Crustal Structure beneath Hudson Bay from Ambient-Noise Tomography

Agnieszka Pawlak

ABSTRACT

Hudson Bay is a shallow inland sea located in north-central Canada. The underlying lithosphere preserves a complex deformational history that dates back to the Archean. The Hudson Bay Lithospheric Experiment, HuBLE, is a collaborative initiative aimed at understanding the lithospheric evolution beneath the Bay.

The recent emergence of a methodology called ambient-noise tomography provides a tool to image the crust and upper mantle beneath the Bay with higher resolution than previously possible. Using ambient-noise generated by the Earth as a source, this technique requires continuous recordings of ground motion. The ambientnoise method is based on the cross-correlations of daily noise signals between station pairs to estimate empirical Green's Functions.

This thesis is made up of there separate studies. The first is an isotropic application of the ambient-noise tomography method to image crustal structure beneath Hudson Bay. Results show crustal thinning beneath the Bay, allowing us to reject a hypothesis for eclogitization and crustal thickening, with support instead for an extensional hypothesis for the formation of the Hudson Bay basin.

The next study focuses on anisotropic variations of velocity within the subsurface. Inversion results show a distinct outline of geologic boundaries in the upper to mid-crust that does not carry through into the lower crust. A significant change in anisotropic fabric is evident across the Trans-Hudson orogen (THO) suture zone, which allows us to establish that tectonic fabrics formed prior to collision.

The third study employs joint inversion of ambient-noise data and teleseismic surface wave data for increasing resolution of the crust and upper mantle. Results show that the THO suture zone dips to the southeast within the crust and becomes vertical in the upper mantle. This feature is interpreted as a zone of weakness that extends through the lithosphere, providing a locus for initiation of localized lithospheric stretching.

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