Dear all,
Ah - the good old brittle ductile transition: a perennial favourite for
discussion and misunderstanding!
Thanks to Tim's kind name-check, I should just confirm his observation
that the Lewisian complex in NW Scotland is a particularly good place to
study the geological complexities and character of the BDT (or whatever
you wish to describe it as). It is of historical importance too since
the geological description of this phenomena came from the
Lewisian-hosted Outer Hebrides Fault Zone (OHFZ) that Rick Sibson used
to propose a simple fault rock classification and to describe a
geological manifestation of the transition from plastic to brittle
deformation in continental basement rocks (both in his seminal PhD
project - one of the best I have ever read - and in his 1977 paper
published in JGSL). The rest of what follows is a plug for the OHFZ as
a superb natural laboratory....
The OHFZ is an extraordinary structure that extends ~180km down the
eastern side of the Outer Hebrides. It is known to cut the entire crust,
has a complex and long-lived history of repeated reactivation, likely
spanning at least 1Ga. It has at various times been a thrust, a
strike-slip fault and, most recently a major basin-bounding normal
fault. Thanks to the effects of footwall uplift, and with a little extra
help from the Iceland plume, it is superbly exposed compared to most
fault zones, with world class exposures of pseudotachylytes,
cataclasites, crush melanges, mylonites, phyllonites, breccias and
gouge. Subsequently work by the likes of Joe White, Chris Butler, Jonny
Imber and others have highlighted some of the joys and complexities of
the BDT and its manifestation in mid- to upper crustal rocks and also
the importance of this transition in controlling fault weakening and
reactivation.
In short, folks, if there were a list of 10 geological things to go look
at and study before you die, this fault zone is one of them. And the
scenery and refreshments are also, of course, world class.
So: get thee to the Outer Hebrides!!
Bob Holdsworth
Some references (not exhaustive)
Butler, C. A., Basement fault reactivation: the kinematic evolution of
the Outer Hebrides Fault Zone, Scotland, Ph.D. thesis, University of
Durham, U.K., 1995.
Butler, C. A., R. E. Holdsworth, and R. A. Strachan, Evidence for
Caledonian sinistral strike-slip and associated fault zone weakening,
Outer Hebrides Fault Zone, Scotland, Journal of the Geological Society,
London, 152, 743-746, 1995.
Imber, J., Deformation and fluid-rock interaction along the reactivated
Outer Hebrides Fault Zone, Scotland, Ph.D. thesis, University of Durham,
U.K., 1998.
Imber, J., R. E. Holdsworth, C. A. Butler, and G. E. Lloyd, Fault-zone
weakening processes along the reactivated Outer Hebrides Fault Zone,
Scotland, Journal of the Geological Society, London, 154, 105-109, 1997.
Imber, J., Holdsworth, R.E., Butler, C.A. & Strachan, R.A. A reappraisal
of the Sibson-Scholz fault zone model: The nature of the frictional to
viscous ('brittle-ductile') transition along a long-lived crustal-scale
fault, Outer Hebrides, Scotland. Tectonics 20, 601-624, 2001.
Imber, J., Strachan, R.A., Holdsworth, R.E. & Butler, C.A. The
initiation and early tectonic significance of the Outer Hebrides Fault
Zone, Scotland. Geological Magazine, 139, 609-619, 2002.
MacInnes, E. A., G. I. Alsop, and G. J. H. Oliver, Contrasting modes of
reactivation in the Outer Hebrides Fault Zone, northern Barra, Scotland,
Journal of the Geological Society, London, 157, 1009-1017, 2000.
Sibson, R. H., Generation of Pseudotachylyte by Ancient Seismic
Faulting, Geophysical Journal of the Royal Astronomical Society, 43,
775-794, 1975.
Sibson, R. H., Fault rocks and fault mechanisms, Journal of the
Geological Society, London, 133, 191-213, 1977a.
Sibson, R. H., The Outer Hebrides Thrust: Its Structure, Mechanism and
Deformation Environment, Ph.D. thesis, University of London, U.K.,
1977b.
Smythe, D. K., A. Dobinson, R. McQuillin, J. A. Brewer, D. H. Matthews,
D. J. Blundell, and B. Kelk, Deep structure of the Scottish Caledonides
revealed by the MOIST reflection profile, Nature, 299, 338-340, 1982.
White, J. C., Transient discontinuities revisited: pseudotachylyte,
plastic instability and the influence of low pore fluid pressure on
deformation processes in the mid-crust, Journal of Structural Geology,
18, 1471-1486, 1996.
