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)
>
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