Dear all,
I have enjoyed following the recent conversations on brittle and ductile, frictional and viscous, and lithosphere and asthenosphere. I welcome the opportunity to see, in writing, how my colleagues view these terms. My views are a bit different, so I thought I would "toss in my two cents".

  1.  The idea of an exhumed brittle-ductile transition has always struck me as odd. The reason is that any ductily deformed rocks have to move through the frictional domain before reaching the surface. (Of course, this assumes that the rocks are at yield when they move through the brittle/frictional domain.)
  2.  It is my understanding that the term plastic means that a material can sustain a finite deviatoric stress. In the ductile domain, rocks deform by thermal activated viscous processes. For dislocation glide, the viscous behavior is stress-dependent, which gives the power-law constitutive equation that we use to represent this process. The term crystal plasticity is widely used, but is otherwise incorrect. This usage originated from the engineering literature because engineering materials that deform by power-law viscosity appear to have a finite strength. This idea does not extend to  geology, because the time-dependent nature of power-law viscosity is readily apparent at geologic time scale. This issue becomes even more confusing given that a frictional rheology is often called a Coulomb plastic, in that the material can sustain a finite shear stress, and will yield or fail when it reaches a threshold "yield" stress. That yield stress is, of course, pressure dependent.
  3.  These terms--plastic, viscous, elastic, ductile, brittle--have been around for a century or more.  My preference is to use them according to their original definition. There was an earlier discussion about the meaning of lithosphere and asthenosphere. Those terms were coined in 1914 by Barrell (J of Geology). Our understanding of the lithosphere has changed with time, but the original definitions are still very much relevant.

Best,
Mark
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Mark Brandon, Professor, Dept. of Geology and Geophysics
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From: Christopher WIBBERLEY <[log in to unmask]<mailto:[log in to unmask]>>
Reply-To: Tectonics & structural geology discussion list <[log in to unmask]<mailto:[log in to unmask]>>
Date: Fri, 9 Sep 2011 18:33:33 +0200
To: <[log in to unmask]<mailto:[log in to unmask]>>
Subject: Re: The ficticious brittle/ductile transition

Dear All,

I have a slightly different take on the BDT argument, which seems to be as long-lived as many of Bob’s faults, and doubtless it will carry on …


-          I prefer to avoid it altogether when referring to the spatial distribution of strain for precisely the reasons John has invoked – this is a scale dependant matter and depends where you’re coming from. The BDT terminology (Ernie and others might like to correct me if I’m wrong) comes from experiments on metals – fracture being brittle, whereas “significant” strain before fracture (or no fracture at all) being ductile. As these are often uniaxial tension tests there tends to be fairly homogeneous elongation away from the end parts of the sample (“necking”), and so the strain is indeed fairly distributed. Personally I prefer to talk in terms of becoming more localized / more distributed through time / strain and avoid all this BD confusion.

-          As Rick Sibson and others, I think, originally used elastic-frictional and plastic terminology, I usually stick with that when discussing rheology of faults (a big nod to the Handy and Schmidt 1991 paper too). However, “brittle” and “ductile” seem perfectly OK for describing fracture and non-fracture (DMT & crystalline plasticity) deformation mechanisms.

I disagree with trying to simplify terminology for the benefit of the wider public – “plastic” is not much more frightening a word than “ductile”, and if we lose robustness in our scientific articles, then how can we expect the next generation to improve, or non-native English speakers to understand and get things right in English ?

I was interested in Ernie’s comment on folding by cataclastic flow, but would just add a distinction I think is important. I understand the “flow” to refer to the movement of particles with no macroscopic velocity discontinuity away from the boundaries (again, someone might like to correct me on this). In this sense, “cataclastic flow” refers to the movement of particles, probably with frictional sliding between them (and micro velocity discontinuities). However, personally I would not also use this term to refer to the GENERATION of these particles by fracturing/breakdown of the initially intact wall rock – you have to fracture the rock / grains first. In the same way, I would separate the brittle jointing creating Ernie’s blocks from the macroscopic flow of these joint blocks which follows when the folding continues to larger strains. So the brittle fracture in the folding example is occurring (at least in any given part of the fold) before the macroscopically “ductile” flow, not at the same time.

