Dimensional and crystallographic fabric development in experimentally deformed synthetic aggregate and natural rocks

Abstract

Calcite Portland-cement aggregate samples were deformed triaxially at 25 deg. 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. Stress orientations in the deformed specimens agree well with the externally imposed stresses. A new method has been successfully used to determine the a, orientation. The method uses 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 preformed with a confining pressure of 200 Mpa, a natural strain rate of 10-5/s and at a temperature of 135°C. 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. The deformation process of ooids in the dry experimental test is rigid rotation of the ooid particles. In the case of wet experimental conditions, it appears that the pore fluid pressure produces particulate flow in the fine grained ooid matrix. Due to a viscosity contrast, between ooids and cement matrix, strain analysis on the ooids exhibits an overestimate of strain compared to the experimental bulk strain. This holds true for both wet and dry experimental conditions. Experimental triaxial deformation was conducted on the China Beach sandstone by pure shear for dry experimental conditions. The temperature was held constant at 25°C, with computer controlled natural strain rates of 10-5/s and a confining pressure of 200 Mpa. Mechanical heterogeneities in the grains of the China Beach sample play an important role in the development of cleavage. Altered feldspar grains and lithic fragments deform by ductile processes, while unaltered feldspar and quartz grain deform by rigid rotation and brittle processes. Strain analysis of each grain type in the China Beach sandstone yield a range of strain estimates depending on the deformation process compared to the experimental bulk strain. Comparison of Robin's method, the linearization method and Harmonic mean method suggest that Robin's method generates the best estimates of the bulk experimental strain ratio.