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Classification of fault breccias and related fault rocks

Published online by Cambridge University Press:  02 April 2008

N. H. WOODCOCK  and
K. MORT
N. H. WOODCOCK*
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
K. MORT
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
*
*Author for correspondence: nhw1@esc.cam.ac.uk
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Abstract

Despite extensive research on fault rocks, and on their commercial importance, there is no non-genetic classification of fault breccias that can easily be applied in the field. The present criterion for recognizing fault breccia as having no ‘primary cohesion’ is often difficult to assess. Instead we propose that fault breccia should be defined, as with sedimentary breccia, primarily by grain size: with at least 30% of its volume comprising clasts at least 2 mm in diameter. To subdivide fault breccias, we advocate the use of textural terms borrowed from the cave-collapse literature – crackle, mosaic and chaotic breccia – with bounds at 75% and 60% clast content. A secondary breccia discriminant, more difficult to apply in the field, is the ratio of cement to matrix between the clasts. Clast-size issues concerning fault gouge, cataclasite and mylonite are also discussed.

Keywords

brecciagougecataclasite
Type
Rapid Communication
Information
Geological Magazine , Volume 145 , Issue 3 , May 2008 , pp. 435 - 440
Copyright
Copyright © Cambridge University Press 2008

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