Foliations and shear sense: A modern approach to an old problem |
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| Authors: | T H Bell B E Hobbs |
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| Institution: | 1.School of Earth and Environmental Sciences,James Cook University,Townsville,Australia;2.CSIRO Exploration and Mining,Bentley,Australia |
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| Abstract: | Argument about shear on foliations began in the mid 19th century and continues to the present day. It results from varying interpretations of what takes place during the development
of different types of foliations ranging from slaty cleavages through differentiated crenulation cleavages, schistosity and
gneissosity to mylonites. Computer modelling, quantitative microstructural work and monazite dating have provided a unique
solution through access to the history of foliation development preserved by porphyroblasts. All foliations involve shear
in their development and most can be used to derive a shear sense. The shear sense obtained is consistent between foliation
types and accords with recent computer modelling of these structures preserved within porphyroblasts relative to those in
the matrix. The asymmetry of curving foliation into a locally developing new one allows determination of the shear sense along
the latter foliation in most rocks. The problem of shear on fold limbs and parallelism of foliation and the flattening plane
of the strain ellipse is resolved through the partitioning of shearing and shortening components of deformation into zones
that anastomose around ellipsoidal domains lying parallel to the XY plane. Conflicts in shear sense occur if multiple reuse
or reactivation of foliations is not recognized and allowed for but are readily resolved if taken into account. |
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