| dc.description.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. | |
