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The Swedish Nuclear Fuel and Waste Management Company (SKB) has recently completed investigations in the Forsmark and Laxemar-Simpevarp areas in the Fennoscandian Shield of central and south-eastern Sweden, respectively. These investigations have aimed to locate and to characterize a possible repository site for the disposal of highly radioactive nuclear waste. In the context of these investigations, anisotropy of magnetic susceptibility (AMS) measurements have been carried out on samples from both areas. The aim of the AMS study was to provide independent, complementary data to standard structural geological data from both the field and from drill cores, as a support to the 3D structural modelling work.
The bedrock in the Forsmark area has been affected by penetrative ductile deformation under amphibolite facies conditions. The AMS study (529 specimens at 134 locations) was focussed on a tectonic lens with NW-SE strike, which mainly consists of metagranite, and a banded complex of metagranitoids and metavolcanic rocks, immediately to the south-west of the lens. This is in contrast with the Laxemar-Simpevarp area (499 specimens at 111 locations), where the bedrock is dominated by well-preserved, 1.80 Ga granite to quartz monzodiorite, which only displays a weak foliation. However, ductile to brittle-ductile deformation zones at different scales are conspicuous.
At Forsmark, the AMS data provided complementary information bearing on the contrasting style and degree of ductile strain within and immediately outside the tectonic lens. Inside the tectonic lens, the rocks are folded, and the inferred fold axis (132º/43º) is parallel to the magnetic lineation. Constrictional deformation with extension parallel to the fold axis is apparent. Stronger and more oblate ductile strain characterizes the subarea with banded and heterogeneous rocks to the south-west of the lens. These results are in excellent agreement with the structural geological field data.
In the Laxemar-Simpevarp area, the degree of anisotropy is generally low. The magnetic foliation shows a variable orientation and the magnetic lineation plunges gently to the north-west. These data are in good agreement with the structural geological data. The AMS fabric is inferred to have developed at a late stage during the magmatic evolution, due to the effects of the regional stress field that prevailed during and after emplacement of the rocks. The AMS data from samples collected across a regional ductile deformation zone with NE-SW strike indicate a sinistral strike-slip component of movement, which is in agreement with independent kinematic data from field and microstructural studies. Furthermore, the AMS data from well outside the shear zone indicate rotation of the magnetic foliation in rocks that do not show any optical sign of ductile strain. This implies the existence of a c. 250 m wide transition zone outside the deformation zone, which is c. 300 m thick inside the investigated section.
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