WHAT DRIVES MOUNTAIN BUILDING IN ACCRETIONARY OROGENS?

In collisional orogens mountain building is related to continent-continent collision. In accretionary orogens, where subduction is on-going, mountain building may be a consequence of terrane accretion (soft collisions), flat-slab subduction, or global plate reorganization resulting in changing convergence rates. In all three processes, deformation in the over-riding plate is driven by a transitory coupling across the plate boundary and evidence from the geological record suggests all have been active in building mountains within the major accretionary orogens of the world. This paper highlights the evidence for deformation and resultant crustal growth and mountain building in accretionary orogens during phases of supercontinent assembly and consequent plate reorganization.

Supercontinent assembly is marked by ocean closure and collisional orogenesis. Age relations for assembly of Gondwana and Pangea indicate that the timing of collisional orogenesis between amalgamating continental bodies was synchronous with subduction initiation and contractional orogenesis within the accretionary orogens (e.g. Terra Australis Orogen) located along the margins of these supercontinents. This relationship probably reflects the global plate kinematic budget within a constant radius Earth where termination of convergence during supercontinent assembly is compensated by an increase in the subduction rate along the margin of the supercontinent resulting in a phase of increased coupling and accretionary orogenesis.

Final assembly of Gondwana occurred at the end of the Neoproterozoic to early Paleozoic, between about 570-510 Ma. This was coeval with a switch from passive margin sedimentation to convergent margin activity along the Pacific margin of the supercontinent followed by Ross-Delamerian orogenesis along the Pacific margin of Gondwana. Similarly, the final stages of assembly of the Pangean supercontinent occurred during the end Paleozoic to early Mesozoic, between ca.320-250 Ma, and involved the accretion of Gondwana, Laurasia and Siberia. This phase of major plate boundary reorganization was accompanied by regional orogenesis along the Pacific margin of Gondwana/Pangea (Gondwanide orogenesis).