RIBBON CONTINENTS: A MARGINAL AFFAIR OF CENTRAL IMPORTANCE

Ribbon continents are major building blocks of continents. They form mainly as migrating arcs atop retreating subduction zones (e.g. the Kipchak Arc in the Altaids, the Cimmerian Continent in the Tethysides) , but some also are generated as marginal strips of Atlantic-type continental margins resulting from re-rifting (e.g. the Lomonosov Ridge) or by transtensional faulting along a continental margin (e.g. the Baja California). Many of the large ribbon continents are hybrid structures and some even include ensimatic arcs that form "in situ" and connect separate "ribbons". By nature, therefore, ribbon continents are continental margin phenomena, but once formed, they may have multifarious destinies depending on the vicissitudes of the evolving global plate boundary systems. Most commonly, they become deformed into more equant continental entities forming their own continents (such as the "Kazakhstan microcontinent") by means of oroclinal bending, arc shaving or arc slicing strike-slip faults. This was probably how the first continents were assembled in the Archaean (or even in the Hadean?) They may also form giant "isthmian links" between two larger continental entities (as did the Kipchak arc betweeen the Russian and the East Siberian cratons) or they may be rifted from one continent and collide with another in a "windshield wiper fashion" as did the Cimmerian Continent in the Tethysides. Once deformed into a continent of a different shape, their continuity is extremely difficult to establish. The best means to do so is to trace continuity of tectonic facies along their deformed remnants and to undeform them in a stepwise fashion, continuously checking the overall tectonic compatibility in and among the generated reconstructions and the palaeomagnetic data from individual segments in the deformed bundle. Assumption of along-strike continuity is a more easily refutable claim than discontinuity and thus preferable in cases of doubt. The continuity can then be further checked by using terrestrial and benthic organisms. Distinguishing ribbon continents from marginal strips that move coastwise without orthogonal separation is very difficult and requires a careful detection of signs of rifting. Some marginal strips may become locally ribbons detached from the main continent and others may have "free ends" in form of ribbons .