THE LATE SVECOFENNIAN GRANITE-MIGMATITE ZONE OF SOUTHERN FINLAND - A BELT OF TRANSPRESSIVE DEFORMATION AND GRANITE EMPLACEMENT

The late Svecofennian granite-migmatite (LSGM) zone in southwestern Fi nland is a similar to 100 km wide and 500 km long belt transecting the southern Svecofennides from WSW to ENE. It was formed in an area of t hin pillow lavas, volcaniclastic sediments and limestones. The area is interpreted as having been an early basin of crustal extension which was the locus of an inherited zone of weakness in the Proterozoic crus t. Early recumbent folding was followed by crustal thickening and intr usions of similar to 1.89-1.88 Ga old plutonics. The LSGM-zone is char acterized by 1.84-1.83 Ga old rhomboidal sheets of late Svecofennian m icrocline granite and is bounded by ductile shears. Amongst the two ma jor phases of deformation defined in the LSGM-zone, the earlier one (D 1) affected only the supracrustals and the 1.89-1.88 Ga old early plut onics. In contrast, the later phase (D2) also deformed the late Svecof ennian migmatites and granites. D1 represents a complex and long-lasti ng deformation event which included overturning and thrusting of the S vecofennian strata. D2 comprised ENE-WSW directed drag accompanied by NNW-SSE compression. The Svecofennian crust was thickened further and anatectic microcline granites intruded along thrusts. The rhomboidal o utline of the late Svecofennian granite sheets indicates a sense of mo vement in agreement with measured dextral strike-slip in the shears de limiting the LSGM-zone. Imbricated feldspar megacrysts in the granites indicate thrusting towards the west during the stage of granitic magm atism. The gently dipping early Svecofennian gneisses and the late gra nite sheets were folded into upright F2 folds with gently plunging axe s. Locally, the F2 axial surfaces were intruded by late Svecofennian g ranite mobilisates.