Tectonophysics, 122 (1986) 381-387 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 381 Letter Section On the nomenclature of mode of failure transitions in rocks E.H. RUTTER Rock ~ef~rmat~on La~orato~, department of Geology, Imperial College, London, S. W. 7. (Great Britain) (Received November 22,1985; revised version accepted December 6,1985) ABSTRACT Rutter, E.H., 1986. On the nomenclature of mode of failure transitions in rocks. Tec- tonophysics, 122: 381-387. Attention is drawn to the increasing identification in the geological literature of the concept of ductility with a particular mechanism of rock deformation, namely intra- crystalline plasticity. It is argued here that “ductility” only describes the capacity of a material to deform to a substantial strain without the tendency to localize the flow into bands (faults), and should not have a mechanistic connotation. A simple diagrammatic visualization of the concepts of mode-of-failure transitions is proposed. By force of repetition, the depth in the Earth to the seismic-aseismic transition is being regarded as a brittle-ductile transition in the sense of a brittle-plastic transition. This tendency is criticized and it is suggested that the nomenclature of mode of failure of rocks be rationalized, in the face of growing prospects of imprecision and misunder- standings of the communication of data and ideas. Increasingly in recent articles (e.g. Kligfield et al., 1984; Mitra, 1984; Passchier, 1984; Smith and Bruhn, 1984; Harper, 1985; Simpson, 1985) and conference contributions (e.g. Vann, 1985), reference is being made to “the brittl~uctile transition” in rocks, even to the expression of the view that it is a discernable, mappable surface within the Earth’s crust (Miller et al., 1983). In the writer’s view, the concept is being oversimplified and mis- used by many geologists, to the extent that significant misunderstandings will progressively arise in the communication of data and ideas. On the other hand, some geologists take great care to ensure that their nomen- clature cannot lead to confusion (Vernon, 1974). This note sets out what is essentially a personal view of the concept of “mode of failure transitions” in rocks, but one which I know to be shared by many colleagues. Equally, I know that others will still disagree with the particular point that the concept of “ductility” should not be dependent on mechanism of defor- mation, but should reflect only the capacity for substantial, non-localized strain. If so, urgent attention should be directed to establishing some con- sensus regarding nomenclature in this area of rock deformation studies. 0040-1951/86/$03.50 0 1986 Elsevier Science Publishers B.V.