I completely agree with Alan on this.
John Ramsay's restoration of the Helvetic nappes is a classic case. The
section cannot really be called a restored balanced section because a
lot of the deformation is ductile folding; however it is an excellent
palinspastic reconstruction of the Morcles, Diableret and Wildhorn
nappes; it doesnt pretend to give precise crustal shortening estimates
like you can in parts of the Moine thrust zone, but does show where each
nappe - thrust sheet lay in respect of the stable foreland.
Likewise in the Greater Himalaya. We have tried to do similar
restorations using the same restoration techniques in both the Indian
Himalaya (Zanskar-Kulu Kashmir region, and in Nepal along the Everest
section; these restored sections of the Himalaya (combined with
protolith signatures) give us a very good indication of where each unit
was in relation to the neighboring units/ thrust sheets prior to
deformation and metamorphism.
I would venture that you can use balanced-restored cross section -
palinspastic restoration techniques in any mountain belt to good use,
including UHP - HP terranes (eg restoring the Oman continental margin
that has been to 23 kbars eclogite facies), not for obtaining precise
amounts of shortening but for restoration. I would also venture that the
same techniques could possibly also be used for 4D restoration of Sheath
folds. In Oman the Wadi Mayh sheath fold has about 20 km of SSW-directed
sheathing and a significant (but unquantified) amount of WNW-ESE
shortening that could potentially be estimated using restoration
techniques (best attempted after several gin and tonics).
Mike Searle
Alan Gibbs wrote:
> In the real world everything balances, so John, Hermann, you are correct
> that you can't "balance" a single section. However, thinking about how it
> might balance and using section techniques on a number of sections and
> orientations to help constrain just how much might have gone out of section
> is certainly worth some effort.
>
> It's definitely not pointless to have a go quantifying shortening and the
> implications of the range of answers you are going to get out of looking at
> area and volume conservation assumptions even if you are not going to end up
> with a unique answer.
>
> Otherwise you might just as well scribble down any old section or isometric
> drawing you like and think looks pretty. That sounds like abstract art and
> not geology to me.
>
> alan
>
> Dr Alan Gibbs
> Director
> Midland Valley Exploration
> 144 West George Street
> Glasgow
> G2 2HG
> tel: 44 (0) 141 332 2681
> fax: 44 (0) 141 332 6792
>
>
> [log in to unmask]
>
> www.mve.com
>
>
> -----Original Message-----
> From: Tectonics & structural geology discussion list
> [mailto:[log in to unmask]] On Behalf Of John F. Dewey
> Sent: 13 February 2012 19:30
> To: [log in to unmask]
> Subject: Re: the effect of superposed folding on estimation of crustal
> shortening and cross section balancing
>
>> Hermann has it right. Balancing of poly-deformed sections or, indeed,
>> any non-plane strain sections is pointless.
>
> Best wishes,
> John Dewey
>
>> Koushik,
>>
>> since there is no true principal section through a fold interference
>> systems (except perhaps in case of perfect type III - coaxial fold
>> superposition) isn't it pointless to balance such sections?
>>
>> Hermann
>>
>> On 2/13/12 4:00 AM, koushik sen wrote:
>>> Apologies for multiple posting
>>>
>>> Hi All,
>>> can anyone provide me with references of some papers or books where
>>> the effect of superposed folding and/or tight isoclinal folding on
>>> cross section balancing and estimation of crustal shortening have been
>>> discussed? papers dealing with significance of superposed folding in
>>> fold and thrust belts will also be helpful. Thanks in advance.
>>>
>>> Best Regards
>>> Koushik
>>>
>>> --
>>> Dr. Koushik Sen
>>> Scientist 'B'
>>> Wadia Institute of Himalayan Geology
>>> Dehra Dun- 248001
>>> India
>
>
> --
> 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)
--
******************************************
Professor Michael P.Searle
Dept. Earth Sciences
Oxford University,
South Parks Road.,
Oxford, OX1 3AN
England
Professor of Earth Sciences, and
Senior Research Fellow, Worcester College, Oxford.
Tel: +44 1865 272022
Fax: +44 1865 272072
Mike Searle's Home Page: http://www.earth.ox.ac.uk/~mikes
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