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Abstract
Late Gothian (c. 1.58 Ga) and Sveconorwegian (1.1–0.9 Ga) structures outline a 35 km long, NNE‐oriented, open gneiss synform in the Varberg‐Horred region of SW Sweden. This is a region of the Southwest Scandinavian Domain, within which a major shear zone and tectonic boundary, the Mylonite Zone, forms a branching shear zone system which converges in the eastern part of the synform.
A subdivision between the Gothian and Sveconorwegian events is made by using the intervening anorogenic intrusions as structural markers. This, and the non‐recognition of a previously assumed orogenic event, results in a geodynamic model which is similar for the crustal segments on both sides of the largely N‐S trending Mylonite Zone, except for the higher grade Sveconorwegian metamorphism to the east. The evolution is characterised by one or more major Gothian gneiss‐forming events, followed by intermittent anorogenic magmatism and a later Sveconorwegian development that, outside discrete shear zones, gave rise to moderate fabric‐forming deformation and only localised formation of migmatitic leucosomes.
The final Gothian orogenic episode at c. 1.58 Ga and three distinct anorogenic events between 1.51 and 1.20 Ga are correlated across the Mylonite Zone, thus supporting models where the Mylonite Zone constitutes an intracratonic Sveconorwegian shear zone. The Sveconorwegian development is interpreted to include eastward thrusting on the Mylonite Zone, followed by dominantly static metamorphism prior to 0.98 Ga, due to the thickened crust. Subsequent uplift and rapid cooling preserved granulite‐facies assemblages in the southern Eastern Segment. Late Sveconorwegian extensional movements occurred until c. 0.92 Ga along the largely west‐dipping Mylonite Zone system.
Åhäll, K.‐L, 1995: Crustal units and role of the Mylonite Zone system in the Varberg‐Horred region, SW Sweden. GFF, Vol. 117 (Pt. 4, December), pp. 185–198. Stockholm. ISSN 1103–5897.
