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On the nomenclature of mode of failure transitions in rocks
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~, S. W. 7. (Great Britain)
department
of Geology,
Imperial
College, London,
(Received November 22,1985;
revised version accepted December 6,1985)
ABSTRACT Rutter, E.H., 1986. On the nomenclature tonophysics, 122: 381-387.
of mode of failure transitions in rocks. Tec-
Attention is drawn to the increasing identification in the geological literature of the concept of ductility with a particular mechanism of rock deformation, namely intracrystalline 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 misunderstandings 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 misused 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 nomenclature 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 deformation, but should reflect only the capacity for substantial, non-localized strain. If so, urgent attention should be directed to establishing some consensus regarding nomenclature in this area of rock deformation studies.
0040-1951/86/$03.50
0 1986 Elsevier Science Publishers B.V.
382
In describing the way in which a rock has been deformed, two concepts are essential elements of the description, deformation mechanism and the degree of ho~oge~~i~~ of the deformation. There are three fundamental deformation mechanisms: (a) Cataclasis, in which crystal structure remains undistorted, but grains or groups of grains become cracked and the fragments may exhibit frictional sliding with respect to one-another. The process necessarily involves dilatancy and is therefore pressure sensitive. (b) Intrac~st~line plasticity, in which grains become internally distorted through dislocation motion or deformation twinning. (c) Flow by diffusive mass transfer, in which shape change of the deforming aggregrate is accomplished by stress induced diffusion of matter away from interfaces sustaining high normal stresses, with the same or different phases being reprecipitat~ at potentially dilatant sites. (b) and (c) can be constant volume processes, hence are pressure insensitive, but both are sensitive to temperature and deformation rate relative to (a). Deformation may be heterogeneous in various ways, but the one of interest here is that due to localization of deformation into a band (a fault or shear zone, in the loose sense) in an otherwise homogeneously deforming (or rigid) body. Each of the fund~en~ deformation mechanisms above can result in localized OR distributed deformation of a rock. Whether localization occurs depends on the rock type and the physical conditions of deformation. It may also be strain dependent, and develop after a certain degree of homogeneous flow. is associated with the formation Everyone will agree that “brittleness” of cracks, and can therefore be defined at the microscopic level. It is essentially a mechanistic concept. In the literature on experimental rock deformation, “ductility” has always been defined as the capacity for more or less uniformly distributed flow (e.g. Paterson, 1978), although there may be significant heterogeneities of strain on the grain scale. It is a concept which must be defined on the macroscopic scale and does not depend on the operative deformation mechanism. In the earliest experimental studies of the “brittle to ductile transition” (e.g. Paterson, 1978; Heard, 1960; Byerlee, 1968), what was observed was the transition from localized cataclasis (brittle faulting) to ductility due to distributed microcracking (cataclastic flow) as confining pressure was increased at constant temperature. Ductility dominated by intracryst~line plasticity, on the other hand, could be obtained by the application of higher confining pressure, or higher temperature, or both. In the geophysical and geological communities at present, what is tending to happen is that “ductility” is being used in a much more restricted way, being identified purely with deformation by intracrystalline plasticity. The term is therefore being made mechanism dependent. Whether the concept of ductility is used in this mechanistic way in metallurgy and materials science is not important, because rocks generally show a much
383
wider spectrum of rheological characteristics than do any engineering materials. To use the term “ductile” in such a way as to imply only the operation of plastic deformation mechanisms, strictly would exclude rocks deformed by cataclastic flow processes from being considered ductile. This is a dangerous tendency. For example, from thin-section observations alone it is possible to misinterpret some natural cataclastic flow textures by attributing them to grain-size reduction by dynamic recrystallization when in fact they are the products of cataclastic granulation followed by cementation by hydrothermal overgrowths (Rutter et al., 1985). At the scale of a single crystal, crack formation implies brittleness but intracrystalline plasticity usually involves the possibility of large, more or less homogeneous distortion of the crystal. Flow by intracrystalline plasticity is almost always associated with macroscopic ductility of the polycrystal, at least over several grain diameters. There may, however, be marked localization of the flow on a larger scale. Rocks deformed in this way tend to be regarded by geologists as “ductile” on the basis of thin section observations alone. Flow of a polycrystal by diffusive mass transfer, however, is heterogeneous even on the grain scale. There may be no significant internal distortion of an individual crystal, but grain boundary sliding accommodated by diffusive redistribution of material may result in the polycrystal exhibiting extreme ductility. Cataclasis without localization can be the