Crustal anisotropy beneath Hudson Bay from ambient-noise interferometry

Agnieszka Pawlak, David W. S. Eaton

Hudson Bay overlays complicated tectonic structures for which new insights have only recently been emerging. Using azimuthal anisotropy (or HTI) results in conjunction with isotropic group velocity maps, we focus our study on the formation and regional crustal structure beneath Hudson Bay. Twenty-one months of continuous ambient-noise recordings have been acquired from 37 broadband seismograph stations that encircle Hudson Bay. These stations are part of the Hudson Bay Lithospheric Experiment (HuBLE), an international project that is currently operating more than 40 broadband seismograph stations around the periphery of Hudson Bay. The inter-station group-velocity dispersion curves found from ambient-noise interferometry, are input into tomographic inversion for azimuthally anisotropic group-velocity maps, which reveal structural fabric in the crust.

This work marks the first study where solely ambient seismic noise data have been considered in azimuthal anisotropy work. As such, extensive parameter and resolution testing is done to assess model feasibility. Results (FIG. 1) show mid-crustal anisotropy (20 s period) patterns correlating well with regional geology, with a profound change along the suture between the Churchill and Superior plates that collided ca. 1.8 Ga during the final stages of assembly of Laurentia: the Trans Hudson Orogen. The lower crust/upper mantle results (30 s period), however, show a significant transition in anisotropic fast direction and a more complicated anisotropic fabric, also with a profound change along the suture zone. The data are also compared with regional crustal magnetics and show a significant similarity at periods sampling the mid-crust, but not the lower crust.