Foliations and shear sense: a modern approach to an old problem

Bell, T.H., and Hobbs, B.E. (2010) Foliations and shear sense: a modern approach to an old problem. Journal of the Geological Society of India, 75 (1). pp. 137-151.

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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.

Item ID: 8491
Item Type: Article (Research - C1)
ISSN: 0974-6889
Keywords: deformation partitioning; slaty cleavage; crenulation cleavage; Porphyroblasts
Funders: Australian Research Council
Date Deposited: 06 Aug 2010 04:28
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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