Convergent margin tectonics in the North American Cordillera : implications for continental growth and orogeny




English, Joseph M.

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Continental growth may be accomplished at active convergent margins through tectonic accretion and orogeny. Accretionary processes believed to add material to continents include the collision of island arcs and other unsubductable crustal blocks. Using the Intermontane belt as a case study for assessing accretionary processes, it is concluded that island-arc collision and accretion was the principal mechanism for continental growth with relatively minor contributions from 'sliced-off oceanic seamounts and/or plateaux. Fold and thrust belt formation in the northern Intermontane belt records a Middle Jurassic orogenic event that can be attributed to the collision of island-arc highlighting the importance of island-arc collision for causing orogenesis in the North American Cordillera. However, not all orogenic events in the North American Cordillera can be readily attributed to a collisional event. The leading model for driving Laramide orogenesis in the United States is flat-slab subduction, and thermal modelling indicates that subduction of a relatively buoyant oceanic plateau/aseismic ridge may have been responsible for the shallow trajectory. In the Canadian and Mexican portions of the Laramide, the coeval development of a magmatic arc within 300 km of the trench refutes the existence of flat-slab subduction in these regions, and therefore the processes responsible for this orogeny remain enigmatic and require resolution.