A crustal-scale seismic refraction survey has been conducted in the Arctic Ocean across the NE Alpha Ridge complex and its northern flank within the SE Makarov Basin. The data from the four reversed profiles have been analysed using 1-D and 2-D synthetic seismogram modelling schemes. Upper crustal velocities beneath these parts of the Alpha Ridge complex and Makarov Basin are surprisingly uniform at 5.0-5.2 km s-l, increasing smoothly and rapidly to ~6 . 5 km sK1 at depths of only 4 3 km. Below this level the velocities continue to increase relatively smoothly, reaching values of ~7 . 0 km s-l at depths ranging from 14 to 19 km. Mantle-type velocities of 23.0 km sK1 are found at depths of 36-44 km beneath some regions of the Alpha Ridge complex and 21-25 km beneath the SE Makarov Basin. Based on these results and other geophysical and geological data from the Arctic Ocean we interpret the Alpha Ridge complex as an Icelandic-type structure generated by Mantle Plume activity, and we suggest that the region of Makarov Basin surveyed is underlain by a thick oceanic crustal section.

Ray tracing through simple three‐dimensional velocity models clearly demonstrates that a localized region of mantle inhomogeneity beneath Hawaii is not the source of large dt/dΔ anomalies. Upper mantle inhomogeneity beneath western North America is shown to be the likely source of most anomalous seismic data recorded there from events in the western Pacific Ocean.