The Canadian Beaufort Sea continental margin of northwestern Canada is a Cenozoic convergent margin, potentially representing a rare case of incipient subduction. Here, we produce P- and S-wave seismic velocity models of the crust and the uppermost mantle using recordings from regional earthquakes. Our models reveal a northwest-dipping very low-velocity anomaly within the crust (δV up to -15%) beneath the Romanzof Uplift. We interpret this low-velocity feature to correspond to a weaker and thicker crust due to shortening and stacking of igneous and sedimentary rocks. The co-location of the thickened crust and lack of present-day seismicity indicates that north-south compression is accommodated by slow, aseismic deformation in the narrow margin beneath the Romanzof Uplift or more broadly offshore. Neither interpretation requires a subduction initiation process.