Strain in ductile shear zones in the Caledonides of northern Sweden: a three-dimensional puzzle

Abstract

Small-scale ductile shear zones are well developed in amphibolites of Caledonian nappes of the Swedish Lapland. Strain in the shear zones is recorded by the shapes of originally sub-equant aggregates of plagioclase feldspar. Classical strain analysis on two-dimensional (xz) profiles across the shear zones leads to results that indicate large volume losses associated with the shear strain. Large volume losses are, however, incompatible with geochemical analysis, which shows little chemical variation across the shear zones. By including data from xy and yz sections, information on the three-dimensional state of strain can be obtained. These data indicate that deformation is not confined to a two-dimensional plane—particle extension within the shear zone, parallel to the shear plane and perpendicular to the shear direction has also occurred. The state of strain and strain path in the shear zone can be modeled in a general way by combining stretches in x, y and z with simple shearing and volume loss. End member models correspond to plane strain with volume loss (kX=kY=1; kZ<1), ‘transpression’ (kX=1; kY=1/kZ), and pure shear (kX=1/kZ; kY=1). These models produce identical strain ratios and orientations on the xz section, but differ in stretch in y and in area of the strain ellipse on the xz section. Extension in y or x allows for reconciliation of the geometrical and geochemical data for part of an individual shear zone, but poses a problem of strain compatibility at the edges and terminations of the shear zones. The shear zones show a complex anastomosing pattern in three dimensions, and their connectivity provides a way in which strain compatibility is maintained.