Calcite Portland-cement aggregate samples were deformed triaxially at 25E with confining pressures of 200 Mpa. The samples were deformed under experimental approximations of pure shear (dry and wet experimental conditions), transpressional shear and simple shear. The pore fluid pressure during the wet pure shear test was less than 195 MPa. Extensive grain rotation accompanied by twinning of the calcite grains occurred.
Optical analyses of calcite crystallographic fabrics have been used to infer the orientation of the maximum principal compressive stress (F1). Stress orientations in the deformed specimens agree well with the externally imposed stresses. A new method has been successfully used to determine the F1 orientation. The method used contouring of the lamellae index associated with the compression direction determined from Turner's Dynamic analysis method.
In pure shear, preferred dimensional orientation (PDO) of the calcite grains are produced more efficiently in the presence of a pore fluid pressure. In dry specimens, transpressional shear is more effective in producing a PDO in the calcite grain than either pure shear or simple shear. Grain shape fabrics do not conform to the symmetry of the bulk deformation when extensive rotation of calcite grains is involved. Mean grain alignment is perpendicular to the shortening in pure shear, initially inclined and later parallel to the shear zone wall in transpressional shear, and inclined to the shear zone wall in simple shear. The mean orientation of the grain-alignment fabrics is, therefore, a reliable kinematic indicator under the conditions investigated. Transpressional shear and dry pure shear exhibit higher lamellae indices than either wet pure shear or simple shear.
Strain analysis of calcite grains by Robin's method (1977), the linearization method (Yu and Zheng, 1984) and Harmonic mean method (Lisle, 1977) yields overestimates of the experimental bulk strain in wet pure shear. These methods fail to take into account interparticle motions that occur in the presence of a high pore fluid pressure.
The triaxial deformation of the Ancaster oolitic limestone was performed with a confining pressure of 200 Mpa, a natural strain rate of 10-5/s and at a temperature of 135EC. The samples were deformed under dry and wet experimental conditions. The pore fluid pressure, during the wet test, was less than 60% of the confining pressure.
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