The Tampere Schist Belt (TSB) in southern Finland is a c. 1.92-1.88 Ga volcano-sedimentary basin that underwent inversion and closure between c. 1.89-1.88 Ga. We present field observations from the Tampere palaeobasin, where the primary structures have been exceptionally well preserved. The TSB, therefore, offers an excellent opportunity to examine the volcano-sedimentary evolution of an ancient marginal basin, and the mechanics of and strain distribution during its subsequent closure.
The aim of this study is to investigate the structural development and the architecture of a part of the TSB in more detail, including the relationships between the volcano-sedimentary sequences, the tectonic structures, and the sulphide-bearing schist horizons. Important insights are gained into understanding the mechanisms of the basin closure and the localisation of the sulphide mineralisation within the basin. We use the observations to construct a new conceptual tectonic model for the closure of the southeastern margin of the Tampere basin.
The observed volcano-sedimentary and structural features suggest a change in the local structural style from thick-skinned inversion to thin-skinned thrusting, in order to accommodate the crustal shortening during basin closure. Furthermore, it is suggested that there is a genetic relationship between the interpreted palaeothrusts and the sulphide-bearing schist horizons in the study area: early, gently dipping thrusts acted as both channels and traps for the mineralising fluids that possibly sourced either locally or from relatively shallow depths from the base of the basin infill. The continued compression caused a subsequent rotation of the thrusts into their present subvertical position.