STRUCTURAL STUDY OF THE EASTERN SEGMENT OF THE SVECONORWEGIAN OROGEN IN THE HALLAND PROVINCE, SW SWEDEN

The Eastern Segment (ES) in SW Scandinavia (Baltica) is the easternmost parautochthonous basement of the Sveconorwegian Province, and is dominated by orthogneisses derived from 1.7 Ga granitic and younger basic intrusive rocks that have been overprinted by a Hallandian tectonothermal event (1.44-1.40 Ga) and Sveconorwegian (0.99-0.95 Ga) high-grade deformation and metamorphism. The Eastern Segment contains a unique and well exposed sequence of dominantly supracrustal rocks (Stensjö association) along a 7 km section of the Halland coast, SW Sweden, characterized by migmatitic sillimanite-bearing and sillimanite-free quartzo-feldspathic gneiss, migmatitic garnet amphibolite gneiss and garnet-poor migmatitic amphibolite gneiss with calc-silicate lenses. Most rocks in this sequence show a predominant N-S to NE-SW gneissic foliation (S₁), in which intrafolial folds (F₁) are locally preserved. Polymineralic aggregates define a prominent ESE-trending mineral lineation (L₁) in the area. S₁, F₁ folds and L₁ are folded by metric to kilometric-scale asymmetric F₂ folds that are characterized by a subhorizontal and a steep limb, angular hinges, and N-S to E-W trending axes. Parasitic F₂ folds are relatively common, particularly in the sillimanite-bearing gneisses, and permit the reconstruction of F₂ larger scale folds. Cm-thick granitic leucosome veins and cm- to metric-thick granitic pegmatites, both subparallel to the main banding, are folded by F₂ folds. Abundant sugary granitic leucosome appears in a syndeformational relation to F₂ folds, and locally cross-cuts F₂ fold hinges. Cm- to km-scale concentric F₃ folds with subhorizontal to steeply E to NE or SW-plunging axes verge to the south or southeast, and affect all the older structures. Cm-thick leucosome veins associated to F₃ crosscut the tectonic banding at high angles, and are subparallel to F₃ axial planes. The structures correlate with D₁-D₃ deformation structures recorded in the surrounding orthogneiss complex. Based on structural and geochronological data, we interpret D_1-3 ductile structures as recording a mainly WNW-ESE contraction followed by E-W shortening of the Eastern Segment during the Sveconorwegian orogeny.