Strain analysis of heterogeneous ductile shear zones based on the attitudes of planar markers

Abstract

Using the simple measurement of the attitude of planar features such as foliation, shear planes and displaced tabular markers (such as veins or dykes), fundamental information is obtained on deformation types and rheology in heterogeneous ductile shear zones. The method outlined in this study is based on the subdivision of the shear zone in n deformed layers, each of them being characterized by approximately homogenous deformation. For each layer, paired measurements of θ′ and Γ are made, where θ′ is the angle that the foliation forms with the shear plane and Γ is the effective shear strain. By means of suitable θ′- Γ grids, constructed according to strain boundary conditions, values of stretch ( k), shear strain ( γ), strain ratio ( R) and kinematic vorticity number ( W k) are calculated. The method has been tested on three ductile wrench zones exposed in a deformed granitoid pluton in the Eastern Alps (Italy). The θ′- Γ data indicate that the shear zones are of dominantly transtensional or transpressional type. Furthermore, the finite shear strain and strain ratio peaked profiles suggest that in all instances shear zone evolution was characterized by strain softening. Analysis of the kinematic vorticity indicates that strain softening essentially affected the simple shear component of the deformation.