EVOLUTION OF RETREATING SUBDUCTION BOUNDARIES FORMED DURING CONTINENTAL COLLISION

Retreating subduction boundaries, formed where the rate of subduction exceeds the rate of overall plate convergence, appear to be commonly d eveloped features within regions of early or incomplete continent-cont inent collision. They are characterized by regional extension within t he overriding plate and, at their leading edge, by thin-skinned arcuat e thrust belts that are concave towards the overriding plate. As is il lustrated by examples from the Mediterranean region, the formation of retreating subduction boundaries is intimately related to the process of continental collision. During the early stages of collision, retrea ting subduction boundaries are commonly formed by lateral ejection fro m zones of crustal shortening along the main collision boundary. Retre ating plate boundaries can also form before the main collision, and th e associated thrust belts emplaced as precollisional accretionary asse mblages. Because the driving mechanism for retreating subduction bound aries appears to be gravity acting on a dense subducted slab (slab pul l), subduction usually ceases when, and only when, thick buoyant conti nental crust enters the subduction zone. Thus differences in the evolu tion and duration of retreating subduction systems can be largely attr ibuted to the size and configuration of the deep water regions availab le to be subducted. In some cases, retreating subduction boundaries ma y ''escape'' into the open ocean, where they form nearly isolated, loc al tectonic systems. In these systems the rate of subduction is approx imately compensated by the rate of upper plate extension, and migratio n of the system across the oceanic region may be very rapid. For examp le, the Horseshoe Seamounts, located about 800 km offshore in the east ern North Atlantic, may be the active expression of an east dipping, w estwardly migrating retreating subduction boundary that has evolved fr om the Betic Cordillera-Rif system active in Miocene time and may now be progressing across the Atlantic at approximately 50 mm/yr. An analo gous situation may be represented by the Scotia Arc system, a westward dipping retreating subduction system located between the South Americ an and Antarctic plates, which may have ''escaped'' into the South Atl antic ocean from a zone of crustal shortening in the Andes and is now progressing across the Atlantic at a rate of about 80 mm/yr.