Seismic Architecture of the Archean North American Mantle and Its Relationship to Diamondiferous Kimberlite Fields

摘要

The seismic architecture of the average Archean-only mantle beneath exposed and subsurface Archean crust of the United States and Canada is presented here in three dimensions for the first time, using a high lateral resolution Rayleigh wave phase velocity model of the upper mantle (30- to 250-km depth). The morphology of the cratonic coherent mantle is compared with other regional and local geophysical models, geologic interpretations,and published xenolith barometric studies. In particular, the kimberlite magma source regions at the Lithosphere-Asthenosphere Boundary (LAB) inferred from xenolith data are consistent with the bottom topography of the Archean seismic mantle signature. The characteristic fast seismic response found beneath much of the exposed Archean crust is also found in Canada beneath some covered terranes, sedimentary basins, and Proterozoic mobile belts.The northeastern and northwestern parts of the Superior craton host, with the central Hearne craton, the deepest mantle roots of North America (225- to 240-km depth). However, the southern portion of the Superior craton is characterized by an east-west channel that is 30 percent slower in seismic velocity than its northern counterpart. This contrasting seismic signature correlates with the location of the southernmost Neoarchean greenstone belts and to their plume-driven subduction zones. The scar in the mantle produced by this early tectonothermal event has been reused by widespread and sporadic carbonatite and kimberlite magmatic events spanning from the Early Proterozoic to the Cretaceous, and as a consequence, the diamond stability field has been partially to totally overprinted. Almost all diamondiferous kimberlites in Canada are located vertically over an interval of 160- to 200-km depth in areas of steep slopes surrounding deep (180-240 km), relatively small, and flat-bottomed Paleo-Mesoarchean cratonic keels.