High-temperature deformation in the basal shear zone of an eclogite-bearing fold nappe, Sveconorwegian orogen, Sweden

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Ductile shear zones associated with emplacement of high-pressure nappes are key features to resolve exhumation mechanisms. The Eastern Segment of the Sveconorwegian orogen hosts an eclogite-bearing fold nappe, whose basal shear zone shows structures associated with emplacement of the eclogite-bearing nappe and decompression under high to intermediate pressure granulite and upper amphibolite facies conditions. Based on detailed structural mapping of a 4km well-exposed section of the basal shear zone, we describe two major phases of deformation. An early deformation stage (D1) formed a penetrative gneissic foliation and tectonic layering, including isoclinal folds (F1). The sequence was subsequently affected by up to km-scale tight south-vergent folds (F2) with sheared out limbs. At the outcrop scale, asymmetric F2 folds are commonly S-vergent, but symmetric folds with different degrees of tightness are also present. Melt was present at all stages of deformation and the structural relations demonstrate mutual feedback between melt localization and fold formation. F2 folds have shallowly E-plunging fold axis parallel to a stretching lineation defined by high-grade mineral aggregates. Both constitute prominent structures of the basal shear zone. F2-folds are associated with an axial planar fabric (S2), defined by upper-amphibolite- and locally granulite-facies mineral assemblages. D2 shear structures are associated with top-to-the-east kinematic indicators throughout the section. The D1 episode was responsible for emplacement of the eclogite-nappe into its present structural position. The subsequently developed lineation-parallel folds are interpreted to form by general shear, where the structures reflect the regional E-directed flow of the entire eclogite-nappe.