There is an important thread of common ground between the two
perspectives provided by John and Alan on the value of cross section
balancing techniques. And that is that it is critical to apply methods
only where they are appropriate, and to not interpret the results too
far beyond the limits of the simple models you are applying to a complex
process.
Line length balancing in 2D sections is a gross simplification of
reality, and is only a reasonable approximation in cases of highly
layered rocks with fairly small amounts of deformation. Even in these
simple cases the errors are commonly seen to be on the order of 5% or
more. Once the deformation approaches foldbelt scale the errors caused
by internal strains begin to overwhelm the technique. Many field studies
in foldbelts have documented that internal deformation (subseismic scale
folds and faults) begin to take up more than 10% of the deformation, all
of which would be lost to line length techniques. Regional restorations
of detached extension-contraction systems, such as those in gravity
driven delta systems, show that the seismically measurable shortening at
the toe is often a minority of the demonstrable extension at the head.
This implies that the bulk of the deformation is happening at subseismic
scale via internal deformation of the units. By the time we get to the
deformation intensity of anything you would call an orogenic system, the
line length methods are clearly pointless as conservation of bed length
is demonstrably not taking place.
Basic area balancing techniques are a bit more art, and a lot less
quantitative, but are the best we can do in cases involving rocks which
we know will not maintain bed length (salt, mobile shale, highly
deformed sections), and conservation of mass (volume in 3D, area in 2D)
is the only anchor we have left. Because of all of the problems of
out-of-plane deformation, 2D application of these methods can only
provide insights as to the order of magnitude of the bulk deformation.
But as loose as these estimates might be, they do provide some level of
sense check on cross section construction.
Over the years I have found a lot of value in applying section balancing
techniques to the construction of cross sections. But it is critical to
recognize the limitations of such simple models. Most of the benefit I
have gained from section balancing has not been from the quantitative
estimates of shortening or extension, but from the simple geometric
logic that you are forced to apply in retrodeforming a section. Do the
faults connect up in logical fashion in time and space? Do the slips on
the faults diminish away from the driving force? Do the folds correspond
in time and space to appropriate fault bends or slip gradients? Do the
implications for what was locally going up or down match the evidence
from growth stratigraphy? The closer you come to constructing a section
where all of these basic observations make sense, the more credence I
would give to the result.
Scot Krueger
Senior structural geologist
BP Americas
-----Original Message-----
From: Tectonics & structural geology discussion list
[mailto:[log in to unmask]] On Behalf Of Alan Gibbs
Sent: Tuesday, February 14, 2012 4:43 AM
To: [log in to unmask]
Subject: Re: the effect of superposed folding on estimation of crustal
shortening and cross section balancing
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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