Interesting discussion... An unspoken assumption underlying Uwe's view
is that brittle and ductile behaviors are largely controlled by
temperature in geologic settings. My comments here are similar to those
of Ernie Rutter, and are meant, I hope, to expand on his thoughts.
I have seen low-temperature "ductile shear zones" in the Mistaken Point
Fm of SE Newfoundland. The maximum temperatures are ~200 to 250 C. The
"ductile shear" is accommodated by pressure solution, not
intra-crystalline glide and climb. Likewise, I have seen very low
temperature ductile shear zones in olistostromal units, such as in the
Apennines and Sicily, where ductile deformation was accommodated by
granular flow while the sediments were unlithfied. The idea of a
temperature-controlled "brittle-ductile" transition in the crust is
widely accepted but the idea tends to fall apart when considering
deformation at an outcrop scale.
Note also that the term "ductile shear zone" is somewhat misleading
since ductile really means that the absence of strain localization. In
the rock mechanics literature (e.g., John Handin and others), the term
semi-ductile was used to refer to cases where strain started to become
weakly localized.... The simplified notation of a "brittle-ductile"
transition in the crust is based on competition between a frictional
faulting and intracrystalline dislocation glide and climb. In fact, it
might be better to call this the seismogenic transition or the base of
the schizosphere (following Scholz, 2002). As we all know, the specific
depth for this transition depends on temperature, ambient strain rate,
mineralogy, and fluid pressure. The situation of course becomes more
complicated when additional rheologies (e.g., pressure solution,
granular flow) are added to the mix and when other types of localizing
mechanisms (e.g., reaction weakening, LPO-softening, etc.) Thus it
should not be surprising to find semi-ductile shear zones in low
temperature settings, and frictional faults in very deep higher
temperature settings (e.g. pseudotachylites in blueschists and eclogites
of Corsica and SW Norway).
Best,
Mark Brandon
On 7/28/10 6:55 AM, Francesca Meneghini wrote:
> I agree with Ernie Rutter and, to avoid "young scientist confusion",
> here is a definition from the Passchier and Truow book:
> One of the most common patterns of heterogeneous
> deformation is the concentration of deformation in planar
> zones that accommodate movement of relatively rigid
> wall-rock blocks. Deformation in such high-strain zones
> usually contains a rotation component, reflecting lateral displacement
> of wall rock segments with respect to each other;
> this type of high-strain zone is known as a shear zone. Deformation
> in a shear zone causes development of characteristic
> fabrics and mineral assemblages that reflect
> P-T conditions, flow type, movement sense and deformation
> history in the shear zone. As such, shear zones are an
> important source of geological information.
> Shear zones can be subdivided into brittle zones or faults,
> and ductile zones (Chap. 3). Ductile shear zones are usually
> active at higher metamorphic conditions than brittle shear
> zones (Figs. 3.44, 5.2).
>
>
> Cheers
> Francesca Meneghini
>
>
> On 28 July 2010 12:04, Ernest Rutter<[log in to unmask]> wrote:
>
>> Hello All,
>> The term 'shear zone' should not be restricted to faults in which there is so-called 'ductile' (meaning intracrystalline plastic) deformation. Look at the magnificent cataclastic shear zones (wide zones of localized cataclastic deformation but a meter or more width) at Durdle Door (S coast of England), for example. Shear zone is (like mylonite) a field term and no mechanism should be implied - it is easy to deduce mechanism wrongly. John is quite correct in his usage.
>> Ernie Rutter
>>
>> -----Original Message-----
>> From: Tectonics& structural geology discussion list [mailto:[log in to unmask]] On Behalf Of Uwe Altenberger
>> Sent: 28 July 2010 10:25
>> To: [log in to unmask]
>> Subject: Re: Anchi-metamorphic shear zones
>>
>> Dear John, dear Graham
>> are these really shear zones? The classical and modern textbooks
>> define shear zones as ductile fault zones. The anchimetamorphic zones
>> are probable brittle faults? I know, it is only a semantic problem.
>> However, younger scientists get be confused.
>>
>> best wishes
>>
>> uwe
>>
>>
>>> Dear John
>>> Have a look at what Bernard and I did with the Orlock Bridge Fault...it's all
>>> anchizone........sorry I don't have a pdf
>>>
>>> Anderson, T B.& Oliver, G J H (1986). The Orlock Bridge Fault: a major Late
>>> Caledonian sinistral fault in the Southern Uplands Terrane, British Isles.
>>> Trans. R. Soc. Edin: Earth Sci. 77, 203-222.
>>>
>>> Cheers
>>> Grahame (Still at the National University of Singapore and loving it)
>>>
>>> Quoting "John F. Dewey"<[log in to unmask]>:
>>>
>>>
>>>> Dear Colleagues,
>>>>
>>>> For some years, I have been working on the anchi-metamorphic shear
>>>> zones of the North Cornwall/Devon coast between Crackington Haven and
>>>> Hartland Quay. Rob Strachan got me going by, generously, giving me a
>>>> sliced pebble through one of the finest shear zones of my experience
>>>> from the beach at Hartland Quay. I have, now, analysed many large
>>>> thin sections across these shear zones, which combine sigmoidal
>>>> tension gashes with solution cleavages. Most are transtensional plane
>>>> strain zones of volume increase. I would be most grateful to hear
>>>> from anyone who has studied such shear zones.
>>>>
>>>> Best wishes,
>>>> John Dewey
>>>> --
>>>>
>>>> -----------------------------------
>>>> Please update your address book to use [log in to unmask] as my
>>>> e-mail address.
>>>>
>>>> Prof. John F. Dewey FRS, Hon. M.R.I.A., Mem. Acad. Eur., For. Mem.
>>>> US Nat. Acad. Sci, Distinguished Emeritus Professor University of
>>>> California,
>>>> Emeritus Professor University College Oxford.
>>>>
>>>> Sherwood Lodge,
>>>> 93 Bagley Wood Road,
>>>> Kennington,
>>>> Oxford OX1 5NA,
>>>> England, UK
>>>>
>>>> University College,
>>>> High Street,
>>>> Oxford OX1 4BH
>>>>
>>>> Telephone Nos:
>>>> 011 44 (0)1865 735525 (home Oxford)
>>>> 011 44 (0)1865 276792 (University College Oxford)
>>>> email: [log in to unmask]
>>>>
>>>>
>>>
>>>
>>>
>>> ------------------------------------------------------------------
>>> University of St Andrews Webmail: https://webmail.st-andrews.ac.uk
>>>
>>
>> --
>> Uwe Altenberger
>> Institute of Earth and Environmental Sciences
>> University of Potsdam
>> Karl-Liebknecht-Str. 24
>> D-14476 Potsdam-Golm
>> Tel. +49 331 977 5806
>> Fax +49 331 977 5700
>> [log in to unmask]
>>
>>
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