>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)
011 44 (0)1865 276792 (University College)
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