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GEO-TECTONICS  August 2011

GEO-TECTONICS August 2011

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Subject:

Re: Brittle-Ductile Transition Confusion...

From:

"John F. Dewey" <[log in to unmask]>

Reply-To:

Tectonics & structural geology discussion list <[log in to unmask]>

Date:

Tue, 30 Aug 2011 09:37:31 +0100

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (139 lines)

>Dear Mark,

No attachment?

Best wishes,
John

>John,
>
>...and here is the one that started us off on the b-d transition some time
>ago. Again, field inspired...
>
>
>Mark
>
>
>>>Dear Scott,
>>
>>  Far from being a naive or foolish question, you have hit upon and
>>  asked one of the central and most important questions in tectonics
>>  and the material science of olivine. There are two splendid
>>  (opposing) publications that address this question head-on in the GSA
>>  magazine, one by Watts and Burov, the other by James Jackson. I have
>>  been working on and worrying about this problem for at least ten
>>  years and am gradually moving towards a "conclusion" but I am
>>  sufficiently far from that "conclusion" to prevent my giving you a
>>  definitive "solution" This is a seriously difficult problem that has
>>  occupied better minds than mine. The best starting point is to think
>>  of the lithosphere as a boundary conduction layer that may deform but
>>  does not internally convect, then think about the
>>  rheology/deformation mechanisms  of olivine and quartz for a PTt
>>  spectrum. It is not simple.
>>
>>  Best wishes,
>>  John  Dewey
>>
>>>Hello Geo-Tectonics Folks,
>>>
>>>I have run across a conundrum of sorts concerning my understanding
>>>of the lithosphere, asthenosphere, and the brittle-ductile
>>>transition. I hope that folks on this discussion list can be of
>>>help. Hopefully, I haven't said something foolish in here...
>>>
>>>In my intro geology course here at Appalachian State University, I
>>>teach about the layers of the Earth (crust, mantle, and core). I
>>>also discuss the lithosphere and asthenosphere and the
>>>brittle-ductile transition. I define the lithosphere as the layer of
>>>the earth that undergoes brittle behavior and the asthenosphere as
>>>the layer that undergoes ductile flow. Later on in the course I talk
>>>about the LVZ and stuff like that, but in the beginning, I just want
>>>them to realize that there is more than one way to subdivide the
>>>layers of the earth and that the tectonic plates are lithospheric
>>>not crustal.
>>>I then talk about the brittle-ductile transition because this marks
>>>the approximate depth of the base of seismicity (and the strongest
>>>portion of the lithosphere/crust) and it therefore controls the
>>>magnitude potential of a given active tectonic region. E.g.
>>>subduction zones can make the biggest earthquakes because they have
>>>the largest potential rupture area. I think this is a useful concept
>>>to teach intro students because I often hear the students saying
>>>things like..."I heard that the next earthquake in California could
>>>be a M9.5 or more."
>>>This is of course not possible (unless we are way off in our
>>>understanding of basic earthquake physics). I think that teaching
>>>about the brittle-ductile transition is therefore useful, even for
>>>intro-level students. The problem is that, as I have defined
>>>lithosphere, the entire lithosphere should all undergo brittle
>>>failure.
>>>
>>>
>>>So, here is my question (Finally!): If the brittle ductile
>>>transition lies within the lithosphere, does this not contradict the
>>>definition of lithosphere? How do folks on this list define these
>>>terms to intro-level students? Should I define
>>>lithosphere/asthenosphere as Fowler's text does (i.e. non-convecting
>>>vs. convecting)?
>>>
>>>Any thoughts or advice list members may have on this would be very
>>>enlightening.
>>>Cheers,
>>>-Scott
>>>
>>>--
>>><><><><><><><><><><><><><><><>
>>>Scott T. Marshall
>>>Department Of Geology
>>>Appalachian State University
>>>572 Rivers St.
>>>Boone, NC 28608
>>>
>>>http://www.appstate.edu/~marshallst/
>>>ftp://pm.appstate.edu/pub/prog/marshallst/
>>
>>
>>  --
>>  Please note that my email address has changed to: [log in to unmask]
>>
>>  Prof. John F. Dewey FRS, M.R.I.A., FAA, Mem. Acad. Eur., Mem.
>>  US Nat. Acad. Sci., Distinguished Emeritus Professor University of
>>  California, Emeritus Professor and Supernumerary Fellow, 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)
>>


-- 
Please note that my email address has changed to: [log in to unmask]

Prof. John F. Dewey FRS, M.R.I.A., FAA, Mem. Acad. Eur., Mem.
US Nat. Acad. Sci., Distinguished Emeritus Professor University of 
California, Emeritus Professor and Supernumerary Fellow, 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)

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