Pre-1.87 Ga development of crustal domains overprinted by 1.87 Ga transpression in the Palaeoproterozoic Skellefte district, Sweden

Abstract

The complex structural evolution within the VMS-hosting Skellefte district, Sweden, has been investigated to provide a solid structural framework for the known mineral deposits in the area. The area occurs in a transition zone between dominantly N–S to NNE–WSW striking structures in the north and approximately WNW–ESE oriented structural trends in the south. The presence of high-strain zones with both the above orientations in the Skellefte district allows constraining their mutual relationship, as well as their significance for the build-up of the Svecokarelian orogen at around 1.89Ga and for the following tectonic overprint between 1.87 and 1.80Ga. The methods used in this study include structural analysis complemented by potential field modeling and SIMS U–Pb geochronology on zircon. Based on the results of this study, the earliest deformation (D1) is constrained at 1.89–1.88 (1.87)Ga and tentatively attributed to crustal extension occurring synchronously with volcanism. Deposition of the Skellefte Group metavolcanic rocks is inferred to have occurred in a pull-apart basin developed due to dextral strike-slip shearing along approximately N–S striking regional-scale shear zones. Variations in the development of deformation fabric across the district indicate that the crust was divided into an upper, un-metamorphosed domain and a lower, strongly metamorphosed domain during D1. We further infer that the transition from the upper to lower crust was locally coupled with development of low-angle crustal-scale detachment zones during D1. The heterogenous crust was subsequently overprinted by transpressional deformation which may be explained by two alternative models. According to the first model, one single SSE–NNW transpressional event with distinct strain partitioning between the coaxially deformed upper crust and the non-coaxially deformed lower crust is largely responsible for the present-day structural geometry. A post-folding rhyolite dyke, here dated at 1871±4Ma, constrains the minimum age of this event (D2). The alternative model includes two separate transpressional events: a SW–NE one at (1.88–) 1.87Ga, followed by SSE–NNW transpression at 1.86Ga. Recognition of the early-orogenic detachment zones allow us to suggest that many of the major crustal-scale shear zones in the central Fennoscandian Shield have originated as 1.89–1.87Ga crustal detachment zones, i.e. earlier than typically considered.