Strain and vorticity patterns in ideally ductile transpression zones

Abstract

The prevalent model for ductile shear zones assumes that they develop by progressive simple shearing, resulting in a monoclinic fabric in which the vorticity vector is parallel to the shear zone and perpendicular to the lineation. But some ductile shear zones exhibit an amount of coaxial flattening, or a fabric pattern which appear to be incompatible with the assumptions of plane strain and progressive simple shear. In certain sections of the Archean Larder Lake—Cadillac deformation zone (LCDZ), for example, vorticity indicators (asymmetric pressure wings, Z-folds, SC fabrics), best seen on horizontal surfaces, indicate dextral transcurrent motion, whereas stretching lineations have variable but steep plunges. In the Proterozoic Mylonite Zone (MZ) of south-west Sweden, vorticity indicators combined with foliation and lineation data suggest a continuous change from reverse dip-slip motion close to the footwall to sinistral transcurrent motion adjacent to the hangingwall of the zone. Such departures from the ideal progressive simple shear zone pattern may in fact be common. Rather than invoke two stages of deformation, we explore the possibility that these patterns could be the result of ductile transpression. Ductile transpression between relatively rigid walls implies an extrusion of material out of the shear zone. When the material cannot slip freely along the boundaries of the zone, the extrusion strain is by necessity heterogeneous. In order to explore these heterogeneous strain distributions, we have developed a continuum mechanics model in which the ‘transpressed’ rock is a linear viscous material squeezed upward between two parallel, rigid, vertical walls. Transpression is further generalized by modelling oblique (i.e. with a dip-slip component) relative displacements of the walls. Models, which can vary in their obliquity and their ‘press’/‘trans’ ratio, are examined for their distributions of K-values, strain rate intensity, ‘lineation’ (direction of maximum principal strain rate), ‘foliation’ (plane perpendicular to the direction of minimum principal strain rate) and vorticity. To quantify the expected petrographic effect of the vorticity when the strain path has triclinic symmetry, we introduce a sectional kinematic vorticity number, W s k. The model predicts ‘foliations’ and ‘lineations’ which vary in orientation and intensity across the zone. In some model zones, the vorticity vector can be nearly parallel to the ‘foliation’ and perpendicular to the ‘lineation’, as expected in progressive simple shear, but it can also be locally nearly parallel to the ‘lineation’, as in the LCDZ. Commonly, however, the vorticity vector is not parallel to any of the principal directions of instantaneous strain, and the deformation has triclinic symmetry. The pattern of foliations and lineations in the MZ can readily be matched to that in an oblique transpression model zone.