CRUSTAL UNITS AND ROLE OF THE MYLONITE ZONE SYSTEM IN THE VARBERG-HORRED REGION, SW SWEDEN

Late Gothian (c. 1.58 Ga) and Sveconorwegian (1.1-0.9 Ga) structures o utline a 35 km long, NNE-oriented, open gneiss synform in the Varberg- Horred region of SW Sweden. This is a region of the South west Scandin avian Domain, within which a major shear zone and tec tonic 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 met amorphism to the east. The evolution is char acterised by one or more major Gothian gneiss-forming events, followed by intermittent anorogen ic magmatism and a later Sveconorwegian development that, outside disc rete 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 be tween 1.51 and 1.20 Ga are correlated across the Mylonite Zone, thus s upporting models where the Mylonite Zone constitutes an intracratonic Sveconorwegian shear zone. The Sveconorwegian development is interpret ed to include eastward thrusting on the Mylonite Zone, followed by dom inantly static metamorphism prior to 0.98 Ga, due to the thickened cru st. Subsequent uplift and rapid cooling preserved granulite-facies ass emblages in the southern Eastern Segment. Late Sveconorwegian extensio nal movements occurred until c. 0.92 Ga along the largely west-dipping Mylonite Zone system.