LATE SVECONORWEGIAN DEFORMATION IN A SECTION ACROSS THE IDEFJORDEN TERRANE-EASTERN SEGMENT BOUNDARY, SVECONORWEGIAN OROGEN, SW SWEDEN

The SE-vergent Mylonite Zone (MZ), in SW Sweden is a crustal scale tectonic boundary that juxtaposed the Idefjorden Terrane (IT) in the hanging wall against the Eastern Segment (ES) in the footwall during the 1 Ga Sveconorwegian orogeny. New structural and petrographic data collected to the east of Göteborg are combined to document Sveconorwegian deformational structures and microstructures preserved in rocks of the IT, the MZ, and the ES.

Granitic to granodioritic mylonitic schist and gneiss in the IT contain a pervasive W to NW-dipping foliation and a mostly dip-slip W or WNW-plunging stretching lineation. Foliation planes are folded by isoclinal (F1) folds (locally preserved), and by asymmetric S- or SW-verging folds (F2) with W or NW-plunging axes. In highly strained areas, F2 folds exhibit sheared out limbs. At the terrane boundary (MZ), F2 folds show syn-kinematic melting. The main stretching lineation is subparallel to F2 axes and to a mylonitic lineation that resulted from a shearing episode consistent with top-down-to-the-W/SW kinematics. Along E-W sections, the main foliation is overprinted by a conspicuous C’ shear band cleavage that also suggests top-down-to-the-W/SW extension. Microlithons typically contain quartzo-feldspathic aggregates and mantled porphyroclastic feldspar with aggregates of mostly fine-grained quartz, feldspar, and biotite filling their pressure shadows. C’ shear bands are defined by oriented biotite and epidote grains. Chlorite is scarce in most of the analized IT samples suggesting that syn-shearing retrogression of biotite to chlorite was not significant. To the east of the MZ, felsic to intermediate coarse-grained metagranitoids record deformation under amphibolite-facies conditions. C’ shear band cleavage defined mostly by aligned biotite grains overprints earlier fabrics and structures (a gneissic/schistose banding folded by F1 and F2 folds) and suggests top-down-to-the-W extension. C’-antithetic shear bands and boudins are consistent with thinning along E-W sections. Quartz and feldspar microstructures and mineral assemblages preserved in the studied samples are compatible with extensional (or transtensional) ductile deformation that occurred predominantly under upper greenschist-facies conditions overprinting earlier fabrics and structures.