Experimental calcite fabrics in a synthetic weaker aggregate by coaxial and non-coaxial deformation

Abstract

Calcite grain aggregates, supported by a weaker, macroscopically ductile, matrix of set Portland cement, were deformed triaxially at room temperature with a confining pressure of 150 MPa and at rates corresponding to a pure shear, natural strain rate of 10 −5 s −1. Extensive grain rotation accompanied the twinning of calcite. Stress orientations inside the specimens, determined from calcite twins, agree well with the stresses imposed upon the specimens. Thus calcite twins are sound kinematic indicators in this study. Various sample configurations were used to simulate pure shear, transpression and simple shear. In pure shear or coaxial strain, preferred dimensional orientations ( PDO) of calcite are produced more efficiently in the presence of high pore fluid pressure. Those grain alignments are stronger than the bulk strain would predict assuming homogeneous strain. Strain is overestimated by methods assuming continuum behaviour because they fail to take into account the extensive interparticle motions that occur in the presence of high pore fluid pressure. Fluid pressure suppresses the twinning of calcite by reducing the intergranular stresses. During deformation, small rapid variations in pore fluid pressure are believed to represent ephemeral dilatations accompanying particulate flow as groups of grains slide past one another. In dry specimens, pure shear is more effective than simple shear in producing a PDO of the calcite grains. Calcite grains are rotated rigidly and somewhat strained to produce L < S alignment fabrics in pure shear but give S-fabrics in transpression and perhaps also in simple shear. Thus grain-shape fabrics do not conform to the symmetry of bulk deformation in this study. Mean grain alignment is perpendicular to shortening in pure shear, initially inclined and later parallel to shear zone walls in transpression, and weak but statistically inclined to shear zone walls in simple shear. The mean orientation of grain-alignment fabrics is therefore a reliable kinematic indicator under the conditions investigated. At comparable strains in dry tests, e-twinning is best developed in transpression, next best developed in coaxial strain and least well shown by simple shear. High fluid pressure considerably reduces the incidence of twinning.