Kinematics of a major fan-like structure in the eastern part of the Sveconorwegian orogen, Baltic Shield, south-central Sweden

Abstract

Abstract The N-S-trending so-called Protogine Zone in the Baltic Shield of south-central Sweden is usually considered to mark a tectonic boundary between the rocks of the Transscandinavian Igneous Belt (TIB) in the east and the Sveconorwegian orogen in the west. Detailed structural mapping in the Karlskoga-Kristinehamn area has shown that an anastomosing network of ductile deformation zones with generally N-S strike extends ca. 40 km east of the traditional “Protogine Zone”. Furthermore, the western boundary of this ductile deformation and the TIB is not constrained in the Kristinehamn area. Reconnaissance studies indicate that they both extend westwards towards the so-called Mylonite Zone. It is suggested that the eastern limit of the Sveconorwegian orogen is located some 40 km east of the present boundary and that the “Protogine Zone” concept is obsolete. The term Sveconorwegian Frontal Deformation Zone (SFDZ) is proposed as a more appropriate alternative in southern Sweden (south of lake Vattern) and to correspond to a younger set of oblique ductile deformation zones with reverse and right-lateral components of movement in the easternmost part of the orogen farther to the north. Ductile deformation zones older than the SFDZ in the Karlskoga-Kristinehamn area display a fan-like geometry in an E-W cross-section, with steep westerly dips in the eastern part of the section, vertical dips farther west and moderate easterly dips in the western part of the section. Kinematic analysis indicates that dip-slip movements predominate with a consistent top-to-the-east sense of movement across the entire fan-like structure. In their present orientation, deformation zones are characterized by reverse movements in the eastern part and normal movements in the western part of the structure. Between Kristinehamn and the Mylonite Zone, the main foliation is gently dipping to subhorizontal, indicating that the regional structure is strongly asymmetric, and that the fan-like structure occurs close to the foreland of the orogen. Deformation zones are spaced to semi-penetrative in the eastern part of the fan-like structure, whereas the deformation is more or less penetrative and the TIB rocks are transformed to orthogneisses west of Kristinehamn. This east to west increase in bulk strain is in accordance with an increase in syn-deformational metamorphic grade across the structure. Younger ductile deformation zones belonging to the SFDZ are responsible for a major change in orientation of the older deformation zones in the easternmost part of the structure. The fan-like structure is best explained by models involving the interference of two separate tectonic events. Deformation occurred after ∼ 1.57 Ga and prior to deposition of Neoproterozoic and younger cover sedimentary rocks. It is not yet clear whether the initial phase of deformation (early Sveconorwegian or older) was related to the build-up of an imbricate thrust stack in a compressional regime, as favoured here, or to regional E-W extension. The younger deformation phase was related to rotation of these older structures into the compressional, late Sveconorwegian SFDZ.