Evidence for a thick oceanic crust adjacent to the Norwegian Margin

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Abstract
The oceanic crust created during this first few million years of accretion in the Norwegian‐Greenland Sea lies at an unusually shallow depth for its age, has a smooth upper surface, and in many places the results of multichannel seismic reflection profiling reveal that its upper layers comprise a remarkable sequence of arcuate, seaward‐dipping reflectors. These have been attributed to lava flows generated during a brief period of subaerial seafloor spreading. We describe the results of inversions of digitally recorded sonobuoy measurements and two‐ship expanded spread profiles collected over the oceanic crust adjacent to the Norwegian passive margin. We find that the crust of the deep Lofoten Basin is indistinguishable from normal oceanic crust in thickness and structure. Closer to the margin we observe up to a four times expansion in thickness of layers with velocities equal to those of oceanic layer 2, while the layer 3 region retains approximately the same thickness. The area over which the seaward‐dipping reflectors can be observed on reflection profiles corresponds to the region of greatest expansion in “Layer 2” thickness. In the very oldest crust immediately adjacent to an escarpment that probably marks the continent‐ocean boundary, we see evidence for a low velocity zone overlying an indistinct reflector that may mark the dyke‐lava interface in the thick crust. Comparing the structure of the thick crust to that of eastern Iceland, we find a strong resemblance, especially in the expansion in thickness of material with layer 2 velocities. These results support the suggestion that during the earliest stages of spreading extrusive volcanism at the ridge crest was unusually voluminous, building a thick pile of lavas erupted from a subaerial spreading center.