dominant process accommodating folding in rocks, especially at high levels in the Earth’s crust. Such cataclastic flow may involve microfracturing of every grain in the rock, or may break the rock into fragments of several cm in dimension. Small movements on the bounding cracks, if homogeneously distributed, are able to accommodate folds (e.g. Stearns, 1969; Hadizadeh and Rutter, 1983). Thus brittleness and ductility can occur together, and the terms should not be used in a mutually exclusive way. Rather, the scale of observation should be indicated, e.g. brittle on the grain scale, macroscopically ductile. Essentially the same point has recently been made by Chester et al. (1985). Figure 1 shows some interrelationships between descriptions of mode of failure, expressed according to dominant deformation mechanism and degree of observed localization of the deformation. For convenience, deformation mechanisms have been separated into only two groups; cataclastic, and plastic in the loose sense, the latter combining flow by dislocation motion and by diffusive mass transfer. The separation is fundamentally into: (a) those mechanisms which result in dilatancy, and are therefore mean stress sensitive in terms of resistance to deformation (cataclasis), and (b) constant volume, pressure insensitive processes. Particular modes of failure then fall into the four main categories shown. Various transitions exist between the modes of failure shown on Fig. 1. Transitions from (1) to (2) or (1) to (3) have been termed brittle to ductile transitions. Other transitions indicated on Fig. 1 depend mainly on strain. Combinations of changes of temperature, confining pressure, pore fluid
384
Fig. 1. Simple diagrammatic representation of mode of failure transitions in rocks, showing how the description of mode of failure depends on the deformation mechanism and whether the strain is localized. Deformation mechanisms are grouped into those involving dilatancy (cataclasis) and those not involving volume change (crystal-plastic in the loose sense). Examples of particular mode of failure transitions are shown together with the main controlling parameters. Transitions between (1) and (4) can be induced through strain rate and/or pore fluid pressure changes.
pressure, strain and strain rate can give rise to a wide range of changes of behaviour. The impression one gains increasingly from current literature is that natural rocks only display transitions of type (1) to (3). Further, it appears to have become fashionable to identify a brittle--ductile transition of this type as characteristic of the Earth’s crust as a whole (e.g. Turcotte, 1983), the transition often being equated with the depth below which shallow earthquakes do not occur (Sibson, 1982). Such a depth is being progressively better defined in places such as central California. It may indeed correspond to the attainment of physical conditions such that the build-up of tectonic stresses, which would otherwise cause seismogenic cataclastic rupture, can be relaxed by plastic flow. To speak of a brittle-plastic transition (purely mechanistic terms) would be more satisfactory in this context. However, processes other than intracrystalline plasticity can also be responsible for such a cutoff effect, yet geophysicists often talk loosely of the “brittle-ductile transition” as if this were a fact of observation directly from geophysical sensing techniques, when they should adhere to the more guarded and factual expression “the base of the shallow seismogenic zone”. Figure 2 shows a data set from rock deformation experiments which indicate the variation in strength and mode of failure of a limestone at a constant strain rate along a constant pressure/temperature gradient corresponding to about 35°C per km depth. It shows the transition from cataelastic faulting to cataclastic flow, which for this material would correspond to the base of the seismogenic layer. This is followed by the upper transi-
385
cataclastic fault~na~ataciastlc
flow tronsltian
f
‘;;
z
5 k
Solnhofen limestone -
z 7100
-
+
Oven dry
A
wet,
Pore water wessure
=
0.1
confining pressure
0
1 0
I
I
100
200
Confining Pressure
300
400
(fiP0)
Fig. 2. Compilation of experimental data (peak stress for specimens which failed by cataclastic faulting, otherwise stress supported at 10% total strain) of Rutter (1972) for Solnhofen limestone tested wet and dry, under conditions of confining pressure increased in step with temperature to simulate natural burial. Strain rate = 10m5 s-l. Note the camelastic faulting (open symbols) to cataclastic flow transition takes place well below the peak strength. The strength peak corresponds to the transition from dominant cataclasis (pressure insensitive, temperature insensitive, d&&ant) to dominant plasticity (pressure insensitive, temperature sensitive, non-di~~nt). Because plasticity is strain rate sensitive, the peak transition also will be rate sensitive, occurrmg at shallower depths at lower strain rates.
tion to flow dominated by constant volume intracrystalline plasticity. The strength peak is caused by the mechanism change from temperature and strain rate insensitive (but pressure sensitive) cataclastic deformation, to temperature and strain rate sensitive (but pressure insensitive) intracrystalline plasticity. However, the development of the strength peak has nothing to do with the macroscopic mode of failure of the rock. I do not suggest that this limestone is particularly representative of the mechanical behaviour of the continental crust, but its behaviour does indicate that while the shape of the strength curve for a rock might be predicted from constitutive flow laws for the rock (e.g. Goetze and Evans, 1979; Sibson, 1983) the mode of failure might not simply correspond.
