Stratification of Seismic Anisotropy Beneath Hudson Bay

The Hudson Bay region has a complex tectonic history spanning ~4 Ga of Earth's evolution. During the ~1.8 Ga Trans-Hudson orogeny, the Archean Superior and Western Churchill cratons collided following the subduction of a Pacific-scale ocean. It is thought that a significant amount of juvenile material is preserved in the Trans-Hudson Orogen, in part due to the complex double-indentor geometry of the Superior-Churchill collision. In the region of interest, the orogen lies beneath a large but shallow Paleozoic intra-cratonic basin. Studies of the crust and upper mantle beneath this region have been enabled through the HuBLE (Hudson Bay Lithospheric Experiment) project, through the deployment of broadband seismographs around the Bay and across the islands to the north. A surface-wave tomography study has taken advantage of the data coverage, providing new information on phase velocity heterogeneity and anisotropy for wave periods of 25-200 seconds (equivalent to depths from the lower crust to ~300 km). On a large scale, our results show that the entire region is underlain by a seismically fast lithospheric lid corresponding to the continental keel. The lithospheric thickness ranges from ~180km in the northeast, beneath a zone of Paleozoic rifting, to ~280km beneath central Hudson Bay. Within the lithosphere, seismic velocities vary laterally, including high-velocity material wrapping around the Bay in the uppermost mantle. In the mid-lithosphere, two high-velocity cores are imaged, with a zone of lower velocity between them beneath the Bay. We interpret these high-velocity structures to represent the strongest central cores of the Superior and Churchill cratons, with more-juvenile material preserved between them. The near-vertical geometry of the lower-velocity zone suggests that it is only the effects of terminal collision of the cratonic cores, rather than any preceding subduction, that is preserved today. The lowermost lithosphere has a more uniform velocity, and may represent a pervasive zone of metasomatism or underplating. Anisotropy patterns across the region also vary with depth, suggesting ~3 layers of stratification of lithospheric fabric. At the shallowest depths, anisotropic fast directions wrap around the Bay in a similar fashion to the patterns of isotropic wavespeed. The upper lithospheric mantle below is characterized by relatively weak and incoherent anisotropy; however the mid-to-lower lithosphere shows stronger anisotropy, with a pattern of fast directions broadly consistent with the tectonics of the Superior-Churchill collision as inferred from potential-field data. This may suggest some degree of coherency of deformation between the crust, uppermost mantle and lower lithosphere. These models of seismic wavespeed variation beneath the Hudson Bay region reveal the preservation of a major collision zone during the assembly of the Laurentian continental mass, and also suggest that the Archean cratons can be subdivided into different lithospheric domains that reflect their tectonic history but do not necessarily correspond to surface geological boundaries.