The Gakkel Ridge in the Arctic Ocean with itsadjacent Nansen and Amundsen Basins is a key region forthe study of mantle melting and crustal generation atultraslow spreading rates. We use free-air gravity anomaliesin combination with seismic reflection and wide-angle datato compute 2-D crustal models for the Nansen andAmundsen Basins in the Arctic Ocean. Despite the permanentpack-ice cover two geophysical transects cross bothentire basins. This means that the complete basin geometryof the worlds slowest spreading system can be analysed indetail for the first time. Applying standard densities for thesediments and oceanic crystalline crust, the gravity modelsreveal an unexpected heterogeneous mantle with densitiesof 3.30 9 103, 3.20 9 103 and 3.10 9 103 kg/m3 near theGakkel Ridge. We interpret that the upper mantle heterogeneitymainly results from serpentinisation and thermaleffects. The thickness of the oceanic crust is highly variablethroughout both transects. Crustal thickness of less than1 km dominates in the oldest parts of both basins, increasingto a maximum value of 6 km near the Gakkel Ridge.Along-axis heat flow is highly variable and heat flowamplitudes resemble those observed at fast or intermediatespreading ridges. Unexpectedly, high heat flow along the Amundsen transect exceeds predicted values from globalcooling curves by more than 100%.