Exploring Seismic Anisotropy of the Eastern Canadian Shield and its Margins: Evidence From Shear Wave Splitting and Surface Wave Tomography

Abstract:

The eastern Canadian Shield and its surroundings record a complex tectonic history of craton formation and evolution, multiple periods of orogenesis, and episodes of rifting. These large-scale tectonic processes likely result in “fossil” fabric preserved within the continental lithosphere, which may manifest as measurable seismic anisotropy. Present-day mantle convective flow below the lithosphere can cause lattice-preferred orientation of intrinsically anisotropic minerals such as olivine and thus presents a second possible source of seismic anisotropy. There is also evidence for anisotropic structure in the lowermost mantle, though this appears not to be ubiquitous.

Since the pioneering work of Silver and colleagues in the late 1980s, many observations of seismic anisotropy have been made across the eastern Canadian Shield and its margins. These include measurements of shear wave splitting as well as regional-scale surface wave tomography studies that solve for azimuthal anisotropy. In northern Canada, evidence from azimuthal variation of shear wave splitting parameters suggests the presence of multiple anisotropic layers in the upper mantle, and the existence of dipping fabrics associated with Paleoproterozoic orogenesis. Anisotropy within the Superior craton is both laterally and vertically variable. The large splitting delay times in the Western Superior appear to be related to an alignment of the fabrics within the cratonic keel and in the sublithospheric mantle. In contrast, anisotropy is weaker in the central Superior, which has experienced multiple episodes of hotspot-lithosphere interaction. In eastern Canada, we see evidence for variations in anisotropy across the Grenville Front, and significant variability within the Appalachian domains. In contrast, along the St. Lawrence valley, the dominant fast-polarisation orientation varies only slightly, and may reflect a more regionally pervasive, perhaps sublithospheric, contribution.

We examine the anisotropy measurements in the context of other geological and geophysical data sets, as well as numerical models of mantle convection, in order to improve our understanding of the sources of seismic anisotropy associated with lithospheric and sublithospheric processes beneath eastern Canada.