappes onshore, and is therefore expected to be the collision zone between Baltica and Laurentia. The southeastern crustal unit aligns with the Lower to Upper Allochthons and their Fennoscandian basement onshore. A compilation of all known thrusts and sutures in the western Barents Sea revealed the complexity of the Iapetus closure. It challenges the concept of two Caledonian main branches (NE and N). The thrusts/sutures are fan‐shaped distributed across the western Barents Sea, resembling the structure of the Late Paleozoic–Mesozoic/Paleocene rift basins and basement highs in the southwestern Barents Sea.

S U M M A R YNew seismic refraction data were collected across the western Svalbard continental margin off Kongsfjorden (NyÅlesund) during the cruise leg ARK15/2 of RV Polarstern. The use of onshore and offshore seismic receivers and a dense air-gun shot pattern provide a detailed view of the velocity structure of Svalbard's continental interior, the continent-ocean transition, and oceanic crust related to the northern Knipovich Ridge and the Molloy Ridge.The proposed Caledonian central and western terranes of Svalbard are not distinguishable on the basis of seismic velocity structure. Below a 7 to 8 km thick Palaeozoic sedimentary cover the crystalline crust reveals a three-layer structure with seismic velocities ranging between 6.1 and 6.9 km s −1 . The geological suture between the terranes is imperceptible. The middle and upper crust below the Tertiary Forlandsundet graben shows low velocities. This can be related to faulting during the Early Palaeozoic movements between Svalbard and northern Greenland, followed by the continental break-up. Moreover, a sedimentary Palaeozoic core is may be buried below the Forlandsundet graben.The continent-ocean transition can be classified as an obliquely sheared (transform) continental margin. The Moho dips with an angle of 45 • eastwards at the continent-ocean transition that exhibits higher seismic velocities of more than 7.2 km s −1 on the continental side. The sheared margin evolution is linked to the Spitsbergen Transform Fault, today located north of the Molloy Ridge spreading segment. During a later evolutionary stage the Molloy Ridge passed the continental margin. The separating boundary between continental and oceanic crust off northwestern Svalbard is today part of the inactive Spitsbergen Fracture Zone. The high seismic velocities at the continent-ocean boundary can be interpreted as minor mantle-derived intrusions, probably induced by interaction of the passing spreading ridge during the sheared margin evolution.The oceanic crust generated at the Knipovich Ridge and the Molloy Ridge is thin (2 to 4 km), compared to the global mean, and is thinner as previously observed. The oceanic crust is characterized by the absence of oceanic layer 3. These observations can be ascribed to conductive cooling of the ascending mantle as a result of the extremely low divergence rate.The underlying mantle is slightly serpentinized below the Knipovich Ridge segment, reflected by low seismic velocities of ∼7.7 km s −1 . A thicker sequence of syn-and post-rift sediments and sedimentary rocks are observed on the Molloy Ridge oceanic segment, which most likely results from greater subsidence relative to the Knipovich Ridge segment.