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
Web:
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
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?
On 7 Sep 2011, at 12:45, Behr, Whitney wrote:
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.
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