STRUCTURAL CHARACTERIZATION OF THE SOUTHERN SECTION OF THE MYLONITE ZONE AND THE EASTERN SEGMENT IN THE BUA AREA, SVECONORWEGIAN OROGEN, SW SWEDEN

The Mylonite Zone (MZ), a c. 1.0 Ga terrane boundary in the Baltic Shield, is a >450 km long and >5 km wide SE-vergent crustal scale Sveconorwegian thrust that juxtaposed the allochtonous Idefjorden Terrane in the hanging wall against the Eastern Segment in the footwall. These two orogenic segments show fundamentally different igneous and metamorphic evolutions in both Sveconorwegian and pre-Sveconorwegian time. We present preliminary structural data as part of an ongoing project that will also include petrological and geochronological analyses. The main focus of the research is to constrain temporal, geometric and kinematic relationships of Sveconorwegian deformation phases along the Idefjorden Terrane-Eastern Segment boundary.

In its southernmost part, the MZ strikes ENE-WSW (Bua segment) and consists of banded granodioritic gneisses with interlayered granitic veins that were sheared under amphibolite-facies conditions. The banded gneisses include asymmetric SW and W-verging angular folds (F2) and winged porphyroclasts that are consistent with a top to the southwest and west transport. F2 fold axes are parallel to the stretching lineations (mainly N-S, E-W and NW-SE trending). In E-W sections, conjugate extensional shear bands and boudins are relatively common. Also in the Bua area and to the south of the MZ (Eastern Segment), the basal shear zone of an eclogite nappe is exposed, and preserves structures and fabrics associated with four main deformation phases (D1-D4). Structures related to D1 are a pervasive gneissic layering that includes asymmetric kinematic indicators and local isoclinal folds. D1 structures were overprinted by a younger deformation phase (D2) characterized by asymmetric tight to isoclinal folds with angular hinges, N-S to E-W axes, and an associated amphibolite-facies axial planar foliation. Coarse-grained leucosome segregated around mafic lenses or was injected along F2 axial planes or sheared F2 limbs. F2 folds and leucosome were strongly sheared. In the research area, the Idefjorden terrane and the basal shear zone were subsequently overprinted by D3 and a D4 structures, characterized by E-W or NW-SE (F3) and N-S to NE-SW (F4) trending open folds.