TECTONICS THROUGH THE AGES: LITHOPROBE PERSPECTIVES ON THE EVOLUTION OF THE NORTH AMERICAN CONTINENT

Syntheses of results acquired during the Lithoprobe program (1984-2005) from orogens across Canada provide evidence that tectonism over the past 2.8-3.0 Ga has been characterized by cyclic processes resembling the Wilson cycle today. Decreasing radioactive heat generation and associated mantle cooling since the Archean were probably responsible for increased lithosphere thickness with orogenic age, decreasing oceanic crustal thickness, changing plate velocities, increasing plate areas, decreasing amounts of volcanically-derived sedimentary rocks, and increasing iron depletion of the mantle lithosphere. Even with all of these long-term changes, however, large-scale tectonic styles in Archean, Proterozoic and Phanerozoic orogens in Canada, and elsewhere, are similar. For example, virtually all of the orogens display varying degrees of development and/or preservation of rifted margins, contractional structures that are more or less parallel to the margins, linear arrangements of structural belts such as fold and thrust belts, foreland basins, large aspect ratio (i.e., wide, but very thin) accreted terranes, and probable relict subduction zones. Localized features, such as ophiolites, blueschists and ultra-high pressure rocks, are rare or even non-existent in Archean and Proterozoic orogens, just as they are in modern orogens. Thus, differences from one orogen to another may be attributed to complexities such as sediment vs. volcanic dominated margins, character and quantity of accreted terranes, or amount of preservation, rather than to tectonic processes that were substantially different in the past than they are today. By the Neoarchean, sufficient mantle cooling had taken place to allow stable lithospheres to form and cratonic nuclei to develop. Global redistribution and interactions led to amalgamation of proto-cratons, arcs and micro-continents into larger masses. During the Paleoproterozoic, these cratonic nuclei were assembled into the recognizable Canadian Shield; thus, formation of the North American craton was essentially completed by 1.8 Ga. Subsequent orogenic activity has produced long and narrow mobile belts that surround the cratonic core; these include the Grenville, Appalachians, Cordillera and other late orogens in Canada.