386
In conclusion I would suggest the avoidance of the imprecise and potentially misleading expression “brittle to ductile transition” or any expression in which the two words are used together, so that when one is dealing with mechanism changes (which can be perceived on the scale of a thin section), one uses expressions like “brittle to plastic” or “cataclastic-plastic”. Description of other aspects of transitions in modes of failure, in which the mechanism is not a necessary part of the description, can be dealt with using terms like “faulting to flow transition”, or where it is possible to give a complete description, expressions like “cataclastic faulting to plastic flow” or “plastic shear zone to cataclastic faulting” might be employed.
REFERENCES Byerlee, J.D., 1968. The brittle-ductile transition in rocks. J. Geophys. Res., 73: 47414750. Chester, F.M., Friedman, M. and Logan, J.M., 1985. Foliated cataclasites. Tectonophysics, 111: 139-146. Goetze, C. and Evans, B., 1979. Stress and temperature in the bending lithosphere as constrained by experimental rock mechanics. Geophys. J. R. Astron. Sot., 59: 463478. Hadizadeh, J. and Rutter, E.H., 1983. The low temperature brittle-ductile transition in a quartzite and the occurrence of cataclastic flow in nature. Geol. Rundsch., 72: 493-509. Harper, G.D., 1985. Tectonics of slow spreading mid-ocean ridges and consequences of a variable depth to the brittle-ductile transition. Tectonics, 4: 379-394. Heard, H.C., 1960. Transition from brittle fracture to ductile flow in Solnhofen Limestone as a function of temperature, confining pressure and interstitial fluid pressure. Mem. Geol. Sot. Am., 79: 193-226. Kligfield, R., Crespi, J., Naruk, S. and Davis, G.I.H., 1984. Extensional Orogens. Tectonics, 3: 577-609. Miller, E.L., Gans, P.G. and Garing, J., 1983. The Snake Range decollement; an exhumed mid-Tertiary brittle-ductile transition. Tectonics, 2: 239-264. Mitra, G., 1984. Brittle to ductile transition due to large strains along the White Rock thrust, Wind River mountains, Wyoming. J. Struct. Geol., 6: 51-62. Passchier, C.W., 1984. The generation of ductile and brittle shear bands in a low angle mylonite zone. J. Struct. Geol., 6: 273-282. Paterson, M.S., 1978. Experimental Rock Deformation: the Brittle Field. Springer, Berlin. Rutter, E.H., 1972. The influence of interstitial water on the mechanical behaviour of calcite rocks. Tectonophysics, 14 : 13-3 3. Rutter, E.H., Peach, C.J., White, S.H. and Johnston, D., 1985. Experimental ‘syntectonic’ hydration of basalt. J. Struct. Geol., 7 : 251-266. Sibson, R.H., 1982. Fault zone models, heat flow and the depth distribution of earthquakes in the continental crust of the United States. Bull. Seismol. Sot. Am., 72: 151-163. Sibson, R.H., 1983. Continental fault structure and the shallow earthquake source. J. Geol. Sot. London, 140: 141-167. Simpson, C., 1985. Deformation of granitic rocks across the brittle-ductile transition. J. Struct. Geol., 7: 503-512.
381 Smith, R.B. and Bruhn, R.L., 1984. Intra-plate extensional tectonics of the eastern Basin-Range: inferences on structural style from seismic reflection data, regional tectonics and thermal-mechanical models of brittle-ductile deformation. J. Geophys. Res., 89: 5733-5762. Stearns, D.W., 1969. Fracture as a mechanism of flow in naturally deformed, layered rocks. Proc. Conf. on Research in Tectonics. Geol. Survey of Canada, pp. 68-80. Turcotte, D.L., 1983. Mechanisms of crustal deformation. J. Geol. Sot. London, 140: 701-724. Vann, I.R., 1985. To stretch a continent. Nature, 316: 293-294. Vernon, R.H., 1974. Controls of mylonitic compositional layering during non-cataclastic ductile deformation. Geol. Mag., 111: 121-123.
