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)
|
