A theory of finite strain variation through contrasting layers, and its bearing on cleavage refraction

doi:10.1016/0191-8141(83)90023-8

Journal of Structural Geology

Volume 5, Issues 3–4, 1983, Pages 351-368

https://doi.org/10.1016/0191-8141(83)90023-8 Get rights and content

Abstract

A theory of finite strain variation in contrasting viscous layers is presented. The theory is applicable to layers which are oblique to two principal strains, but parallel to the third, and is not restricted to plane strain. Results may be obtained algebraically or by use of the Mohr diagram for strain. Solutions are given for various examples of layering attitude and viscosity ratio, measured with respect to a reference layer. It is shown that the finite strain ellipsoid changes in shape and orientation across contrasting layers, and in some cases the principal axes may be exchanged. Low strain is indicated in relatively more viscous layers, and high strain with extension at a smaller angle to the layering, in less viscous layers; the latter may approximate simple shear parallel to layering if the strain is sufficiently high.

The geological implications of the theory are strong strain variations in layered rock sequences. It is suggested that, in general, strain will be inhomogeneous from layer to layer both in orientation and amount. The relationship of strain and cleavage are reviewed in the Introduction to establish the validity of predicting cleavage patterns from strain data. Qualitative comparisons between the theoretical results of strain variation and natural cleavage refraction in layered rocks would appear to justify the assumption that cleavages of varied morphologies are sub parallel to the XY planes of strain. With this assumption, some three-dimensional features of planar and linear fabric refraction in contrasting rocks are predicted.

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