Structural development of the Mjelde-Skorelvvatn Zone on Kvaløya, Troms: a metasupracrustal shear belt in the Precambrian West Troms Basement Complex, North Norway
1, 2
pp. 117-132

The Mjelde-Skorelvvatn Zone is one of several metamorphosed and deformed volcanosedimentary belts in the Archaean-Palaeoproterozoic West Troms Basement Complex. The Mjelde-Skorelvvatn Zone is a belt of steeply dipping metasupracrustal rocks (Skorelvvatn Formation) of presumed Palaeoproterozoic age, lying between Archaean (?) anorthositic gneisses (Gråtind Migmatite) and a mafic plutonic complex (Bakkejord Diorite). These units occur on the west limb of a large north-south trending, gently plunging antiform, and all three units share common groups of structures. The structural development of the zone is characterised by three-phase, amphibolite-facies ductile deformation (D1-D3) and a phase of brittle/semi-ductile faulting of probable post-Caledonian age. D1 generated a penetrative foliation (S1) which is axial-planar to isoclinal folds of variable orientation (F1). S1 strikes NNW, dips steeply west and exhibits a moderately plunging stretching lineation (L1). D2 involved macroscale folding of S1 about a gently SSW-plunging axis (F2). The axis is perpendicular to L1, suggesting that D2 may have been a late stage of D1. D3 structures are represented by subvertically plunging macrofolds (F3) with sinistral geometry and a network of moderately to steeply dipping, lateral shear zones (S3) of dominantly sinistral sense and subordinate dextral sense. The geometric and kinematic relationship between S3 shear zones suggests that D3 structures were generated by sinistral transpression with associated minor lateral extrusion. Granitoid pegmatite dykes with a U-Pb titanite age of 1768 ± 4 Ma are shown to have intruded at a late stage of this ductile event, thus constraining a Palaeoproterozoic deformation age. The overall structural character of the Mjelde-Skorelvvatn Zone suggests that it developed during: (i) WSW-ENE crustal shortening (D12) and later sinistral transpression (D3); or (ii) a progressive transpressional event with partitioned east-west crustal contraction (D1-D2) and NW-SE lateral shearing (D3). The proposed kinematic model is comparable with models for other shear zones of the West Troms Basement Complex and is linked with tectonic processes in the Palaeoproterozoic terrains of the northernmost Fennoscandian Shield.

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