PALAEOPROTEROZOIC INTRAPLATING EXPOSED BY RESULTANT GRAVITY OVERTURN NEAR KIRUNA, NORTHERN SWEDEN

The tectonic history of the Kiruna area is reexamined on the basis of four newly available digitised geophysical maps, a literature survey and some first-hand field observations. A basement complex of Archaean,gneisses was unconformably overlain by a Palaeoproterozoic sequence of metabasalt, andesite and keratophyric porphyries with interstratified tuffs and sediments. The cover was then detached from its basement by an orogeny with a northerly tectonic grain. A pile of ductile nappes separated by mylonites in the east pushed westward an imbricate fold-thrust belt that may have decoupled over an evaporite unit of regional extent. Conglomerates and quartzites accumulated in a foredeep migrating in front of a wedge of emergent thrusts. The allochthonous cover then developed a vertical north-trending foliation in high-T/low-P conditions as it foundered among buoyant diapirs of remobilised granitoid basement. The near-constant lateral spacing of the centres of these diapirs, their mushroom shapes, and the absence of contemporaneous volcanic rocks, suggest that these diapirs rose in the solid state. The region continued to distort episodically even after diapir emplacement. Ductile shear zones evolved through vein systems to faults as the region cooled and was uplifted before the Cambrian. The youngest fault movements clearly displace 9000-years-old glacial deposits. Post- or late-orogenic plutons in the Kiruna area appear to be contemporaneous with syn-Svecofennian plutons further south. We therefore suggest reinstating the label Svecokarelian for the orogeny in the Kiruna area. Heat advected by melts from the mantle is invoked to account for the Archaean basement rising in two convective stages, the first as porphyritic lavas, the second as solid-state diapirs. Melts rose rapidly and erupted before the Svecokarelian orogeny, while the slower diapirs ended it. We attribute the current southwestern dip of the diapir zone to extension of Svecokarelian crust before the Svecofennian magmatic are was built above it.