DEFORMATION OF YULE MARBLE. PART VII: DEVELOPMENT OF ORIENTED FABRICS AT 300°C–500°C
Yule marble has been deformed at 400° and 500° C and a confining pressure of 5000 atmospheres. Its strength, relative to that at room temperature, is 47 per cent at 400° C and 41 per cent at 500° C, measured in all cases at 3 per cent strain. Textures of the deformed rock are similar to those of marble comparably deformed at 300° C.

Sander's method of “axial-distribution analysis” has been used to test homogeneity of fabric in the original marble and in a specimen shortened 19 per cent at 300° C. In both cases aggregates of a few tens of grains show local very strong preferred orientation of the lattice. In this respect there is no effect that can be correlated with deformation.

Petrofabric analysis of specimens shortened 40 per cent or elongated 90–120 per cent demonstrates strong preferred orientation of the c axis at 10°–30° to the axis of compression or at 60°–80° to the axis of extension. The symmetry of the deformed fabrics corresponds with the symmetry of strain, which is determined by the orientation of the axis of extension or compression in relation to the preferred orientation pattern of the original fabric.

Behavior of grains is predicted on the basis of a hypothesis of approximately homogeneous deformation, involving twin gliding on ![Graphic][1] and translation gliding on ![Graphic][2] . Behavior of individual grains is also reconstructed from evidence of observed internal rotation of early e lamellae, interpreted in the light of previous observations on single crystals of calcite. The same data are used to compute shortening or elongation of individual grains parallel to the axis of applied force. The behavior and strain so deduced and the details of observed preferred-orientation patterns agree with prediction. It should prove possible to compute strain in geologically deformed rocks on the basis of petrographically measured effects of internal rotation. However, at present this can be attempted only for those few minerals, such as calcite and dolomite, whose glide systems have been ascertained experimentally. Possible analogy between a natural marble fabric and a fabric resulting from experimental compression is noted.

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