Volcanic and sedimentary rocks of the Västervik and Valdemarsvik groups along the southern margin of the 1.90–1.85Ga south Svecofennian crustal province in Sweden have been migmatised and intruded by granitoids and gabbros during extension (deformation phase D1), at a depth of at least 10km. Peak metamorphic conditions were reached around 1825Ma. Extension switched to compression before cooling. The partially molten middle crust facilitated shortening and thickening by isoclinal and sheath folds on scales from a decimetre to 10km (D2). A pervasive mylonitic S2 foliation developed. These structures were refolded into eastward plunging crustal scale upright folds with E(SE)–W(NW) trending axial planes (D3). D2–D3 deformation structures are colinear and developed during top-to-the-WNW slip along S2. Locally, zones of high ductile strain in garnet-bearing migmatite assemblages record considerable vertical displacement (from 850 to 300MPa), possibly in nappe roots. Some major D2 sheath folds are embraced by D3- like, but NNW-trending megafolds (D4), apparently resulting from shortening around the former. D2–D4 deformations are interpreted to be broadly coeval and related to transpressive crustal shortening against the northern Svecofennian continental back-stop.After cooling to high greenschist-facies conditions, early structures underwent renewed transpression in north–south direction, expressed in localized, retrogressive shear zones (phase D5). Much D5 strain was accommodated by the 10km wide, NW–SE striking, right-lateral Loftahammar–Linköping Deformation Zone (LLDZ), operative around 1800–1780Ma. Regional strain partitioning caused transverse shortening in 15–20km wide border zones of the LLDZ. Later overprints (D6) involve narrow, subparallel, NW–SE striking and steeply NE dipping low greenschist-facies mylonite zones, some of which merge into the regional, polyphase Åsbro–Norrköping Deformation Zone. D6 was transpressive; oblique, left-lateral, NE-block-up slip suggests a W(SW)–E(NE) orientation of σ1.Peak metamorphism and partial melting in the supracrustal rocks was approximately coeval with ≥1800–1835Ma old subduction-related magmatism in the Oskarshamn–Jönköping Belt (OJB), 100km to the south-west. Seismic reflection profiling off the Baltic coast (BABEL line B) suggests northward polarity of subduction beneath the OJB and a back-arc environment of the Västervik and Valdemarsvik groups between the OJB arc and the margin of the >1850Ma old Svecofennian orogen. We suggest that D1–D5 deformation was caused by intracontinental back-arc extension and subsequent closure of the back-arc basin by oblique accretion of the OJB marginal arc onto the northern continent. Accretion involved northward indentation by mid-crustal wedges, one of which coincides with and possibly caused the nucleation of the D5 stage LLDZ.40Ar/39Ar data from hornblende and white mica identify a prolonged cooling history of the area (1810–1490Ma), influenced by the nearby, late- and postorogenic Transscandinavian Igneous Belt and, possibly, by largely hidden, c. 1530Ma old anorogenic rapakivi-type intrusions.