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.
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
> Dr Alan Gibbs
> Midland Valley Exploration
> 144 West George Street
> G2 2HG
> tel: 44 (0) 141 332 2681
> fax: 44 (0) 141 332 6792
> [log in to unmask]
> -----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,
>> 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.
>> Dr Alan Gibbs
>> Midland Valley Exploration
>> 144 West George Street
>> G2 2HG
>> tel: 44 (0) 141 332 2681
>> fax: 44 (0) 141 332 6792
>> [log in to unmask]
>> -----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
>>> 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?
>>> 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
>>>> Dr. Koushik Sen
>>>> Scientist 'B'
>>>> Wadia Institute of Himalayan Geology
>>>> Dehra Dun- 248001
>> 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,
>> 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
> Sherwood Lodge,
> 93 Bagley Wood Road,
> 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)