Structural evolution of the accretional continental margin of the Paleoproterozoic Svecofennian orogen in southern Sweden

dVolcanic and sedimentary rocks of the Vastervik and Valdemarsvik groups al ong the southern margin of the 1.90-1.85 Ga south Svecofennian crustal prov ince in Sweden have been migmatised and intruded by granitoids and gabbros during extension (deformation phase D-1), at a depth of at least 10 km. Pea k metamorphic conditions were reached around 1825 Ma. Extension switched to compression before cooling. The partially molten middle crust facilitated shortening and thickening by isoclinal and sheath folds on scales from a de cimetre to 10 km (D-2). A pervasive mylonitic S-2 foliation developed. Thes e structures were refolded into eastward plunging crustal scale upright fol ds with E(SE)-W(NW) trending axial planes (D-3). D-2-D-3 deformation struct ures are colinear and developed during top-to-the-WNW slip along S-2. Local ly, zones of high ductile strain in garnet-bearing migmatite assemblages re cord considerable vertical displacement (from 850 to 300 MPa), possibly in nappe roots. Some major D-2 sheath folds are embraced by D-3- like, but NNW -trending megafolds, (D-4), apparently resulting from shortening around the former. D-2-D-4 deformations are interpreted to be broadly coeval and rela ted to transpressive crustal shortening against the northern Svecofennian c ontinental back-stop. After cooling to high greenschist-facies conditions, early structures under went renewed transpression in north-south direction, expressed in localized , retrogressive shear zones (phase D-5). Much D-5 strain was accommodated b y the 10 km wide, NW-SE striking, right-lateral Loftahammar-Linkoping Defor mation Zone (LLDZ), operative around 1800-1780 Ma. Regional strain partitio ning caused transverse shortening in 15-20 km wide border zones of the LLDZ . Later overprints (D-6) involve narrow, subparallel, NW-SE striking and st eeply NE dipping low greenschist-facies mylonite zones, some of which merge into the regional, polyphase Asbro-Norrkoping Deformation Zone. D-6 was tr anspressive. oblique, left-lateral, NE-block-up slip suggests a W(SW)-E(NE) orientation of sigma (1). Peak metamorphism and partial melting in the supracrustal rocks was approxi mately coeval with greater than or equal to 1800-1835 Ma old subduction-rel ated magmatism in the Oskarshamn-Jonkoping Belt (OJB), 100 km to the south- west. Seismic reflection profiling off the Baltic coast (BABEL line B) sugg ests northward polarity of subduction beneath the OJB and a back-arc enviro nment of the Vastervik and Valdemarsvik groups between the OJB arc and the margin of the > 1850 Ma old Svecofennian orogen. We suggest that D-1-D-5 de formation was caused by intracontinental back-arc extension and subsequent closure of the back-are basin by oblique accretion of the OJB marginal arc onto the northern continent. Accretion involved northward indentation by mi dcrustal wedges, one of which coincides with and possibly caused the nuclea tion of the D-5 stage LLDZ. Ar-40/Ar-39 data from hornblende and white mica identify a prolonged coolin g history of the area (1810-1490 Ma), influenced by the nearby, late- and p ostorogenic Transscandinavian Igneous Belt and, possibly, by largely hidden , c. 1530 Ma old anorogenic rapakivi-type intrusions. (C) 2001 Elsevier Sci ence B.V. All rights reserved.