The Gakkel Ridge in the Arctic Ocean with its<br/>adjacent Nansen and Amundsen Basins is a key region for<br/>the study of mantle melting and crustal generation at<br/>ultraslow spreading rates. We use free-air gravity anomalies<br/>in combination with seismic reflection and wide-angle data<br/>to compute 2-D crustal models for the Nansen and<br/>Amundsen Basins in the Arctic Ocean. Despite the permanent<br/>pack-ice cover two geophysical transects cross both<br/>entire basins. This means that the complete basin geometry<br/>of the world’s slowest spreading system can be analysed in<br/>detail for the first time. Applying standard densities for the<br/>sediments and oceanic crystalline crust, the gravity models<br/>reveal an unexpected heterogeneous mantle with densities<br/>of 3.30 9 103, 3.20 9 103 and 3.10 9 103 kg/m3 near the<br/>Gakkel Ridge. We interpret that the upper mantle heterogeneity<br/>mainly results from serpentinisation and thermal<br/>effects. The thickness of the oceanic crust is highly variable<br/>throughout both transects. Crustal thickness of less than<br/>1 km dominates in the oldest parts of both basins, increasing<br/>to a maximum value of 6 km near the Gakkel Ridge.<br/>Along-axis heat flow is highly variable and heat flow<br/>amplitudes resemble those observed at fast or intermediate<br/>spreading ridges. Unexpectedly, high heat flow along the<br/>Amundsen transect exceeds predicted values from global<br/>cooling curves by more than 100%
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