GARNET PORPHYOBLASTS AS MARKERS FOR STRAIN PARTITIONING IN THE HANGING-WALL OF A MAJOR OUT-OF-SEQUENCE THRUST

Within the stack of Caledonian crystalline thrust sheets of northern Scandinavia, a single amphibolite facies lithotectonic unit, the Småtinden nappe, is identified as a major, basement-coupled (“stretching”) shear zone. This dominantly pelitic unit achieved peak metamorphic conditions of 535-550°C and 8-9kbars, and the stretching geometry suggests that this most likely occurred in response to overthrusting of a hot, pre-assembled Caledonian thrust stack. Porphyroblast-fabric relationships in the shear zone, especially involving garnet, show marked variations along strike and indicate the role of strain partitioning during late-stage decoupling of the Småtinden unit from the basement.

Southern sections through the shear zone are characterised by relatively late, static growth preserving concordant S_i-S_e relationships, and typically symmetrical external fabrics consistent with formation under dominantly pure shear conditions. The north of the area is characterised by early garnet growth, with discordant S_i-S_e relationships and asymmetric external fabric geometries consistent with formation during ESE-directed simple shear. Further, geometrical relationships between internal (S_i) and external (S_e) fabric domains indicate that the curved inclusion trails characteristic of the Småtinden nappe represent overgrowths of ESE-vergent microfolds, formed during bulk ESE-directed non-coaxial deformation. This model is supported by chemical zonation patterns within garnet porphyroblasts which are consistent with nucleation controlled by local dilation along the short limbs of evolving asymmetric crenulations.