Very high anisotropies of the magnetic susceptibility in ductile shear zones: first quantitative results from a metamorphic nappe in the Central Scandinavian Caledonides, Sweden

Abstract

Shear zones in the Seve crystalline basement nappes of the central Scandinavian Caledonides contributed to exhumation and translation of these highgrade metamorphic rocks. The Seve unit is considered to represent the former distal passive margin of the continent Baltica, which was subducted beneath an island arc during Ordovician times and subsequently collided with the continent Laurentia during Silurian and early Devonian times. Strongly textured mylonitic garnet mica schists with well developed mica fish and S–C-fabrics from a shear zone within the Seve unit show unusual high anisotropies of the magnetic susceptibility (AMS). The corrected degrees of anisotropy of the magnetic susceptibility (P0) range from 1.78 to 4.24. Bulk susceptibilities range from 2.8 × 10−3 to 96.9 × 10−3. The shape factors range from 0.32 to 0.62, documenting an oblate shape. Magnetic foliation is subparallel with metamorphic foliation. Magnetic lineation scatters due to permutations of the maximum and intermediate principal susceptibility axes...