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.

[1] In year 2000, an ocean bottom seismometer (OBS) profile was acquired across the Møre margin to the Aegir Ridge, an extinct seafloor spreading axis. The margin is an early Eocene volcanic passive margin, located between the Faeroe-Iceland Ridge (FIR) and the East Jan Mayen Fracture Zone (EJMFZ). The P wave data were modeled by ray tracing to give a crustal transect showing a 10-11 km thick igneous crust created by breakup magmatism, tapering off to magma-starved seafloor spreading by C23 time (51.4 Ma). The location of the EJMFZ was reinterpreted from a satellite derived gravity map, and spreading direction in the Norway Basin reevaluated. No other fracture zones were confirmed, and both thin oceanic crust (4-5 km) and lack of fracture zones resemble ultraslow spreading on the Arctic Gakkel Ridge. Magnetic seafloor spreading anomalies were identified from the magnetic track recorded with the OBS profile, and half spreading rates were derived. Early seafloor spreading was slow (15-32 mm yr À1 ), approaching ultraslow (6-8 mm yr À1 ) by C20 time (42.7 Ma). A V-shaped pattern seen in the gravity field located only around the northern part of the Aegir Ridge corresponds to increased crustal thickness in the seismic model, recording northeast transport (3-6 mm yr À1 ) of more melt-fertile asthenosphere zones. The magma-starved character of the Norwegian Basin seen also during slow seafloor spreading may be the result of depletion of the asthenosphere when the Iceland plume constructed the FIR to the south, as the asthenosphere is subsequently transported into the Norway Basin.

An extensive seismic survey using ocean‐bottom seismographs (OBS) was performed in the area across the Jan Mayen Basin, North Atlantic, from the Jan Mayen Ridge to the Iceland Plateau. The Jan Mayen Ridge and surrounding area are considered to be a fragment of a continent which was separated from Greenland just prior to magnetic anomaly 6. This study presents the crustal structure of the Jan Mayen microcontinent and the ocean/continent transition to the west of the Jan Mayen Ridge. The crustal structures from the centre of the Jan Mayen Ridge to the Jan Mayen Basin are characterized by a deep sedimentary basin, a thin basaltic layer within the sedimentary section and extreme thinning of the continental crust towards the Iceland Plateau. The OBS data indicate that a continental upper crust (V p=5.8–6.1 km s−1) and lower crust (V p=6.7–6.8 km s−1) underlie the deep sedimentary basin. The thickness of the continental lower crust varies significantly from 12 km beneath the Jan Mayen Ridge to almost zero thickness beneath the northwestern part of the Jan Mayen Basin. An ocean/continent transition zone is found at the western edge of the Jan Mayen Basin. Within the 10 km wide transition zone, crustal velocities increase towards the Iceland Plateau, and approach the velocities of the oceanic crust obtained at the Iceland Plateau, that is 3.8–5.1 km s−1 (oceanic layer 2A), 5.9–6.5 km s−1 (oceanic layer 2B) and 6.8–7.3 km s−1 (oceanic layer 3). The crustal model indicates very thin oceanic crust (5 km) immediately oceanwards of the ocean/continent transition zone. Beneath the Iceland Plateau, the oceanic crust is thicker (9 km) than the typical thickness of normal oceanic crust. This might imply that the oceanic crust at the Iceland Plateau has been generated by asthenospheric material slightly hotter than normal. From the crustal structure obtained by the present study, it is proposed that the western part of the Jan Mayen Ridge may be referred to as a non‐volcanic continental margin, generated by a long duration of rifting. Even if the asthenospheric material upwelling along the margin were hotter than normal, only small amounts of magmatic intrusions and extrusions would have been generated because of significant conductive cooling under the long duration of rifting.

[1] A 180 km long seismic wide-angle profile was acquired across the Vøring Plateau, NE Atlantic, using a tuned air gun array and three-component ocean bottom seismometers deployed with $5 km spacing. The seismic P wave data have been modeled by ray tracing/inversion, and the model has been constrained by S wave and gravity modeling. The data and modeling have allowed us to depict the crustal structure and nature of the continent-ocean transition (COT) in a classical volcanic margin case. The P wave velocity near the top of the main crustal layer is estimated to $6.0 km/s landward of the $25 km wide COT. The seaward increase to $6.5 km/s in the COT is conformable with heavily intruded continental crust within this zone. Farther seaward, the velocity increase to $6.9 km/s in the same layer suggests the presence of oceanic crust. The abundant magmatism landward of, and within, the COT is primarily observed as extrusives forming wedges of seaward dipping reflectors and mafic lower crustal intrusions/underplating. The maximum thickness of the oceanic crust is measured to $23.5 km at the seaward termination of the COT.Citation: Mjelde, R., T. Raum, B. Myhren, H. Shimamura, Y. Murai, T. Takanami, R. Karpuz, and U. Naess (2005), Continent-ocean transition on the Vøring Plateau, NE Atlantic, derived from densely sampled ocean bottom seismometer data,

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