Thermochemical Structure and Stratification of the Hudson Bay Lithosphere, Northern Canada: Evidence from Multi-Observable Probabilistic Inversion

Abstract:

The Paleoproterozoic Trans-Hudson Orogeny (THO) was a Himalayan-style collision that marked an important stage of assembly of the Canadian Shield. Today, the THO is largely concealed beneath the Hudson Bay intracratonic basin. Regional seismic tomography shows a thick, high-wavespeed cratonic keel beneath the region, but also includes significant local heterogeneity that may be associated with the imprint of the THO, providing clues to Precambrian plate-tectonic processes.

In this study, we use multi-observable probabilistic inversions to investigate the thermal and compositional state of the Hudson Bay lithosphere, to explain the seismic wavespeed variations and to constrain in more detail potential signatures of the oldest cratonic cores and the THO collision. Rayleigh wave dispersion curves, surface heatflow, geoid anomalies and topography are jointly inverted to give a pseudo-3D model of the upper mantle beneath the region.

Low temperatures are pervasive across the region, leading to a thick thermal lithosphere whose base lies at depths of 250 km or greater. The data are best explained by stratification of the lithosphere into (at least) two layers, with the top layer extremely depleted and the bottom layer generally more fertile, though still depleted with respect to the sublithospheric mantle. Across the Bay and Hudson Strait, a narrow zone of lowered depletion is observed in the top layer. The position of this anomaly coincides geographically with the THO and with the wavespeed reduction noted in previous seismic studies. It is likely that this feature represents juvenile material trapped between the cratonic cores in the final stages of the THO.

We also find evidence for anomalous mid-lithospheric compositions in certain areas, notably west of Hudson Bay. Additionally, some of the long-period surface wave data requires lower than average seismic wavespeeds below the lithosphere, suggesting localised regions of higher temperature/attenuation in the upper mantle.