Caledonian deformation in Norway

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Abstract
Geological research and mapping over the last decade has led to a reassessment of the Caledonian orogen in Norway. Two principal stages of orogenesis are now established: (1) a late Cambrian to early Ordovician Finnmarkian or Grampian event recognized mainly in Finnmark and probably also developed in western Norway; and (2) the widespread Middle-Upper Silurian Scandinavian stage of central and southern areas. Another significant finding, aided by radiometric studies, is that of incorporation of Precambrian crystallines as discrete tectonic slices within the Caledonian nappe complexes, reminiscent of the Lewisian-within-Moine relationship of Scotland. In the extensive gneiss terrain of western districts and of Lofoten the effects of Caledonian tectonometamorphic reconstitution have been minimal. Biostratigraphic and geochronological indicators provide evidence that diachroneity is a major feature common to both the above evorogenic stages, with deformation younging progressively towards the southeastern quadrant. In S Norway, folding and local thrusting continued into the Devonian. In terms of geometry and strain, individual nappes are generally extensive, thin, sheet or westward-thinning elements in which protracted high strains in basal zones have produced complex, banded mylonitic and pseudo-psammitic lithologies. Indications of an overall, primary, progressive simple shear deformation are ubiquitous, commonly with evidence of fold rotation into the stretching lineation trend in areas of high ductile strain; this is masked by important episodic increments of flattening and extensional strain producing lensoid, mega-boudin structures and leading to excision of some nappe units. This vertical shortening and gravitational spreading is recognized in both the Cambrian and the Silurian stages of the orogenesis and affects calculations of nappe translation, estimated in hundreds of km for central and southern areas. In conclusion, some comparisons are outlined between the Scandinavian and East Greenland Caledonides; in these two segments of the orogen, nappe displacement is in opposite directions.