Identification of Upper--Mantle Velocity and Anisotropy Structures Beneath the Archean Superior Province Using Surface Waves

The Canadian Archean Western Superior Province (WSP) is composed of a number of east--west trending subprovinces progressing in age from south to north. Lithoprobe's Western Superior Transect aimed to better understand the significance of such geological architecture for Archean tectonics and included a portable broadband teleseismic array to complement conventional crustal geophysical studies. This N--S array of 17 instruments traversed subprovince boundaries and involved two more stations further north in the Trans--Hudson orogen (THO). Body wave teleseismic tomography and shear--wave splitting analysis have suggested the existence of laterally--varying lithospheric--mantle structures that may be associated with frozen Archean oceanic lithosphere and a high degree of anisotropy. In order to acquire additional constraints on this interesting lower--lithospheric environment we have examined surface waves from high--magnitude teleseismic events in two stages: a standard dispersion analysis along two near--orthogonal paths and a Love and Rayleigh polarization study. The W--E path was examined by incorporating data from CNSN station ULM. Variations in 15--80s Rayleigh fundamental mode group and phase velocities for S--N propagation indicate that the linear profile is best treated in three segments reflecting changes in lithospheric structure. Corresponding shear--wavespeed models inferred from independent isotropic inversions of Love (20--70s) and Rayleigh fundamental phase velocity are broadly coherent with the coincident R/WAR model of Musacchio et al. (in press, JGR, 2004). Both studies indicate crustal thinning moving northward (from 45 to 39 km), faster velocities in the E--W direction and the presence of a thin anisotropic high--velocity layer just below the Moho. While this layer is detected along the E--W path and in the middle and northern parts of the N--S line, there is no surface--wave indication of it in the southern section. It has been interpreted as a relic of oceanic lithosphere accreted during the Kenoran orogeny. Anisotropy is suggested both by comparing the observed Rayleigh phase velocities along the orthogonal paths and by a Rayleigh/Love discrepancy in each of the N--S and E--W isotropic inversions. In fact, inspection of the Rayleigh/Love particle motions in various frequency bands for surface--waves arriving from various azimuths brings into question the assumptions in isotropic analysis. Indicators such as quasi--Love waves and Love elliptical particle motion are seen at most of the stations. Numerical modelling indicates that plausible uppermost--mantle anisotropy has the potential to explain such observatons, especially given the large SKS splitting along the main array. Interestingly, despite a null SKS splitting result at the far north THO station BPW, surface--wave polarizations there provide evidence of significant anisotropy. Overall, the surface--wave data reinforce other techniques suggesting significant lateral variations and anisotropy in the WSP, with the upper 200km of the lithosphere capable of accounting for most of the latter. A true areal array extending across the WSP can be expected to reveal major structures.