Perpendicular Linear Fabrics in Granite: Markers of Combined Simple Shear and Pure Shear Flows?

Abstract

Recent results of numerical modelling of crystallographic fabrics of biotite and feldspar found in the porphyritic granite of the borehole EPS-1 at Soultz-sous-Foret, France, are described and interpreted. Preferred orientations of biotite (001) planes and feldspar (010) and (100) cleavages, were measured by means of an optical goniometer. The poles of (001) biotites show stable orientations throughout the core with maximum eigenvector v oriented vertically, and minimum eigenvector v3, or biotite-lineation, constantly oriented in the NE-SW direction. Within the subhorizontal magmatic foliation plane, also defined by feldspar (010) planes, two mutually orthogonal linear fabrics were found to be NE and NW trending, defined by the feldspar long axes. Variations in biotite fabric intensity and symmetry in different sections of the vertical profile were used to determine a plausible flow geometry in these sections. Numerical modelling involving a component of coaxial flattening, with various amounts of extension parallel and perpendicular to stretch, in addition to simple shearing, agrees qualitatively with the observed data. The models demonstrate that, under combined coaxial and simple shear flow regimes, mutually orthogonal linear fabrics may result from progressive rotation of the shortest crystals.