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Classification of fault breccias and related fault rocks

Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

^* Author for correspondence: nhw1{at}esc.cam.ac.uk

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.

Key Words: breccia • gouge • cataclasite

This article has been cited by other articles:

				N.H. WOODCOCK, N.J. SAYERS, and J.A.D. DICKSON Fluid flow history from damage zone cements near the Dent and Rawthey faults, NW England Journal of the Geological Society, July 1, 2008; 165(4): 829 - 837. [Abstract] [Full Text] [PDF]