Lastly, it might be worth thinking about how this argument relates to stick-slip and instability. A higher rate of strength drop than driving stress release on a fault / fracture leads to instability (and this is the case in brittle joints for example), whereas a higher rate of stress release than rate of strength drop makes it stable. Of course, this is not the same discussion as the BDT one but there are parallels to be drawn (and probably more scope for mis-understanding!), such as the dynamic stress drop with brittle fracturing vs granular sliding and Coulomb plasticity in a supposedly stable creeping fault for example, both being in the shallow crust above the “BDT”.

Best regards,
Chris Wibberley

Ps. I agree with Bob about the OHFZ and NW Scotland in general, but Bob, you forgot to add the weather to the list of attractions!

De : Tectonics & structural geology discussion list [mailto:[log in to unmask]] De la part de Ernest Rutter
Envoyé : mercredi 7 septembre 2011 17:29
À : [log in to unmask]<mailto:[log in to unmask]>
Objet : Re: The ficticious brittle/ductile transition

I’m afraid I must take issue with some of John Platt’s views. There is no way a narrow fault (as little as a centimetre) that is accommodated by high temperature plastic flow in the granulite facies can be regarded as brittle, yet such things are quite common. The mode of failure is localized (commonly called a plastic shear zone though still a fault) but the mechanism is plastic.  Also , cataclastic flow in nature can be by distributed mesofracture, such as accommodates large-scale folding of limestones in central Italy. Multi-cm cracks allow small displacements and dilatancy that allows long-wavelength bending to be accommodated. The mechanism is brittle fracture but the mode of failure is a ductile flow. We have not (yet) been able to reproduce this in lab experiments.   On the other hand, cataclastic flow in experiments is seen in porous rocks, where pore collapse permits local hardening that causes the deformation to be distributed.  Exactly the same kind of flow occurs when a reservoir collapse as a result of pore fluid withdrawal, so cataclastic flow is definitely not restricted to lab experiments.
So death to the BDT in the way it is sometimes used, and I don’t think the alternatives can be pulled apart as easily as John believes!
Ernie Rutter

From: Tectonics & structural geology discussion list [mailto:[log in to unmask]] On Behalf Of John Platt
Sent: 07 September 2011 15:31
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: Re: The ficticious brittle/ductile transition

As someone who has been guilty of using the term brittle-ductile transition (BDT) - ugh - I would like to leap to its defence as a useful practical term for conveying the concept of a level in the crust above which deformation occurs mainly by discontinuous brittle faulting, and below which deformation takes place by a variety of ductile processes without loss of continuity at the scale of observation. And that's the key - the scale of observation.  Consider this:  flow of a viscous fluid (air, say) can be described at the microphysical level entirely by the elastic and frictional interactions among molecules: they bounce off each other and swap neighbours on picosecond timescales.  The deformation is pressure-dependent too:  the viscosity scales linearly with pressure.  And at the other extreme, since the insightful work of David Elliott, Bill Chapple, and others, we have learnt to treat large-scale tectonic features such as thrust belts as macroscopically ductile structures - we use the formalism of continuum mechanics, and a constitutive relationship (Coulomb plasticity) to describe their bulk deformation.  This is valid at a scale of observation larger than the individual faults, just as crystal plasticity is valid at scales larger than the discontinuities (dislocations).  The concept of the BDT depends on the scale of observation, which is the human one:  if we can see and measure a fault displacement, we call it brittle.
The issue of cataclastic flow as seen at the grain-scale in rock-mechanics experiments, is, in my opinion, a red herring.  In nature, cataclastic flow is almost entirely limited to narrow zones directly associated with large displacement brittle faults.  There is no evidence in nature of a transition from dicontinuous faulting to continuous cataclastic flow with depth:  major faults rupture right through the BDT into the underlying plastic zone.
So long live the BDT as a useful, if slightly fuzzy, way of conveying a simple concept.  If we want to pick nits, we can pull apart all the alternatives just as easily.
John Platt.

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Professor J.P. Platt
Department of Earth Sciences
University of Southern California
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