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.

Mapping absolute P-wavespeeds in the Canadian and Alaskan mantle will further our understanding of its present-day state and evolution. S-wavespeeds are relatively well constrained, especially across Canada, but are primarily sensitive to temperature while complimentary P-wavespeed constraints provide better sensitivity to compositional variations. One technical issue concerns the difficulties in extracting absolute arrival-time measurements from often-noisy data recorded by temporary seismograph networks. Such processing is required to ensure that regional Canadian datasets are compatible with supplementary continental and global datasets provided by global pick databases. To address this, we utilize the Absolute Arrival-time Recovery Method (Boyce et al., 2017). We extract over 180,000 new absolute arrival-time residuals from seismograph stations across Canada and Alaska that include both land and ocean bottom seismometers. We combine these data with the latest USArray P-wave arrival-time data from the contiguous US and Alaska. Using an adaptively parameterised least-squares tomographic inversion we develop a new absolute P-wavespeed model, with focus on Canada and Alaska (CAP21). Initial results suggest fast wavespeeds characterise the upper mantle beneath eastern and northern Canada. A sharp transition between the slow wavespeeds below the North American Cordillera and the fast wavespeeds of the stable continental interior appears to follow the Cordilleran Deformation Front (CDF) in southwest Canada. Slow wavespeeds below the Mackenzie Mountains may extend further inland of the CDF in northwest Canada. In Alaska, CAP21 illuminates both lithospheric structure and the along strike morphology of the subducting slab. The newly compiled data may also improve resolution of subducted slab remnants in the mid-mantle below the North American continent, crucial to help constrain the formation of the Alaskan peninsular at ≥50Ma.