Abstract

Planar deformation features inclined along planes of high shear stress and along which cataclasis is concentrated are here called “Lüders' bands.” In Coconino Sandstone (deformed dry and with pore pressure at effective confining pressures to 2.4 kb, room temperature, and at a strain rate of 10−4 per sec) the bands begin to develop in the transitional regime and are the major deformation feature in the macroscopically ductile regime. At axial shortenings of 5 percent and more they pervade the specimen and become closer spaced and thicker with increasing strain. The bands are formed by two or more layers of moderately to highly fractured quartz grains. They are markedly different from shear fractures (or faults) of similar size that typically contain quartz gouge. The average angle between conjugate sets of bands bisected by the greatest principal compressive stress σ1 increases with effective confining pressure from 75° to 109°. The corresponding angles between conjugate macroscopic shear fractures average 60°. The angle between Lüders' bands is essentially independent of strain at fixed effective confining pressure.

In Solenhofen Limestone (deformed dry at confining pressures to 3.0 kb, 24°C, and strain rates from 10/sec to 10−4/sec) the Lüders' bands are developed only in the outer shell of the solid cylinders; however, in coarser grained limestones, the bands are pervasive as in the sandstone. In cylinders of Solenhofen, the bands are best developed in the transitional regime, as noted previously by Heard (1960). They do not form in the ductile regime. The average angle between conjugate sets is independent of strain and strain rate; but as for the sandstone, it increases with confining pressure from 75° to 103°, and it is at least 20 degrees larger than the corresponding angle between conjugate shear fractures which form after the Lüders' bands. Optical and SEM studies indicate that the features in both rocks are zones of intergranular and intragranular cataclasis, along which shear displacements are negligible.

Because the angle between conjugate Lüders' bands is a function of the effective pressure, the bands might be used to derive depth of burial at time of deformation, provided that (1) the pore fluid pressure is assumed to be hydrostatic, and (2) the orientation of CTI, which can be the acute or obtuse bisector, is independently known. If (1) is unwarranted, then the angle between the bands could be used to infer the pore fluid pressure, provided the depth of burial is known. This approach is illustrated with reference to an occurrence of Lüders' bands in naturally deformed Entrada Sandstone, Trachyte Mesa, Utah.

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