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GEO-TECTONICS  February 2012

GEO-TECTONICS February 2012

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Subject:

Re: the effect of superposed folding on estimation of crustal shortening and cross section balancing

From:

Hermann Lebit <[log in to unmask]>

Reply-To:

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

Date:

Wed, 15 Feb 2012 20:50:53 -0600

Content-Type:

text/plain

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text/plain (218 lines)

Alan,
would appreciate if you could share with us an example (publication, 
etc.) demonstrating the 3D "balancing"
approach. This might help Koshik and others in analyzing fold 
interference patterns and poly-phase deformation systems.

Thanks
Hermann

On 2/14/12 4:43 AM, Alan Gibbs wrote:
> John, we have all witnessed cars being badly driven but that does not put us
> off driving, or the derived benefits.
>
> The balance problem is 4D. 3d space + geologic time. If, as you imply, you
> simply take a restrictive number of 2d sections (commonly one) and then make
> some very simple end member assumption (eg line length conservation) you can
> end up with some forced solution that is not geologically valid. However if
> you take the same assumption and same section(s) and recognise that the
> solution is not geologically valid you have learned something, hopefully
> identified one or more key uncertainties in your knowledge and
> interpretation and also placed some numerical bounds on your observation.
> That in itself is worthwhile.
>
> By iterating, using different end member assumptions and using more sections
> you should be able to see your interpretation converge on something
> predictive and useful. If you have access to the full range of existing toys
> you can begin to use geomechanical rather than geometric constraints
> releasing you from plane strain assumptions and you can also balance in full
> 3d using both geometric and geomechanical constraints. Of course you may
> need add and subtract volume through chemical and thermal process too where
> your geological history dictates but these too must be broadly quantifiable.
>
>
> Personally, I have yet to see an interpretation that hasn't been improved by
> the geoscientist using a systematic approach to quantifying kinematics and
> using "balance" as one of the key techniques. The recognition of problems
> and where the interpretation is under-constrained that comes from doing this
> is invaluable.
>
> Indeed the recent contribution in Geology, 2012;40;70-78 by Bond et al
> clearly shows that interpretation accuracy is improved by a factor of three
> as soon as tests for geometric and evolutionary feasibility are applied.
>
> That has to be a gain worth trying for, doesn't it? And Koushik should be
> congratulated for trying to constrain his interpretation in this way even
> though his area of superposed isoclinal folding will be a tough nut to
> crack.
>
> 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: 14 February 2012 09:59
> To: [log in to unmask]
> Subject: Re: the effect of superposed folding on estimation of crustal
> shortening and cross section balancing
>
> Dear Alan,
>
>    My point is similar to yours and I do not think that there is real
> disagreement. Perhaps the commonly implied goal that section analysis has to
> balance stratal or crustal unit length, and that there is no internal,
> commonly differential strain, is the problem. Most of the sections with
> which I am familiar indicate that material has moved into or out of the line
> of section (s), and that there has been differential internal strain,
> rendering the construction of unique depth sections difficult or impossible.
> Another problem is that seismic sections, for example, are, commonly not
> thought of as being made of rock  with a great range of possible small-scale
> structures that, collectively, can add up to substantial strains. I have
> witnessed balanced section drawing that has forced the composer into
> impossible contortions because stratal length balancing has been assumed.
> This is another form of abstract art or section scribbling as is the
> assumption of a particular fold style such as fault-bend-folding. The arid
> arguments that surround the planar versus listric fault "problem", and the
> granite origin and emplacement "problem", are further examples of diversity
> and that there are several or many solutions and ways of tacking a problem.
> The value of section balancing, at all scales, is that it gives one rough
> ideas, limits,  constraints, and raises problems. It is difficult in simple
> sections let alone in polyphase-deformed rocks.
>
> Best wishes,
> John
>
>> 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)
>
> --
> 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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