381
Letter Section On the nomenclature of mode of failure transitions in rocks
E.H. RUTTER Rock ~ef~rmat~on La~orato~, S. W. 7. (Great Britain)
department
of Geology,
Imperial
College, London,
(Received November 22,1985;
revised version accepted December 6,1985)
ABSTRACT Rutter, E.H., 1986. On the nomenclature tonophysics, 122: 381-387.
of mode of failure transitions in rocks. Tec-
Attention is drawn to the increasing identification in the geological literature of the concept of ductility with a particular mechanism of rock deformation, namely intracrystalline 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 misunderstandings 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 misused 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 nomenclature 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 deformation, but should reflect only the capacity for substantial, non-localized strain. If so, urgent attention should be directed to establishing some consensus regarding nomenclature in this area of rock deformation studies.
0040-1951/86/$03.50
0 1986 Elsevier Science Publishers B.V.
382
In describing the way in which a rock has been deformed, two concepts are essential elements of the description, deformation mechanism and the degree of ho~oge~~i~~ of the deformation. There are three fundamental deformation mechanisms: (a) Cataclasis, in which crystal structure remains undistorted, but grains or groups of grains become cracked and the fragments may exhibit frictional sliding with respect to one-another. The process necessarily involves dilatancy and is therefore pressure sensitive. (b) Intrac~st~line plasticity, in which grains become internally distorted through dislocation motion or deformation twinning. (c) Flow by diffusive mass transfer, in which shape change of the deforming aggregrate is accomplished by stress induced diffusion of matter away from interfaces sustaining high normal stresses, with the same or different phases being reprecipitat~ at potentially dilatant sites. (b) and (c) can be constant volume processes, hence are pressure insensitive, but both are sensitive to temperature and deformation rate relative to (a). Deformation may be heterogeneous in various ways, but the one of interest here is that due to localization of deformation into a band (a fault or shear zone, in the loose sense) in an otherwise homogeneously deforming (or rigid) body. Each of the fund~en~ deformation mechanisms above can result in localized OR distributed deformation of a rock. Whether localization occurs depends on the rock type and the physical conditions of deformation. It may also be strain dependent, and develop after a certain degree of homogeneous flow. is associated with the formation Everyone will agree that “brittleness” of cracks, and can therefore be defined at the microscopic level. It is essentially a mechanistic concept. In the literature on experimental rock deformation, “ductility” has always been defined as the capacity for more or less uniformly distributed flow (e.g. Paterson, 1978), although there may be significant heterogeneities of strain on the grain scale. It is a concept which must be defined on the macroscopic scale and does not depend on the operative deformation mechanism. In the earliest experimental studies of the “brittle to ductile transition” (e.g. Paterson, 1978; Heard, 1960; Byerlee, 1968), what was observed was the transition from localized cataclasis (brittle faulting) to ductility due to distributed microcracking (cataclastic flow) as confining pressure was increased at constant temperature. Ductility dominated by intracryst~line plasticity, on the other hand, could be obtained by the application of higher confining pressure, or higher temperature, or both. In the geophysical and geological communities at present, what is tending to happen is that “ductility” is being used in a much more restricted way, being identified purely with deformation by intracrystalline plasticity. The term is therefore being made mechanism dependent. Whether the concept of ductility is used in this mechanistic way in metallurgy and materials science is not important, because rocks generally show a much
383
wider spectrum of rheological characteristics than do any engineering materials. To use the term “ductile” in such a way as to imply only the operation of plastic deformation mechanisms, strictly would exclude rocks deformed by cataclastic flow processes from being considered ductile. This is a dangerous tendency. For example, from thin-section observations alone it is possible to misinterpret some natural cataclastic flow textures by attributing them to grain-size reduction by dynamic recrystallization when in fact they are the products of cataclastic granulation followed by cementation by hydrothermal overgrowths (Rutter et al., 1985). At the scale of a single crystal, crack formation implies brittleness but intracrystalline plasticity usually involves the possibility of large, more or less homogeneous distortion of the crystal. Flow by intracrystalline plasticity is almost always associated with macroscopic ductility of the polycrystal, at least over several grain diameters. There may, however, be marked localization of the flow on a larger scale. Rocks deformed in this way tend to be regarded by geologists as “ductile” on the basis of thin section observations alone. Flow of a polycrystal by diffusive mass transfer, however, is heterogeneous even on the grain scale. There may be no significant internal distortion of an individual crystal, but grain boundary sliding accommodated by diffusive redistribution of material may result in the polycrystal exhibiting extreme ductility. Cataclasis without localization can be the dominant process accommodating