On 28 Jul 2010, at 21:50, John F. Dewey wrote:
>> Dear Colleagues,
>
> Many thanks for your valuable and enlightening contributions, which
> have been very useful. Please keep them coming. For me, a shear zone
> is a localized deformation zone with a component of non-coaxial
> strain. It can have any rheology.
>
> Best wishes,
> John
>
>> 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]
>>>>
>>>>
>>>
>>>
>>>
>
>
> --
>
> -----------------------------------
> Please update your address book to use [log in to unmask] as my e-
> mail address.
>
> John F. Dewey FRS, UC Distinguished Emeritus Professor of Geology
> Department of Geology
> UC Davis
> One Shields Avenue
> Davis CA 95616
>
> Telephone Nos:
> 530 752 5829 (UC Davis)
> 011 44 (0)1865 735525 (home)
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