A 3D study of aligned porphyroblast inclusion trails across shear zones and folds

Inclusion trails (Si) within 741 equant garnet porphyroblasts show a consistent orientation across a 100 m2 outcrop of Precambrian metapelite with later (D2) folds and shear zones in Posio, Finland. Variation in Si orientations is not systematic with respect to these later folds and shear zones, but instead represents microfolding and warping of S1 before the formation of D2 structures. The development of the D2 folds and shear zones is investigated by comparing the pattern of Si in the outcrop to those predicted for porphyroblasts in tangential longitudinal strain, axial slip, flexural flow, pure shear, partitioned flexural flow and partitioned pure shear folds. Si patterns are also compared with models for non-coaxial and coaxial shear zones. Along with corroborating evidence from structures in the outcrop and matrix, the comparisons indicate that D2 folds and shear zones in the outcrop formed in response to NW–SE axially symmetric shortening during which garnet porphyroblasts did not rotate relative to each other, the axial plane of F2 folds, and D2 shear zone boundaries.