THE APPARENT ABSENCE OF ULTRAHIGH-PRESSURE TERRANES IN NORTH AMERICA

During the accretion of Pangea ultrahigh-pressure (UHP) suites developed where accreting continental blocks were subducted under Eurasia, but none have been found in North America despite Paleozoic orogenies on all sides of the continent.

This apparent absence of UHP terranes in North American orogenic belts is consistent with the absence of concurrent magmatism in North America and its presence in the arriving terranes or along collisional boundaries that developed into embryonic rift zones, including: Avalonian suites on the eastern side of North America; a buried terrane south of the Ouachitas; the Pearya terrane north of the Innuitian orogen, and island arc suites in the western Cordillera. Furthermore, North America was subducted beneath Africa when they collided in the Permian.

North America was in the center of Rodinia and was left completely surrounded by passive margins when Rodinia broke up from approximately 900 to 600 Ma. This position made North America comparable to modern Africa, which was in the center of Pangea and is now surrounded by passive margins developed at various times since 200 Ma.

The separation of North America and Africa left most of the thinned crust that became continental shelves and coastal plains in North America, and all of Africa's continental shelves are very narrow. By analogy with modern Africa, North America may have had very narrow shelves abutting continental lithosphere of normal thickness. This lithospheric configuration may have prevented subduction of colliding blocks beneath North America and thus precluded development of UHP terranes.