folding in rocks, especially at high levels in the Earth’s crust. Such cataclastic flow may involve microfracturing of every grain in the rock, or may break the rock into fragments of several cm in dimension. Small movements on the bounding cracks, if homogeneously distributed, are able to accommodate folds (e.g. Stearns, 1969; Hadizadeh and Rutter, 1983). Thus brittleness and ductility can occur together, and the terms should not be used in a mutually exclusive way. Rather, the scale of observation should be indicated, e.g. brittle on the grain scale, macroscopically ductile. Essentially the same point has recently been made by Chester et al. (1985). Figure 1 shows some interrelationships between descriptions of mode of failure, expressed according to dominant deformation mechanism and degree of observed localization of the deformation. For convenience, deformation mechanisms have been separated into only two groups; cataclastic, and plastic in the loose sense, the latter combining flow by dislocation motion and by diffusive mass transfer. The separation is fundamentally into: (a) those mechanisms which result in dilatancy, and are therefore mean stress sensitive in terms of resistance to deformation (cataclasis), and (b) constant volume, pressure insensitive processes. Particular modes of failure then fall into the four main categories shown. Various transitions exist between the modes of failure shown on Fig. 1. Transitions from (1) to (2) or (1) to (3) have been termed brittle to ductile transitions. Other transitions indicated on Fig. 1 depend mainly on strain. Combinations of changes of temperature, confining pressure, pore fluid
384
Fig. 1. Simple diagrammatic representation of mode of failure transitions in rocks, showing how the description of mode of failure depends on the deformation mechanism and whether the strain is localized. Deformation mechanisms are grouped into those involving dilatancy (cataclasis) and those not involving volume change (crystal-plastic in the loose sense). Examples of particular mode of failure transitions are shown together with the main controlling parameters. Transitions between (1) and (4) can be induced through strain rate and/or pore fluid pressure changes.
pressure, strain and strain rate can give rise to a wide range of changes of behaviour. The impression one gains increasingly from current literature is that natural rocks only display transitions of type (1) to (3). Further, it appears to have become fashionable to identify a brittle--ductile transition of this type as characteristic of the Earth’s crust as a whole (e.g. Turcotte, 1983), the transition often being equated with the depth below which shallow earthquakes do not occur (Sibson, 1982). Such a depth is being progressively better defined in places such as central California. It may indeed correspond to the attainment of physical conditions such that the build-up of tectonic stresses, which would otherwise cause seismogenic cataclastic rupture, can be relaxed by plastic flow. To speak of a brittle-plastic transition (purely mechanistic terms) would be more satisfactory in this context. However, processes other than intracrystalline plasticity can also be responsible for such a cutoff effect, yet geophysicists often talk loosely of the “brittle-ductile transition” as if this were a fact of observation directly from geophysical sensing techniques, when they should adhere to the more guarded and factual expression “the base of the shallow seismogenic zone”. Figure 2 shows a data set from rock deformation experiments which indicate the variation in strength and mode of failure of a limestone at a constant strain rate along a constant pressure/temperature gradient corresponding to about 35°C per km depth. It shows the transition from cataelastic faulting to cataclastic flow, which for this material would correspond to the base of the seismogenic layer. This is followed by the upper transi-
385
cataclastic fault~na~ataciastlc
flow tronsltian
f
‘;;
z
5 k
Solnhofen limestone -
z 7100
-
+
Oven dry
A
wet,
Pore water wessure
=
0.1
confining pressure
0
1 0
I
I
100
200
Confining Pressure
300
400
(fiP0)
Fig. 2. Compilation of experimental data (peak stress for specimens which failed by cataclastic faulting, otherwise stress supported at 10% total strain) of Rutter (1972) for Solnhofen limestone tested wet and dry, under conditions of confining pressure increased in step with temperature to simulate natural burial. Strain rate = 10m5 s-l. Note the camelastic faulting (open symbols) to cataclastic flow transition takes place well below the peak strength. The strength peak corresponds to the transition from dominant cataclasis (pressure insensitive, temperature insensitive, d&&ant) to dominant plasticity (pressure insensitive, temperature sensitive, non-di~~nt). Because plasticity is strain rate sensitive, the peak transition also will be rate sensitive, occurrmg at shallower depths at lower strain rates.
tion to flow dominated by constant volume intracrystalline plasticity. The strength peak is caused by the mechanism change from temperature and strain rate insensitive (but pressure sensitive) cataclastic deformation, to temperature and strain rate sensitive (but pressure insensitive) intracrystalline plasticity. However, the development of the strength peak has nothing to do with the macroscopic mode of failure of the rock. I do not suggest that this limestone is particularly representative of the mechanical behaviour of the continental crust, but its behaviour does indicate that while the shape of the strength curve for a rock might be predicted from constitutive flow laws for the rock (e.g. Goetze and Evans, 1979; Sibson, 1983) the mode of failure might not simply correspond.
