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Precambrian Research Volume 72, Issues 3–4, April 1995, Pages 199–225
Palaeoproterozoic intraplating exposed by resultant gravity
overturn near Kiruna, northern Sweden
Christopher J. Talbot, Hemin Koyi
Abstract
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/lowP conditions as it foundered among buoyant diapirs of remobilised granitoid basement. The nearconstant 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 synSvecofennian 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 risking 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 arc was built above it.