NJG74-4-01
1994
Structural evolution of the Precambrian Bjerkreim-Sokndal intrusion, South Norway
74
4
pp. 185-198

The structures of the northwestern part of the Precambrian Bjerkreim-Sokndal intrusion are studied from micro- to macro-scale. Meso-scale structures are dominated by (1) modal layering of igneous origin (S0), and (2) prominent linear and planar fabrics (L1/S1 ), defined by the shape-preferred orientation of minerals and mineral aggregates. In most places S0 and S1 are parallel, and together they define an isoclinal syncline with a southeastwards plunging fold axis. The linear fabric (L1) is subparallel to the fold axis in the hinge zone and converges towards this zone in the limbs. The intensity and type of fabric vary across the fold. In the binge zone the L-fabric is dominant, whereas the rocks in the limbs display a combined S- and L-fabric. The intensity of the fabric is closely tied with microstructural indications of plastic deformation, such as lattice hending and kinking, polygonization and recrystallization. All of this points to one major phase of solid state deformation. Estimates of the strain state are based on regional variations in the shape of olivine grains and aggregates from two igneous units. Extensions of up to about 100% are observed in the hinge zone, and flattening around 40% is observed in the southern limb. Rheological considerations suggest that these are minimum values. In our interpretation, the deformation was the result of passive subsidence of the intrusive rocks, induced by gravitational instabilities in the area (inverse diapirism). A convergent flow pattern led to the formation of L-tectonites in the hinge zone and an additional shear component along the limbs led to the formation of L/S-tectonites. A series of monzonoritic dykes (the Lomland dykes) and a number of minor ·shear zones all post-date the major deformation. The dykes and shear zones may be coeval.

Johanne Paludan, Ulla B. Hansen & Niels Ø. Olesen, Department of Earth Sciences, Aarhus University, C. F. Møllers Allé, DK-8000 Århus C, Denmark.

0029-196X