Inherent in the arguments presented by both Ernie and John is the
assumption of an homogenous medium. One therefore has to question John's
statement that the brittle-ductile transition is "...a useful practical
term for conveying the concept of a level in the crust...". Convergent
orogens, for example, are made up of very different rock types juxaposed
at all levels of the crust. These have different mechanical responses to
deformation. Therefore, forming a descrete LEVEL in the crust above which
the deformation mechanisms are different from those below seems to me to
be a difficult thing to do. I hope that Ernie, has a mapping geologist,
will some insights into this.
Cheers
Dennis
> 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]
> 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.
>
> --
> Professor J.P. Platt
> Department of Earth Sciences
> University of Southern California
> 3651 Trousdale Parkway
> Zumberge Hall 315
> Los Angeles, CA 90089-0740
> email: [log in to unmask]
> phone: +1-213-821-1194
>
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