SUMMARY The Marvin Spur is a 450-km-long east–west trending escarpment along the northernmost periphery of the Alpha Ridge, starting about 500 km from the coasts of Ellesmere Island and Greenland off the Arctic Ocean margin of North America and running subparallel to the Amerasian margin of the continental Lomonosov Ridge. This region was investigated as part of the Canada–Sweden Polar Expedition in 2016, from which two seismic profiles are presented. The first is a 165-km-long line along the crest of the Marvin Spur. The second is a 221-km-long line extending southwestward from the spur to the northern flank of the Alpha Ridge within the Cretaceous High Arctic Large Igneous Province (HALIP). Multichannel seismic reflection data were acquired along both lines using a 100-m-long streamer, and the airgun shots were also recorded using 16 sonobuoys and 5 stations on the sea ice to calculate a velocity model for the crust from forward modelling of seismic traveltimes. The Marvin Spur profile reveals up to 1100 m of sedimentary rocks on top of a 1-km-thick series of basalts (4.5–5.1 km s−1). Upper and lower crust have velocities of 5.8–5.9 km s−1 and 6.2–6.3 km s−1, respectively, with the upper crust being 1–2 km thick compared to around 13 km for the lower crust. A wide-angle double seismic reflection manifests the top and base of a 6-km-thick lower crustal layer that we interpret as magmatic underplating beneath the continental crust of the Marvin Spur. We correlate a high-amplitude magnetic anomaly on Marvin Spur with a comparable anomaly on Lomonosov Ridge by invoking 110 km of dextral strike-slip motion. Assuming that HALIP-related magmatic deposits generate these anomalies, the strike-slip motion pre-dates the main phase of magmatism (latest Cretaceous, 78 Ma). On the northern Alpha Ridge, sediments are around 1-km-thick and cover a 700 to 1700-m-thick series of basalts with velocities of 4.4–4.8 km s−1. Below is a 3-km-thick layer with intermediate velocities of 5.6 km s−1 and a lower crust with a velocity of 6.8 km s−1. Moho depth is not resolved seismically, but gravity modelling indicates a total thickness of 13 or 18 km for the igneous crust except for the Fedotov Seamount where Moho deepens by about 5 km. Construction of the seamount occurred in multiple magmatic phases, including flow eruptions during deposition of the Cenozoic sedimentary succession post-dating the main HALIP magmatism.