Abstract
Palaeoproterozoic fold-thrust belt structures and steep, lateral shear zones characterize the foreland deformation of Neoarchaean basement tonalites in Vanna, West Troms Basement Complex, northern Norway. Low-grade par-autochthonous and allochthonous cover units (2.4–2.2. Ga) with sandstones and calcareous metapelites exist in separate areas of the foreland. They were formed as intracontinental rift- and/or deltaic shelf deposits, and subsequently intruded by a diorite sill at c. 2.2 Ga. The basement and cover units were folded and inverted along low-angle thrusts and steep reverse faults during two late/post Svecofennian (1.77–1.63 Ga) orthogonal shortening events (D1-D2). The D1 event involved NE-SW shortening, folding, ENE-directed thrusting, and dextral lateral shearing, controlled by pre-existing, N-S striking mafic dykes (c. 2.4 Ga) and basin-bounding normal faults. The D2 event involved SE vergent nappe translation, flat-ramp thrust propagation in a frontal duplex above a basement-seated detachment, and sinistral lateral reactivation in a partitioned orogen-parallel, transpressive setting. Hydrothermal fluid circulation affected all the shear zones. New aeromagnetic data show the basement-involved fold-thrust belt architecture well. The orthogonal Vanna Island fold-thrust belt styles of deformation resemble other inverted rift-basin deposits in northern Fennoscandia, deformed during the Svecofennian Orogeny (1.92–1.79 Ga), Alta-Kautokeino and Karasjok greenstone belts in northern Norway, Central Lapland, Peräpohja, Kittilä and Kuusamo belts of Finland, and in the Norrbotten province of Sweden. Westward younging of the orogenic events explain the younger age span of deformation on Vanna Island.
Highlights
Foreland fold-thrust belts are the most common way to accommo date crustal shortening in frontal parts of orogens
We focus on the Neoarchaean basement rocks and several overlying Palaeoproterozoic cover/basin sequences, preserved as par-autochtonous and allochthonous units
We further argue that the extent and geometry of the pre-existing basin units, their boundary normal faults, and an associated 2.4 Ga dyke swarm (Kullerud et al, 2006), may have controlled inver sion that resulted in folding and thrust propagation above basementseated detachments, as well as hydrothermal alteration and quartzcarbonate precipitation in thrusts and strike-slip shear zones (Paulsen, 2019)
Summary
Foreland fold-thrust belts are the most common way to accommo date crustal shortening in frontal parts of orogens. Such belts are worldwide distributed, typically involve basement rocks and overlying sedimentary cover rocks deposited in rift- and/or shelf basins of former continental margins, and have formed in time span from the Archaean to present (Poblet and Lisle, 2011). The study of basement cover relationships in foreland fold-thrust belts is fundamental to resolving the ore potential in such settings. The study of Archaean and Proterozoic fold-thrust belts, is hindered by the low preservation potential, since commonly only the deeply eroded core parts remain
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