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The preceding chapters, which deal with myriad facets of the Precambrian geology of the conterminous United States, necessarily emphasize local descriptions and details. In this chapter we consider some broader regional and global relations. Hamilton discusses some of the difficulties with conventional views of the early evolution of the crust, and speculates that the geology of Venus, as elucidated by the new Magellan imagery, may provide a model for terrestrial Archean tectonics. Farmer and Ball describe the Nd-Sm isotopic system and discuss the applications of Nd-Sm studies to the determination of model crustal ages and delineation of major crustal provinces. Finally, we summarize the development of the southern part of Laurentia, the Late Proterozoic to early Paleozoic supercontinent that was fragmented during the Paleozoic and Mesozoic to produce the North American craton.

Geologic processes have changed greatly as the Earth has progressively lost heat, and plate-tectonic models that work well for the Phanerozoic (where my own expertise mostly lies) become progressively less applicable to successively older Precambrian assemblages. These models nevertheless can be fitted with reasonable success to Proterozoic geology, as Hoffman has been the leader in demonstrating; his 1989a paper provided excellent syntheses of the geology and tectonic significance of the Proterozoic tracts of Canada and the United States in such terms. Accretionary wedges, ophiolites, fore-arc and back-arc basins, oceanic and continental magmatic arcs, and rifted-margin sedimentary wedges are all recognizable in Proterozoic assemblages although different in important ways from modern ones.

Archean rock and tectonic assemblages are markedly less yet like modern ones. Although an increasing consensus (as, Hoffman, 1989a; until recently, I concurred) holds that Archean geologic processes also were dominated by plate tectonics, major features now appear to me inexplicable in such terms.

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