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MPN-12 Sederholm symposium on high-grade metamorphism, crustal melting, migmatites and granites - Part 1

 

Thrusting in a hot Palaeoproterzoic orogen: examples from the west-central Fennoscandian Shield

 

Håkan Sjöström, Uppsala University (Sweden)
Karin Högdahl, Lund University (Sweden)
Erik Ogenhall, Uppsala University (Sweden)
 

 

The west-central part of the Fennoscandian Shield is characterized by several steep shear zone systems, partly accompanied by migmatite belts and separating different plate tectonic units. Some of these shear zones are therefore considered to be terrane boundaries. Shallow dipping terrane boundaries or thrusts typical of many younger orogens have seldom been identified. However, thrusts appear to be common in the deeply eroded west-central part of the Svecofennian orogen in central Sweden, but subsequent processes obscure their signatures.

In the continental margin Ljusdal Batholith (LJB), metasedimentary screens show complex internal folding that is sheared out along the screen boundaries. The granitoids do not truncate the metasediments and recrystallised mylonites on both sides may reflect thin-skinned thrusting onto the granitoids. Neosomes formed within the screens truncate the folds, showing that metamorphism outlasted the deformation.

The deepest thrusts-like structures exist in the NE part of the LJB (Hornslandet). Granulites and charnockites are exposed in the hanging wall and stromatic migmatites and diatexites in the footwall of shear zones dipping to the east. Two, possibly three levels of such shear zones have been recorded. West-vergent asymmetric folds in one of the sheets indicate top-to-the west thrusting.

The kinematically best defined stacked sequence exists in the SE part of the LJB. The well-preserved Hamrnge supracrustal formation is separated from granitoids by tectonic contacts. Internal mylonite zones truncate recumbent E-W folds. The volcanic rocks have an oceanic island signature and the tectonic contacts record the island arc accretion to the continental margin. Shear bands indicate top-to the west kinematics and are overgrown by post-tectonic garnets showing prograde chemical zoning.

Thrusting is also indicated in the 1.91-1.87 Ga Bergslagen region to the south of a shear zone-migmatite system. A structural break exists between large-scale recumbent folding accompanied by pronounced L-tectonites and the steep structural pattern in the interior of the arc to the W. Fragments of the L-tectonites exist in anatectic granitoids.

The timing of inferred thrusting in the Hornslandet area is constrained by the 1.86-1.85 Ga LJB granitoids and the 1.82 Ga metamorphic age. In Hamrnge, the 1.89 Ga volcanic rocks are overlain by ≤ 1855 Ma quartzites affected by the thrusting; a 1836 Ma granite probably approximates age of the "post-tectonic" metamorphic overprint. In eastern Bergslagen anatectic 1.82 ? 1.80 Ga granites are frequent.

All the examples show LPM-metamorphism outlasting deformation, strong linear features related to recumbent folds and refolding by regional E-W fold, and subsequent strain partition into steep shear zones. This indicates that a fragmented < 1.86 Ga fold-thrust belt resulting in delayed 1.82 (1.84) Ga migmatisation exist in the west-central part of the shield.

 

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