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Orogenic Style and the Configuration of Supercontinents
Two contrasting end-member types of orogenic belts characterize the episodic amalgamation of supercontinents. Interior orogens define suture zones between individual cratonic blocks within a supercontinent. Orogenic activity commences with Marianas-and Andean-type subduction associated with the contraction of “interior oceans”, whose closure is responsible for supercontinent assembly. Orogenesis terminates with continent-continent collision during supercontinent amalgamation. Andean margins are produced as interior oceans contract because the rate of overall plate convergence exceeds the rates of subduction. Abundant calc-alkalic and post-collisional granitoid magmatism, crustal thickening and re-working, large scale thrusting and nappe development, uplift and exposure of high grade metamorphic rocks and regional unconformities with post-orogenic sequences are typical of interior orogens. Variations in orogenic style occur with distance from pole of closure and climatically controlled rates of erosional unloading. Peripheral orogens record subduction at the supercontinent margin prior to and following destruction of interior oceans. Orogenic activity commences with Marianas-type subduction, back-arc basin formation and juvenile magmatism as rates of subduction exceed the overall rate of plate convergence in an expanding “exterior ocean”. Orogenesis continues with Andean-type subduction following supercontinent amalgamation and terminates in arc-arc/arc-continent collisions and transform activity associated with oblique convergence and supercontinent breakup. Orogenic activity is outlived by the exterior ocean, which survives supercontinent amalgamation and dispersal. Inter-arc and intra-arc basin development, localized and dominantly strike-slip deformation, variable crustal thickening and uplift, the preservation of low grade, juvenile, arc-related (synorogenic) volcanic-sedimentary successions and only local unconformities with post-orogenic sequences are typical of peripheral orogens. Significant style variations along peripheral orogenic strike likely reflect variations in longevity of subduction, plate margin geometry, slab profile, rates of plate convergence and the number, size and nature of accreting terranes.
Interior orogenic activity during the Paleozoic assembly of Pangaea is exemplified by the Ouachita, Appalachian, Caledonian, Hercynian, Uralian, Angaran and Tien Shan orogens, which lay in the interior of the supercontinent following its amalgamation. Peripheral orogenic activity, focused at the margins of Panthalassa, is represented in the orogens of the present circum-Pacific as exemplified by the North American Cordillera. Interior orogens associated with the assembly of the Neoproterozoic III supercontinent (Vendia) include the Mauritanide-Rokelide, Trans-Saharan, Brasiliano, Damaride, Mozambique and Paterson belts. Contemporary peripheral orogenesis is recorded in the Avalonian-Cadomian terranes and the pan-African belts of the Arabian-Nubian shield and in Neoproterozoic III segments of the Tasman-Transantarctic system. Recognition of these contrasting end-member orogens can aid reconstruction of past supercontinents by permitting component cratons and supercontinent margins to be identified.
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