Archaean cratonic roots, mantle shear zones and deep electrical anisotropy

THE extent to which the mantle participated in the growth and stabilization of ancient cratons is central to our understanding of the evolution of the continents¹. The detection of seismic anisotropy beneath Precambrian North America, for example, has been interpreted as showing that strain-induced orientation of mantle minerals in subcontinental lithospheric mantle can preserve a record of ancient episodes of deformation². Here we present mag-netotelluric measurements from the Superior Province of the Canadian shield, which reveal pronounced electrical anisotropy in the upper 100 km of the underlying mantle. We argue that this anisotropy is best explained by conducting graphite films, oriented within fractures or on grain boundaries, and associated with metasomatism of the mantle roots of major Archaean shear zones which transect the entire Superior Province. The uppermost mantle beneath the Canadian shield has therefore remained fixed to the crust and isolated from significant tectonic reworking since the late Archaean.