Cover/basement relationships in the SW Swedish gneisses near Varberg

Abstract

The tectonic history of the rocks of the Varberg group SE of the Mylonite Zone near Varberg in SW Sweden is longer and more complicated than that of the Bua group to the NW. Two phases of folding and an early migmatisation occurred before parts of the Varberg group melted to form the Varberg Igneous Suit during the c. 1.4 to 1.5 Ga Hallandian orogeny. Rocks of charnockitic character remained at depth amongst the pyroxene granulites of the Varberg group while more buoyant melts rose as the Torpa granite into the Bua group which was then migmatised for the first time during the development of large recumbent folds facing west. Magmatism occurred in the Varberg area at c. 1.4 Ga when overthrusting was to the west, but had migrated westward by 1.25 Ga when the Mylonite Zone involved overthrusting to the southeast. Still later (c. 1.1 Ga) NNW-trending steep transextension accompanied uplift and erosion of the unconformity on which the Visingsö sediments were deposited. Whether the Bua group was deposited or thrust onto the Varberg group is obscured by the intrusion of an intervening sheet of Torpa granite and, still later, shears. The mutual contact between the two groups originated as either a simple unconformity or a terrane boundary, but both cases can be referred to as a cover/basement relationship. The simpler, unconformable, relationship is assumed in a scenario interpreting the two main phases of sedimentation, two or three suites of igneous intrusion, and two migmatisations in the region, in terms of three docking of a succession of microcontinents onto the westward margin of Svecofennia.

Key Words:

• Bua group
• Varberg group
• Varberg Igneous Suite
• Gothian
• Hallandian
• Sveconorwegian
• Sweden