The lithospheric root beneath Hudson Bay, Canada from Rayleigh wave dispersion: No clear seismological distinction between Archean and Proterozoic mantle

We characterise the nature and thickness of the seismological lithosphere beneath Hudson Bay, using dispersion analysis of fundamental-mode Rayleigh waves along two-station paths crossing the region. Hudson Bay is largely underlain by the Paleoproterozoic Trans-Hudson Orogen, and is surrounded by Archean cratons. This study region thus provides an opportunity to investigate whether fundamental differences exist in the nature of the subcontinental lithosphere between Archean and Proterozoic terranes.Dispersion measurements for Hudson Bay, carried out in the ∼ 15–220 s period range, show seismic phase velocities consistently higher than those derived from global reference models. In addition, the phase velocities are higher than those of the Canadian Shield average, and of many other cratonic regions worldwide. Monte-Carlo modelling is used to estimate 1D shear wave velocity profiles for each of the dispersion curves. The models are characterised by a high-velocity ‘lid’, interpreted as the seismological expression of the subcontinental lithosphere, beneath which velocities decrease gradually toward global reference values. We find lithospheric thicknesses in the range 185–260 km beneath the Hudson Bay region, with maximum lithospheric shear wave velocity perturbations of 4.5–7.3% above global reference values.By treating the geotherm as a free parameter, the average dispersion curve for Hudson Bay can be fit using laboratory based mineral–physics relations with simple compositional models, including constant-composition and layered-mantle models. Low mantle temperatures, corresponding to surface heat-flow values between 37 and 40 mW/m2, are required to match the observed data. A layered model based on the Archean Slave craton is favoured as it provides a lithospheric root that is close to the isopycnic state (i.e., neutrally buoyant for all depths).In common with studies of other continental shields (particularly Fennoscandia and central Australia), the velocity structure modelled for the Hudson Bay region suggests that there is no systematic difference between the velocity structure and thickness of Archean and Proterozoic seismological lithosphere. This result may have important implications for models of lithospheric stabilisation and evolution between 2.8 and 1.8 Ga.