Upper mantle structure of Arctic Canada from Rayleigh wave dispersion

Abstract

Models of upper mantle structure across the Canadian High Arctic region, from 62° North to 82° North, and 134° West to 56° West, are presented in this study. Teleseismic Rayleigh waves were recorded by an array of temporary and permanent broadband seismographs across the region. Fundamental-mode phase velocity dispersion curves were calculated for 84 two-station paths. The period range for the majority of the dispersion curves was ∼ 25–170 s, allowing the resolution of mantle structure to a depth of ∼ 250–300 km. A tomographic inversion was used to combine dispersion data from 61 stable paths, to give a set of phase velocity maps for the region. The maps show a persistent high-velocity anomaly across the central–northern mainland, strongest in the period range 50–110 s, and a low-velocity anomaly along the Arctic continental margin at periods ≤ 90 s. A number of dispersion curves from two-station paths were chosen for further analysis, together with localised dispersion curves derived from the phase velocity maps, for areas with good path coverage. Analysis of these dispersion curves was carried out, using a Monte-Carlo method, to obtain 1-D models for shear wave velocity structure across the region, assuming an isotropic upper mantle. The average velocity of the upper 100–200 km of the mantle is compared to that of the iasp91 global reference model, and the variations in velocity perturbation are examined. The highest velocity perturbations, ∼ 4–6% above iasp91, occur in the southern part of the region and are, for the most part, associated with the Canadian Shield and southern Arctic Platform. In contrast, velocity perturbations of only 1–3% above iasp91 characterise the northwestern Arctic islands, with the lowest velocities occurring beneath the Sverdrup Basin. The characteristics of the upper mantle velocity–depth sections also vary significantly. For paths crossing the central–northern mainland and Baffin Island, the Monte-Carlo models give near-constant velocity to at least 240 km depth in the upper mantle. The best match to the dispersion curves can be obtained by the inclusion of a shallower (∼ 200 km depth) low-velocity zone, but this feature is not required to match the data within physically realistic error bounds. In the central Arctic islands, the models show a ‘lid’ of relatively high velocities in the upper section of the mantle model, underlain by a low-velocity zone. This velocity pattern is consistent with previous interpretations of the character of the seismological lithosphere and asthenosphere. The base of the lid is difficult to identify, but negative velocity gradients are modelled over the ∼ 100–200 km depth range. Along the Sverdrup Basin margins, where the velocity anomalies are smallest, the models are similar in character to the iasp91 reference profile, showing a gradual increase in velocity with depth.