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

Small-scale ductile shear zones are well developed in amphibolites of Caled onian 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 t he shear strain. Large volume losses are, however, incompatible with geoche mical analysis, which shows little chemical variation across the shear zone s. By including data from xy and yz sections, information on the three-dime nsional state of strain can be obtained. These data indicate that deformati on is not confined to a two-dimensional plane-particle extension within the shear zone, parallel to the shear plane and perpendicular to the shear dir ection 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 w ith simple shearing and volume loss. End member models correspond to plane strain with volume loss (k(x) = k(r) = 1; k(z) < 1), 'transpression' (k(x) = 1; k(y) = 1/k(z)), and pure shear (k(x) = 1/k(z); k(y) = 1). These models produce identical strain ratios and orientations on the xz section, but di ffer in stretch in y and in area of the strain ellipse on the xz section. E xtension in y or x allows for reconciliation of the geometrical and geochem ical data for part of an individual shear zone, but poses a problem of stra in compatibility at the edges and terminations of the shear zones. The shea r zones show a complex anastomosing pattern in three dimensions, and their connectivity provides a way in which strain compatibility is maintained. (C ) 2001 Elsevier Science Ltd. All rights reserved.