DEEP CRUSTAL STRUCTURE AND MODEL OF NEORCHAEAN EVOLUTION OF THE NORTH AMERICAN CRATON

Abstract

The structure of the Archaean crust of the North America has been studied based on the synthesis of geolo‐ gical and geophysical data, including seismic sections along LITHOPROBE Geotransects, magnetic and gravity anomaly maps, and seismic tomography data. The authors rely on the experience gained in the Russian Program of the deep geological and geophysical studies of the East European Craton. The juvenile Neoarchaean crust, containing the frag‐ ments of reworked Meso‐ and Paleoarchaean rocks, forms an asymmetric round‐oval‐shaped domain, wherein the geophysical, structural, and metamorphic parameters display a concentric zoning pattern. The Central zone occupies the Hudson Bay basin. The Internal zone (the northeastern and northern Superior Province) is mainly composed of the granulite facies of metaplutonic, metavolcanic and metasedimentary rocks. The External zone encompasses the southern Superior Province together with Hearne and Rae Provinces. This paper presents 3D crustal models of sou‐ thern Superior Province. The crust development resulted from rifting and a partial disruption of the continental crust, short‐term opening of the linear oceans, successive northward subduction and accretion of the ancient continental and juvenile Neoarchaean oceanic and island‐arc terranes between ~2.78 and ~2.70 Ga. Subsequent events in the epicontinental environment, including formation of the metasedimentary belts, granulite facies metamorphism and intense ore formation processes, took place within the range from ~2.71 to ~2.63 Ga. The SCLM morphology within the limits of the Archaean North American Craton can be represented as a flattened overturned cone with a vertical axis (down to a depth of ~350 km). The Hudson Bay basin is located right above the lithospheric keel. A number of the main features of the structure and evolution of the Archaean crust of the North American Craton, primarily the oval‐ concentric zoning, the important role of high‐temperature magmatic and metamorphic processes and mainly in‐ tracontinental magmatism and sedimentation, indicates the leading role of the mantle‐plume type processes. The Neoarchaean evolution of the North American craton represents the plate‐tectonic processes initiated by a super‐ plume. The Neoarchaean North American Craton is one of a series of similar phenomena that occurred ~2.75 Ga ago in a number of continental regions. The most important features, repeated to a certain degree in tectonic units of this type, are: (1) synchronous formation between 2.79 and 2.58 Ga; (2) mainly intracontinental development; (3) the prevalence of oval‐shaped synformal tectonic structures of different ranks with some form of concentric zoning; (4) high‐temperature magmatism (usually with the participation of enderbite‐charnockites and gabbro‐anorthosites) and metamorphism of the granulite facies; (5) a frequently repeated combination of high‐grade (granulite and high‐ temperature amphibolites facies) and low‐ or moderate‐grade (greenschist and epidote‐amphibolite facies) meta‐ morphic rocks; (6) the lower‐crust granulite‐basaltic layer that had formed and was deformed at the final stage of endogenic activity; (7) a thick lithosphere (the lithospheric keel reaches a depth of 250–350 km).