Triclinic transpression zones with inclined extrusion

Abstract

Data from natural transpressional shear zones reveal a large range of lineation orientations that can spread along complete girdles. One possible way to account for such a type of fabrics is variation in the extrusion direction. This work presents a new theoretical model of oblique transpression with inclined extrusion. The model is kinematic and the flow is considered as homogeneous and steady. The angle ζ between the simple shear direction and the extrusion direction is one of the main parameters controlling the final fabric. Low values of ζ (<40°) yield plane-strain ellipsoids, while high ζ values (>40°) can generate strain ellipsoids very close to the abscissa axis of the Flinn plot. Depending on the vorticity of the flow, the finite strain and the actual value of the ζ angle, the model predicts the generation of lineation fabrics ranging from point maxima to patterns with opposite plunge senses. These patterns can display monoclinic, nearly monoclinic or clearly triclinic symmetry. Lineation patterns spreading along complete girdles can be obtained by allowing the extrusion direction to deviate less than 30° about the vertical in either direction.