386
In conclusion I would suggest the avoidance of the imprecise and potentially misleading expression “brittle to ductile transition” or any expression in which the two words are used together, so that when one is dealing with mechanism changes (which can be perceived on the scale of a thin section), one uses expressions like “brittle to plastic” or “cataclastic-plastic”. Description of other aspects of transitions in modes of failure, in which the mechanism is not a necessary part of the description, can be dealt with using terms like “faulting to flow transition”, or where it is possible to give a complete description, expressions like “cataclastic faulting to plastic flow” or “plastic shear zone to cataclastic faulting” might be employed.
REFERENCES Byerlee, J.D., 1968. The brittle-ductile transition in rocks. J. Geophys. Res., 73: 47414750. Chester, F.M., Friedman, M. and Logan, J.M., 1985. Foliated cataclasites. Tectonophysics, 111: 139-146. Goetze, C. and Evans, B., 1979. Stress and temperature in the bending lithosphere as constrained by experimental rock mechanics. Geophys. J. R. Astron. Sot., 59: 463478. Hadizadeh, J. and Rutter, E.H., 1983. The low temperature brittle-ductile transition in a quartzite and the occurrence of cataclastic flow in nature. Geol. Rundsch., 72: 493-509. Harper, G.D., 1985. Tectonics of slow spreading mid-ocean ridges and consequences of a variable depth to the brittle-ductile transition. Tectonics, 4: 379-394. Heard, H.C., 1960. Transition from brittle fracture to ductile flow in Solnhofen Limestone as a function of temperature, confining pressure and interstitial fluid pressure. Mem. Geol. Sot. Am., 79: 193-226. Kligfield, R., Crespi, J., Naruk, S. and Davis, G.I.H., 1984. Extensional Orogens. Tectonics, 3: 577-609. Miller, E.L., Gans, P.G. and Garing, J., 1983. The Snake Range decollement; an exhumed mid-Tertiary brittle-ductile transition. Tectonics, 2: 239-264. Mitra, G., 1984. Brittle to ductile transition due to large strains along the White Rock thrust, Wind River mountains, Wyoming. J. Struct. Geol., 6: 51-62. Passchier, C.W., 1984. The generation of ductile and brittle shear bands in a low angle mylonite zone. J. Struct. Geol., 6: 273-282. Paterson, M.S., 1978. Experimental Rock Deformation: the Brittle Field. Springer, Berlin. Rutter, E.H., 1972. The influence of interstitial water on the mechanical behaviour of calcite rocks. Tectonophysics, 14 : 13-3 3. Rutter, E.H., Peach, C.J., White, S.H. and Johnston, D., 1985. Experimental ‘syntectonic’ hydration of basalt. J. Struct. Geol., 7 : 251-266. Sibson, R.H., 1982. Fault zone models, heat flow and the depth distribution of earthquakes in the continental crust of the United States. Bull. Seismol. Sot. Am., 72: 151-163. Sibson, R.H., 1983. Continental fault structure and the shallow earthquake source. J. Geol. Sot. London, 140: 141-167. Simpson, C., 1985. Deformation of granitic rocks across the brittle-ductile transition. J. Struct. Geol., 7: 503-512.
381 Smith, R.B. and Bruhn, R.L., 1984. Intra-plate extensional tectonics of the eastern Basin-Range: inferences on structural style from seismic reflection data, regional tectonics and thermal-mechanical models of brittle-ductile deformation. J. Geophys. Res., 89: 5733-5762. Stearns, D.W., 1969. Fracture as a mechanism of flow in naturally deformed, layered rocks. Proc. Conf. on Research in Tectonics. Geol. Survey of Canada, pp. 68-80. Turcotte, D.L., 1983. Mechanisms of crustal deformation. J. Geol. Sot. London, 140: 701-724. Vann, I.R., 1985. To stretch a continent. Nature, 316: 293-294. Vernon, R.H., 1974. Controls of mylonitic compositional layering during non-cataclastic ductile deformation. Geol. Mag., 111: 121-123.