Abstract

Two fundamentally different types of orogenic belts may reflect the accretion and breakup of a Late Proterozoic supercontinent. Interior orogens (e.g., trans- Saharan fold belts) lie within the supercontinent after its amalgamation. Orogenic activity commenced with ca. 850-800 Ma Cordilleran-type subduction associated with contraction of interior oceans and terminated in ca. 650-600 Ma continent-continent collision and the amalgamation of an Afro-Brazilian supercontinent. Orogenic activity produced abundant calc-alkalic granitoid magmatism, crustal thickening, large-scale thrusting, uplift and exposure of medium-to high-grade metamorphic rocks, and regional angular unconformities with postorogenic sequences. In contrast, peripheral orogens (e.g., Avalonian-Cadomian belt, Arabian shield) lie at the extremity of the supercontinent following its amalgamation. Orogenic activity commenced with ca. 820-800 Ma subduction in an exterior, expanding ocean that generated a diverse assemblage of arcs and back-arc basins as the plate retreated from the subducting slab. Voluminous ca. 600 Ma ensialic tectonothermal activity in peripheral orogens may reflect renewed subduction at the periphery of the supercontinent following its amalgamation and the destruction of interior oceans. Peripheral orogenic activity produced synorogenic, low-grade volcanic-sedimentary successions, inter-arc and intra-arc rifts, localized and predominantly strike-slip deformation, limited uplift, and only local unconformities with postorogenic successions. Basin closure and termination of peripheral orogens may reflect supercontinent breakup and dispersal.

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