Seismic profiling across the Mendeleev Ridge at 82°N:: evidence of continental crust

In year 2000, a ship-based expedition carried out a ca. 500-km-long geophysical profile ('Arctic-2000') across the Mendeleev Ridge at 82 degrees N, from the Podvodnikov Basin to the Mendeleev Basin. A crustal-scale refraction experiment was combined with shallow reflection and complementary gravity measurements. Bottom samples were also collected. The reflection survey provided data on the depth to the seafloor and the thickness of the sedimentary cover, the latter being divisible in some areas into three layers (I, II and III) and reaching a maximum thickness of 3.5 km in the Podvodnikov Basin. The composition of the underlying bedrock was investigated by the refraction survey, with the uppermost unit (layer IV) having a velocity Vp of 5.0-5.4 km s(-1) and a thickness of up to 4 km, being greatest below the ridge. A sharp increase in velocity marks the boundary to the underlying, layer V rocks (Vp = 5.9-6.5 kms(-1)), which are inferred to be crystalline basement, with a general thickness of 1-3 km, reaching 4 km below the axis of the ridge. Layer VI has a velocity Vp ranging downwards from 6.7 to 7.3 km s(-1) over a thickness of 19-20 km below the arch of the ridge; this decreases to 5-16 km below the western slope towards the Podvodnikov Basin and 7-14 km beneath the eastern slope towards the Mendeleev Basin. These velocities may correspond to the composition of basic granulites. The lowermost unit above the Moho (layer VII), with a Vp of 7.4-7.8 km s(-1), is thought to be of mixed crust-mantle composition, perhaps the result of underpating; it has a maximum thickness of 7 km beneath the ridge and thins rapidly to east and west. The base of the crustal section is taken at the boundary with a Vp = 7.9-8.0 km s(-1), which defines the Moho. The overall thickness of the crust along the 'Arctic-2000' profile varies from a maximum of 32 km below the ridge to 13 km below the Mendeleev Basin and 20 km below the Podvodnikov Basin. Based on bottom sampling by piston coring and dredging, the lithologies of layer IV have been inferred to be dominated by carbonate and terrigenous sedimentary rocks, with some igneous intercalations. This evidence, taken together with the identification of the immediately underlying layer V with a Vp velocity of 5.9-6.5 km s(-1) suggests that the Mendeleev Ridge may be composed of continental material that has been substantially altered during the development of the deep Arctic Basin and associated magmatism. The gentle gradient southward across the Kucherov Terrace to the continental shelf suggests that it is an extension of the Eurasian margin and can be compared with other margins with highly attenuated continental crust.