Prof Bob Holdsworth,
NERC KE Fellow
Dept of Earth Sciences,
University of Durham,
Durham DH1 3LE,
UK
Tel +44(0)1913342299
Fax +44(0)1913342301
e-mail (including jsg business): [log in to unmask]
<mailto:[log in to unmask]>
Web:
Dept: http://www.dur.ac.uk/earth.sciences
<http://www.dur.ac.uk/earth.sciences>
Geospatial Research Ltd (GRL) - www.geospatial-research.com
<http://www.geospatial-research.com/>
From: Tectonics & structural geology discussion list
[mailto:[log in to unmask]] On Behalf Of Tim Wynn
Sent: 07 September 2011 17:21
To: [log in to unmask]
Subject: Re: The ficticious brittle/ductile transition
Malcolm,
I can provide an example of discrete vs. distributed deformation in the
same area. There are nice examples of very narrow (1-2 metre wide) sub
vertical E-W trending shear zones formed at amphibolite / upper
greenschist facies within the otherwise granulite facies Lewisian
Central District of NW Scotland. These features are a few metres across
at best, occasionally with tight folding of the earlier subhorizontal
gneissic banding on their margins. Apparent displacements are clearly
shown by offsets of the various Scourie dykes and offsets are several
hundred metres to 1-2 kilometres in magnitude. Movement was
predominantly in a sinistral strike-slip sense assuming the entire area
hasn't been rotated. The 1907 Peach and Horne maps show the features up
beautifully.
From West to East, toward the larger WNW to NW trending Laxford Shear
Zone, these shears appear to widen, re-orientate to WNW-NW and become
less distinct as the rock in which they reside also becomes more
deformed and amphibolitised. The change in nature of the deformation is
shown by the affect on the Scourie dykes. It is possible that the narrow
shears formed as a swarm of discrete features across the entire area and
were then affected by more pervasive (distributed) deformation in the
east. Alternatively, they could represent discrete shears that were
accommodating deformation within the western part of a stronger basement
block that was also undergoing more distributed deformation in the
Laxford Shear zone at the same time. Either way, these narrow shears
clearly have metamorphic assemblages that demonstrate they were formed
at similar PT conditions to the Laxford Shear Zone and were not formed
at shallower depths. Later, lower greenschist facies and cataclastic
reactivation has also affected the narrow shears, presumably because
they remained as zones of weakness during subsequent exhumation.
At least, that was my understanding from working on them 15-20 years
ago. I know Bob Holdsworth ,amongst others, has been supervising
research in this area more recently so data and interpretations may have
moved on.
Regards
Tim
From: Tectonics & structural geology discussion list
[mailto:[log in to unmask]] On Behalf Of Malcolm McClure
Sent: 07 September 2011 13:09
To: [log in to unmask]
Subject: Re: The ficticious brittle/ductile transition
Hello All,
It must be of wider interest to know of a mapping area where a
recognizable formation sequence occurs as its brittle and ductile
variants in reasonably adjacent outcrops. Can someone please nominate a
candidate area that exhibits both characteristics?
Cheers
Malcolm
On 7 Sep 2011, at 12:45, Behr, Whitney wrote:
Hi Ernie,
With all due respect, while I agree with you that the term
brittle-ductile transition is a misnomer, I wanted to point out that
some of us still use it in an effort to reach the broader earth science
community rather than targeting our work toward rock experimentalists.
Most earth scientists will know the term BDT and will, as you say,
associate it with a place in the crust (e.g. the base of the seismogenic
layer). In some cases, this association is sufficient to get one's
point across in a paper, whereas the use of terms like 'brittle-plastic'
or 'frictional-viscous' transition may alienate or deter broader earth
science readers. In other words, although it is incorrect and
imprecise, there are practical reasons for continuing its use, just as
there are with hundreds of other misnomers in the English language.
Cheers,
Whitney Behr
On Wed, Sep 7, 2011 at 6:01 AM, Ernest Rutter
<[log in to unmask]> wrote:
Hello All,
I just returned for a field trip to find this discussion about
brittle ductile transition. In 1986 Tectonophysics I published a short
article about the brittle-ductile transition in rocks to try to clarify
nomenclature. In summary, there are two types of transition; between
deformation mechanisms, such as deformation by fracturing and frictional
sliding (which can be distributed and is always pressure dependent and
relatively temperature independent) and by (pressure independent but
more temperature dependent) plastic or diffusive transfer processes.
Both of these can lead to either localized deformation (faults, shear
zones) or to distributed flow (whether cataclastic flow or flow by
crystal plasticity). The second type of transition is therefore a mode
of failure transition, from localized to distributed (ductile
deformation). Thus the term brittle-ductile transition is a misnomer - I
never use it, preferring faulting (or localized deformation) to
distributed flow transition (mechanism not specified).
In rock mechanics, ductility is the capacity for flow without
localization (as defined by Hugh Heard in 1960 GSA special publication
vol 73, in which there is no prejudice about mechanism), and after a few
percent strain the mode of failure can change from distributed to
localized deformation. Look how much more carefully experimentalists
talk about these concepts than do others. In my 1986 paper there is a
diagram that illustrates the difference between mode of failure and
deformation mechanism transitions. I hate it when geolophysicists talk
about THE brittle ductile transition in the Earth, as if it is at a
specific place or particular depth, or independent of mineralogy. The
concept gains credence through careless and unwarranted repetition -
ugh!
Ernie Rutter
--
Whitney Behr, Ph.D.
Post-doctoral Researcher
Department of Geological Sciences
Brown University
324 Brook Street
Providence, RI 02912
http://www.geo.brown.edu/geopeople/Postdocs/Behr
<http://www.geo.brown.edu/geopeople/Postdocs/Behr/Behr/Home.html>
