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S U M M A R YIn year 2000, a ship-based expedition carried out a ca. 500-km-long geophysical profile ('Arctic-2000') across the Mendeleev Ridge at 82 • 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 km s −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.

The ArcCRUST model consists of crustal thickness and estimated crustal thinning factors grids for the High Arctic and Circum‐Arctic regions (north of 67°N). This model is derived by using 3‐D forward and inverse gravity modeling. Updated sedimentary thickness grid, an oceanic lithosphere age model together with inferred microcontinent rifting ages, variable crystalline crust and sediment densities, and dynamic topography models constrain this inversion. We use published high‐quality 2‐D seismic crustal‐scale models to create a database of Depths to Seismic Moho (DSM) profiles. To check the quality of the ArcCRUST model, we have performed a statistical analysis of misfits between the ArcCRUST Moho depths and DSM values. Systematic analysis of the misfits within the Arctic sedimentary basins provides information about tectonic processes unaccounted by the assumed model of pure‐shear lithospheric extension. In particular, our model implies a less dense and/or thin mantle lithosphere underneath microcontinents in the deep Arctic Ocean where the ArcCRUST depth to Moho values exceed the DSM. A systematically larger gravity‐derived crustal thickness (~3 km) under the western and northern Greenland Sea points to a hotter upper mantle implied by the seismic tomography models in the North Atlantic.

S U M M A R YThe Canada Basin and the southern Alpha-Mendeleev ridge complex underlie a significant proportion of the Arctic Ocean, but the geology of this undrilled and mostly ice-covered frontier is poorly known. New information is encoded in seismic wide-angle reflections and refractions recorded with expendable sonobuoys between 2007 and 2011. Velocity-depth samples within the sedimentary succession are extracted from published analyses for 142 of these records obtained at irregularly spaced stations across an area of 1.9E + 06 km 2 . The samples are modelled at regional, subregional and station-specific scales using an exponential function of inverse velocity versus depth with regionally representative parameters determined through numerical regression. With this approach, smooth, non-oscillatory velocity-depth profiles can be generated for any desired location in the study area, even where the measurement density is low. Practical application is demonstrated with a map of sedimentary thickness, derived from seismic reflection horizons interpreted in the time domain and depth converted using the velocity-depth profiles for each seismic trace. A thickness of 12-13 km is present beneath both the upper Mackenzie fan and the middle slope off of Alaska, but the sedimentary prism thins more gradually outboard of the latter region. Mapping of the observed-to-predicted velocities reveals coherent geospatial trends associated with five subregions: the Mackenzie fan; the continental slopes beyond the Mackenzie fan; the abyssal plain; the southwestern Canada Basin; and, the Alpha-Mendeleev magnetic domain. Comparison of the subregional velocity-depth models with published borehole data, and interpretation of the station-specific best-fitting model parameters, suggests that sandstone is not a predominant lithology in any of the five subregions. However, the bulk sand-to-shale ratio likely increases towards the Mackenzie fan, and the model for this subregion compares favourably with borehole data for Miocene turbidites in the eastern Gulf of Mexico. The station-specific results also indicate that Quaternary sediments coarsen towards the Beaufort-Mackenzie and Banks Island margins in a manner that is consistent with the variable history of Laurentide Ice Sheet advance documented for these margins. Lithological factors do not fully account for the elevated velocity-depth trends that are associated with the southwestern Canada Basin and the Alpha-Mendeleev magnetic domain. Accelerated porosity reduction due to elevated palaeo-heat flow is inferred for these regions, which may be related to the underlying crustal types or possibly volcanic

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