A first detailed look at the Greenland lithosphere and upper mantle, using Rayleigh wave tomography

Abstract

We present the results of a surface wave study carried out across Greenland as part of the ‘GLATIS’ (Greenland Lithosphere Analysed Teleseismically on the Ice Sheet) project. Rayleigh wave phase velocity dispersion curves were estimated for 45 two‐station paths across Greenland, using data from large teleseismic earthquakes. The individual dispersion curves show characteristics broadly consistent with those of continental shields worldwide, but with significant differences across the Greenland landmass. Reliable phase velocity measurements were made over a period range of 25–160 s, providing constraint on mantle structure to a depth of ∼300 km. An isotropic tomographic inversion was used to combine the phase velocity information from the dispersion curves, in order to calculate phase velocity maps for Greenland at several different periods. The greatest lateral variation in phase velocity is observed at intermediate periods (∼50–80 s), where a high‐velocity anomaly is resolved beneath central‐southwestern Greenland, and a low‐velocity anomaly is resolved beneath southeastern Greenland. The results of the phase velocity inversion were used to construct localized dispersion curves for node points along two parallel north–south profiles in southern Greenland. These curves were inverted to obtain models of shear wave velocity structure as a function of depth, again with the assumption of isotropic structure. A similar inversion was carried out for two two‐station dispersion curves in northern Greenland, where the resolution of the phase velocity maps is relatively low. The models show a high‐velocity ‘lid’ structure overlying a zone of lower velocity, beneath which the velocity gradually increases with depth. The ‘lid’ structure is interpreted as the continental lithosphere. Within the lithosphere, the shear wave velocity is ∼4–12 per cent above global reference models, with the highest velocities beneath central‐southwestern Greenland. However, the assumption of isotropic structure means that the maximum velocity perturbation may be overestimated by a few per cent. The lithospheric thickness varies from ∼100 km close to the southeast coast of Greenland to ∼180 km beneath central‐southern Greenland.