THREE-DIMENSIONAL NUMERICAL MODELING OF LATTICE- AND SHAPE-PREFERRED ORIENTATION OF ORTHOPYROXENE PORPHYROCLASTS IN PERIDOTITES

Abstract

Orthopyroxene porphyroclasts in peridotite mylonites are elongated to variable extents depending on the crystallographic orientation. This paper presents a 3D numerical model of the development of lattice- and shape-preferred orientations of orthopyroxene porphyroclasts in a fine-grained olivine matrix for matrix deformations of simple shear, pure shear and uniaxial shortening. In this model, orthopyroxene grains are assumed to deform by dislocation glide on a unique slip system (100)[001] with additional rigid-body rotation. This rotation and deformation is determined so as to minimize the difference in displacement between the matrix and grains for an imposed incremental matrix deformation. The lattice- and shape-preferred orientations of orthopyroxene porphyroclasts in three mutually orthogonal sections of a peridotite mylonite sample are consistent with the simulation results for simple shear. The proposed model can be regarded as an extension of the rigid inclusion model to an anisotropically deformable model, and may represent a mechanism for the development of strong shape-preferred orientation with antithetic inclination from the shear direction. The proposed 3D model accounts for both lattice- and shape-preferred orientation, and may therefore be applicable to identifying finite strain geometry and the orientation of vorticity vectors independently in natural shear zones.