2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 6
Presentation Time: 2:45 PM


JOHNSTON, Stephen T., School of Earth & Ocean Sciences, Univ of Victoria, PO Box 3055 STN CSC, Victoria, BC V8P 3P6 and MAZZOLI, Stefano, Istituto di Dinamica Ambientale, Universita di Urbino, Urbino, 61029, Italy, stj@uvic.ca

Modern ribbon continents in the Western Mediterranean and the Melanesian region of the Southwest Pacific provide us with an opportunity to understand the lithospheric geometry, origins and tectonic significance of these enigmatic tectonic entities. The Western Mediterranean ribbon continent extends south from the Alps along the length of the Italian/Apennine peninsula, through the Calabrian orocline into Sicily, across northern Africa as the Maghrebides and finally around the Rif – Betic orocline into southern Spain. Along its entire length the ribbon continent consists of a structurally lower, Mesozoic to Tertiary “External zone” northward-facing continental margin that has been overthrust from the north by a Paleozoic to Tertiary “Internal Zone” that is characterized by Variscan and Alpine assemblages and deformation. These disparate zones are separated by an intervening panel of flysch and ophiolite. Miocene and younger buckling of the West Mediterranean ribbon continent resulted in the opening of the Alboran and Tyrrhenian Seas behind, and subduction initiation in front of the outwardly migrating buckles. The Melanesian ribbon continent extends north from the Northland Peninsula, New Zealand, through New Caledonia and the Loyalty Islands into the submarine d’Entrecasteaux ridge to where it terminates in the Vanuatu arc. The ribbon continent consists of a lower (external) Permian-Jurassic east-facing continental margin assemblage that was overthrust from the east by an (internal) arc assemblage. These disparate assemblages are separated by an intervening panel of flysch and ophiolite. The ribbon continent is buckling and detaching from adjacent oceanic lithosphere in response to southward migration of the Vanuatu arc. Clockwise rotation of the Vanuatu arc results from pinning against the ribbon continent. These two modern analogues tell us that ribbon continents: (1) are common and should be well represented in the geological record; (2) deform through buckling giving rise to oroclines; and (3) are lithospheric (subduction occurs in front of, and basin formation behind buckling ribbon continent). Ophiolite characterizes most known ribbon continents, including the two discussed here, and the processes responsible for obduction are inferred to also be responsible for ribbon continent formation.