THE SVECONORWEGIAN SHEAR-ZONE NETWORK OF SW SWEDEN IN RELATION TO MIDPROTEROZOIC PLATE MOVEMENTS

The Mid-Proterozoic Sveconorwegian orogenic province of Scandinavia, usually correlated with the Grenville province of North America, is divided into several sub-provinces separated by major shear zones. In the Swedish part of the province, three sub-provinces, from west to east, the Western, Median and Eastern segments, are separated by, respectively, the Dalsland boundary thrust/Gota Alv zone and the Mylonite zone. These shear zones cut a mainly gneissose 1.7-1.2 Ga basement and c.1.1 Ga supracrustal cover (the Dal group). Detailed mapping in the Western and Median segments has shown that instead of one major zone separating the two segments, there is a complex branching network of zones on which the major displacement appears to be transferred onto successively more easterly zones from north to south. The zones in the west are mainly steep with strike-slip displacements, whereas further east, they are moderately or gently inclined with dip-slip displacements. Both types are folded in younger Sveconorwegian deformations. Early SE-directed thrust movements on the major Lerdal, Gota Alv and Mylonite zones were succeeded by E-directed thrust movements, particularly on the Dalsland boundary and Mylonite zones, and on numerous minor zones. In the west, sinistral strike-slip movements took place on N-S steep zones and dextral on NE-SW steep zones. It is suggested that oblique NW-SE convergence between Scandinavia and North America during Sveconorwegian collision produced the early SE-directed movements and was later partitioned into sinistral strike-slip in the west and orthogonal E-W convergence further east. This kinematic pattern is explained with reference to the paleomagnetically determined rotation of Scandinavia with respect to North America between c. 1200 and 1100 Ma. During the main Sveconorwegian collision at c. 1100 Ma, the geometry of the restoration implies that orthogonal convergence in the Grenville province should be accompanied by oblique convergence in SW Sweden. Subsequent movements may have involved sinistral transpression across SW Sweden.