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The Turku migmatite complex in southwestern Finland is a representative area for the type of tectonic and metamorphic evolution seen within the Palaeoproterozoic Svecofennian Orogen in southern Finland. The orogeny can be divided into early, late and postorogenic stages. The early orogenic structural evolution of the crust is expressed by a D,/D, deformation recorded as bedding-parallel S i mica foliation deformed by tight to isoclinal D 2 folds with subhorizontal axial planes and a penetrative S 2 axial plane foliation. Syntectonic ca. 1890-1870 Ma tonalites were emplaced during D, as sheet intrusions. This deformation is attributed to thrust tectonics and thickening of the crust. The late orogenic structural evolution produced the main D, folding, which transposed previous structures into a NE-SW trend. The doubly plunging fold axis produced dome-andbasin structures. The attitude of the F 3 folds varies from upright or slightly overturned to locally recumbent towards the NW. Granite dikes were intruded along S, axial planes. Large D 3 fold limbs are often strongly deformed, intensively migmatized and intruded by garnet-and cordierite-bearing granites. These observations suggest that these potassium-rich granites, dated at 1840-1830 Ma, were emplaced during D,. This late orogenic NW-SE crustal shortening further contributed to crustal thickening. Subvertical D 4 shear zones that cut all previous rock types possibly controlled the emplacement of postorogenic granitoids. Steeply plunging lineations on D 4 shear planes suggest vertical displacements during a regional uplift stage.
Väisänen et al. (Tue,) studied this question.