Syn-extensional faulting controlling structural inversion – Insights from the Palaeoproterozoic Vargfors syncline, Skellefte mining district, Sweden

The Vargfors basin in the central Skellefte district, Sweden, is an inverted sedimentary sub-basin within a Palaeoproterozoic (1.89 Ga) marine volcanic arc. The sub-basin formed from upper-crustal extension and subsequent compression, following a period of intense marine volcanism and VMS ore formation. Detailed mapping and structural analysis reveals a pattern of SE–NW-striking normal faults and interlinked NE–SW-striking transfer faults, which define distinct fault-bound compartments, each with an individual structural geometry and stratigraphy. Constraints on the deformation style and mechanisms achieved by 2D forward modelling are in agreement with the previously inferred inversion of the early normal faults during a regional crustal shortening event. A rheologically weak carbonate-rich layer at the base of the sedimentary sequence favoured the fault inversion over more distributed shortening as the controlling deformation mechanism. Transposition of sedimentary strata into the approximately SE–NW faults led to formation of asymmetric synclines that were tightened during progressive shortening. Structural analysis infers a progressive opening of the basin towards SE and NW with time. Furthermore, it is inferred that a displacement gradient was developed along the main structural grain, with decreasing dip-slip displacements towards SE and NW, both during the extension and the structural inversion.VMS deposits in the vicinity of the contact between the volcanic and the overlying sedimentary rocks were formed along early normal faults, which reacted as fluid conduits. Subsequently, the deposits were transposed into the inverted faults during crustal shortening. Consequently, the inverted faults provide a useful tool for mineral exploration in the district.

► Extensional normal faults and interlinked transfer faults define fault-bound compartments.
► 2D-forward modelling is in agreement with previously inferred inversion and folding.
► A carbonate-rich layer at the base of the basin favoured fault inversion.
► Continued inversion results in steepening of the faults, break-back faults and overturning.