Receiver function analysis of crustal and upper mantle layering across the western Superior Province
The Superior Province is the Earth’s largest Archean craton. Its western portion in Canada represents the nucleus of the North American continent, and has a lineated structure with well-preserved supracrustal rock sequences, mineral resources, and greenstone-granite terranes. Its strong east-west tectonic fabric is most commonly attributed to the formation and widespread accretion of island arcs and accretionary prisms ~ 2.6 Ga ago (Lucas et al., 1998). The Superior Province is underlain by lithospheric mantle that exhibits strong regional variations in anisotropy and velocity structure (Darbyshire et al., 2007). The stratigraphy, velocity structure and thickness of the crust and upper-mantle beneath the western Superior Province, were examined through the analysis of seismic discontinuities on the radial and transverse components of P-wave receiver functions. Global earthquakes that occurred between 2003 and 2008 and recorded by 13 broadly spaced FedNor/POLARIS and CNSN three-component broadband seismic stations across western Ontario were used to create receiver functions. Receiver functions were calculated using a panel deconvolution approach (using inter-trace regularization constraints) to improve signal-to-noise ratio. Inversion for lithospheric parameters was carried out through a directed Monte-Carlo search method that uses the neighbourhood algorithm of Sambridge (1999). The receiver function data show indications of crustal and mantle layering. Generally, it was observed that in the western Superior Province, seismic stations in the southern portion of the study area (south of ~ 51° N): LDIO, EPLO, PNPO, TIMO and NANO reveal a uniform crust, but a complicated and layered mantle; whereas stations in the northern portion of the study area (north of ~ 51° N): KASO, RLKO, SILO, VIMO and PKLO reveal a more uniform mantle layer, but a stratified crust. The only exception is ATKO, which displays dominant crustal layering, but is located south of ~ 51° N. Other observations include: crustal discontinuities which lose continuity laterally, possibly due to subducting structures and/or regions of velocity gradients, and lobes of opposite polarities on the radial and transverse components of the receiver functions, which are indicative of either azimuthal anisotropy or dipping interfaces. Inversion of the receiver function data showed: 1) that crustal thickness beneath the western Superior Province varies between 39 and 46 km, similar to results from other studies such as Darbyshire et al. (2007), 2) a ubiquitous, anisotropic 15-20 km thick sub-Moho layer similar to results from studies such as Musacchio et al. (2004) and 3) two 20-25 km thick, anisotropic uppermost mantle layers observed only beneath certain stations. Modeling of the data for dip and anisotropy showed that observed back-azimuthal variation at stations with dominant crustal layering, is mainly in response to NE-SW dipping layer interfaces; while in the mantle, inherent azimuthal anisotropy is interpreted to result from frozen fabric due to regional tectonic stresses. Unlike other geophysical studies (Ferguson et al., 2005; Frederiksen et al., 2007; Darbyshire and Lebedev, 2009) in the western Superior Province that reflect the Province’s regional E-W fabric, the NE-SW anisotropic results from this study are in response to small-scale, local structures.