Paper No. 2
Presentation Time: 1:30 PM
THE ROLE AND SIGNIFICANCE OF RIBBON CONTINENTS IN ACCRETIONARY OROGENS AND THE SUPERCONTINENT CYCLE
JOHNSTON, Stephen T., School of Earth and Ocean Sciences, University of Victoria, Bob Wright Centre, PO Box 3065 STN CSC, Victoria, BC V8W 3V6, Canada, stj@uvic.ca
The Paleozoic Variscan orogen of Europe and the Mesozoic Cordilleran orogen of western NA provide us with the opportunity to determine the role of ribbon continents in the supercontinent cycle. In the Cordillera, a >2000 km wide basin separated cratonic NA from an ~8000 km long ribbon continent in the Triassic. Entry of the ribbon continent into a subduction zone in the Upper Triassic: post-dated accretion of Tethyan terranes to the arc; gave rise to an Accretionary Orogen (AO) with arc obduction, UHP metamorphism and doubling of the ribbon continent crust; and precipitated subsequent collapse of the landward basin. Basin collapse: was facilitated by subduction beneath the AO; yielded a foreland-verging thrust belt with syn-kinematic I- and S- type arc magmatism that youngs continentward; and was coeval with opening of the central Atlantic and the demise of Pangea. Basin closure by oblique subduction gave rise to the NA stand still, caused cessation of Atlantic opening, and was followed by 3000 km of dextral orogen-parallel translation at rates of 10 cm/a, forming the coupled oroclines of the Great Alaskan terrane wreck. Subduction of oceanic lithosphere drove translation and facilitated orocline formation.
During the Variscan, a wide basin separated a Gondwana-derived, >8000 km long, Armorican ribbon continent from the craton in the Devonian. Entry of Armorica into an oceanward dipping subduction zone in the Lower Carboniferous gave rise to an AO with arc obduction, UHP metamorphism, thickening of the ribbon continent crust and sinistral shearing, and precipitated subsequent closure of the Gondwana-side basin. Basin closure by subduction beneath Armorica yielded an Upper Carboniferous foreland-verging thrust belt, and syn-kinematic granitic intrusions that young toward the foreland. Subsequent orogen-parallel translation at 300 Ma shortened the 2300 km long Iberian segment of the Variscan by 1100 km at 10 cm/a forming the Iberian oroclines, and requires subduction of oceanic lithosphere to drive and facilitate orocline formation.
These observations indicate that: AOs form during supercontinent assembly and dispersal by ribbon continent subduction; and that subsequent oblique collapse against the continent by subduction beneath the AO is followed by margin-parallel translation during which oroclines form.
