Stratigraphy of the Archean western Superior Province from P- and S-wave receiver functions: Further evidence for tectonic accretion?

The Archean western Superior Province in Canada represents the nucleus of the North American continent whose origin has been speculated to be the result of widespread crustal accretion some 2.7 Ga ago. In this paper, crustal and upper-mantle seismic discontinuities beneath the western Superior Province of the Canadian shield are imaged with teleseismic P-to-S and S-to-P converted phases using the receiver function method. Three crustal discontinuities are observed: the Moho, ranging in depth between 38 and 47 km and dipping to the south; and two intra-crustal discontinuities having depths of approximately 15 and 30 km. The crustal discontinuities undulate laterally and often lose continuity, possibly indicating an imbricated structure and/or regions of velocity gradients. In the shallow lithosphere, a positive discontinuity is imaged at approximately 65 km depth and is consistent with earlier refraction and wide-angle reflection results. Additionally, two zones of negative receiver function amplitudes at 55 km depth are observed and are coincident with a region of anomalous tomographic low P- and S-wave velocities as well as a zone of high electrical conductivity. The images for the crust and shallow upper-mantle, when integrated with previous geophysical studies, are consistent with ideas of continental root formation due to imbrication of Archean subducted material and accretion of island arcs observed in surface geology.