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EUR-01 Three billion years of geological history of the Baltic Shield and its shelf
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Kilometre-scale sheath fold and associated faults in the Keivy terrane: Implications for Early Precambrian geology in the northeastern Baltic shield
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Victor Balagansky, Geological Institute, Kola Science Centre, Russian Academy of Sciences (Russian Federation)
Alexei Raevsky, Geological Institute, Kola Science Centre, Russian Academy of Sciences (Russian Federation)
Vadim Tyuremnov, Geological Institute, Kola Science Centre, Russian Academy of Sciences (Russian Federation)
Sergei Mudruk, Murmansk State Technical University (Russian Federation)
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Archaean metasedimentary kyanite-, staurolite- and garnet schists widely occur in the Keivy terrane. They overlie 2.87 Ga felsic metavolcanics everywhere and are overlain by a Palaeoproterozoic amphibolite-facies sequence in the western Keivy terrane. Stratigraphic relationships between these units including an unconformity between the Neoarchaean and Palaeoproterozoic are thought to have been preserved, and this well studied Early Precambrian succession is interpreted as a standard for stratigraphy in the northeastern Baltic shield. An idea that the succession is severely destroyed by thrusts has not been supported by strong evidence.
Geological and magnetic mapping combined with structural study has confirmed that Palaeoproterozoic rocks make up an isoclinal fold 8 km long and 2 km wide. Dispersed grains and veins of magnetite formed at metamorphism, and a tentative palaeomagnetic study has shown that in these rock the main NRM component corresponds to the time of the Lapland-Kola orogeny (1.9?2.0 Ga). The rocks dip deeply to the west in the western hinge and occur subvertically in the eastern hinge, which suggests subvertical hinge lines rather than subhorizontal as thought before. Thus, this is a sheath fold, the long axis of which plunges to the north in general parallel to stretching lineation. The southern limb of this structure is almost entirely cut off and the northern boundary cuts marble layers in the northern limb. Therefore, the sheath fold occurs within a lens limited by reverse faults and thrusts. Most of Palaeoproterozoic rocks are strongly magnetic (Κ up to 25010-3) whereas the host Archaean schists are almost not magnetic (Κ < 110-3). Based on a technique specially designed for interpretation of magnetic field, the amount and direction of dip of Palaeoproterozoic rocks as well as their extension to depth have been estimated in the western, central and eastern parts of the fold. These reconstructions also suggest the sheath synform morphology. Magnetic mapping has revealed the structure of Palaeoproterozoic rocks in details and shown that tectonic lenses and faults (sub)parallel to layers are folded into this sheath fold. Thus, early thrusting preceded the main folding and associated thrusting. The lens formed by the cut off sheath fold in turn occurs within a ca. 15 km long tectonic lens built up by Archaean garnet schists. Therefore, the Keivy terrane cannot be a standard for Early Precambrian stratigraphy in the Kola region and we deal with tectonostratigraphy as in Phanerozoic orogens. A discovery of tectonostratigraphy in an area believed to be basic for stratigraphy confirms a conclusion made by many geologists that Early Precambrian regions (firstly Archaean) need re-mapping combined at least with structural and geophysical studies.
This study is supported by programme ONZ-6 of the Russian Academy of Sciences.
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