A detailed 3-D tomographic model of the whole mantle beneath the circum-Arctic region is obtained by applying an updated global tomography method to a large amount of P-wave arrival time data. Our model clearly shows the subducted Izanagi and Farallon slabs penetrating into the lower mantle beneath Eurasia and North America, respectively. In the region from Canada to Greenland, a giant stagnant slab lying below the 660-km discontinuity is revealed. Because this slab has a texture that seems to be due to subducted oceanic ridges, the slab might be composed of the Izanagi, Farallon, Kula and Vancouver slabs that had subducted during ~80−20 Ma. During that period, a complex rift system represented by division between Canada and Greenland was developed. The oceanic ridge subduction and hot upwelling in the big mantle wedge above the stagnant slab caused a tensional stress field, which might have induced these complex tectonic events.