Mantle fabric at multiple scales across an Archean–Proterozoic boundary, Grenville Front, Canada

Abstract

In eastern Ontario and southwest Québec, Canada, the Proterozoic Grenville province abuts against the Archean Superior province. The complex tectonic history of the region is reflected in the pattern of electrical and seismic anisotropy within the lithosphere, while asthenospheric anisotropy is expected to reflect current patterns of mantle flow. Magnetotelluric and teleseismic data from the POLARIS and FedNor experiments and the Lithoprobe Abitibi–Grenville transect are examined for SKS splitting and geoelectric strike, and receiver functions are generated at selected stations, in order to characterise both vertical and horizontal variations in anisotropy in the eastern Ontario upper mantle. The average shear-wave split direction coincides with the direction of plate motion. Split times are found to be strongest in the southern part of the Grenville province, where asthenospheric flow is enhanced by the presence of a lithospheric divot. The Ottawa–Bonnechere graben and the immediate vicinity of the Grenville Front are regions of altered split direction indicative of a lithospheric component of anisotropy. North of the Grenville Front, there is a gradual reorientation of the split direction from ESE to ENE which is not easily attributable to crustal tectonics, and may represent the northern limit of lithospheric deformation produced by the Grenville orogen. Electrical anisotropy is pervasive in the study area. The pattern of magnetotelluric strikes is more complex than the SKS pattern, though after correction for local distortion, the geoelectric strikes correlate fairly well with SKS measurements at nearby stations. Obliquity between SKS and magnetotelluric results shows no consistent orientation across the study area. Receiver-function analysis at three selected stations is indicative of a subcrustal anisotropic layer with a consistent SSE fast direction, underlain at station GAC by a sequence of anisotropic layers with varying directions; this sequence is not observed at station SADO. Combining these results, we interpret the strength and direction of anisotropic fabric in the Grenville to vary strongly with depth. The upper part of the lithosphere contains thin anisotropic layers perhaps related to eclogitization and relict slabs, but which are insufficient to explain the observed SKS splits. The lower lithosphere is likely to be more ductile and uniformly anisotropic, and may be an important control on magnetotelluric strike directions as well as a significant contributor to SKS splitting. The largest contributor to SKS splitting in this region is interpreted to be asthenospheric anisotropy related to absolute plate motion.