Two gravity-driven tectonic processes are common in orogenic belts, such as the Scandinavian Caledonides. (1) The rise of buoyant granitoid basement in the form of domes and diapirs into the column of geosynclinal metasediments and metalavas. In the Scandinavian Caledonides some of the nappes also acted as a buoyancy-inducing overburden. This is particularly true for the heavy Seve nappe. (2) The gravitational collapse of the pile of supracrustals and nappes. The collapse is associated with a horizontal spreading which contributes to the horizontal displacement of the thrust sheets. In the Scandinavian Caledonides there are numerous examples of basement diapirism, particularly among the ‘Nordland granites’, but also within the huge Möre-gneiss complex in SW Norway. Closer to the eastern edge of the chain the basement was less hot and plastic during the orogenesis, and cataclastic conditions prevailed during the rise of the row of peripheral uplifts extending parallel to the belt. Evidence of horizontal spreading and vertical shortening of some of the nappes is found in the form of boudinage, mineral lineation and prevailing horizontal schistosity over large regions within the nappes. The collapse-spreading process has been treated theoretically and experimentally, but experimental results are only briefly mentioned.

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