"DUNK" TECTONICS AS A MODEL FOR THE EVOLUTION OF THE SCANDINAVIAN CALEDONIDES

Mountain systems evolve through multiple collisions involving continents, microcontinents, arcs, ocean plateaus etc. before the terminal continent-continent collision that ends the Wilson Cycle. Some collisions occur through the subduction and buoyant return (eduction) of crustal slabs, a process provisionally called "dunk" tectonics. Dunked slabs are recognized as high-grade metamorphic terranes bounded by thrust faults at the base and low angle normal faults at the top. Some slabs are subducted into the mantle where they are invaded by peridotite from the overlying mantle wedge. The presence of peridotite masses are diagnostic of deeply subducted terranes. Variables such as the rate, angle and depth of subduction, "hang-time" in the mantle, and exhumation rate determine the nature of metamorphism and deformation, the degree of retrogression, and the mineralogy of the introduced mantle fragments. Slabs that reach deep mantle levels (>60Km) are metamorphosed under high pressure (HP) conditions to form eclogite-facies terranes. Extreme subduction (>120 km) results in ultra high pressure (UHP) metamorphism. The evolution of the Caledonides of Scandinavia included three and probably more events of crustal subduction that resulted in HP and UHP eclogite-facies assemblages accompanied by the introduction of mantle peridotite. Two are the classic Finnmarkian (arc-continent) and Scandian (continent-continent) orogenies at ca. 500 and 410 Ma. We present evidence for a third HP event at ca. 455 Ma that involved the subduction of the western edge of Baltica beneath a microcontinent. We suggest that other orogens such as the Alps and Himalayas would be better understood through dunk tectonic mechanisms.