In accordance with Hobbs, 1971 the geometrical classification of fold morphology is important in expressions of strain.

As far as meaningful conclusions of Ramsay classification (1967) is concerned, the folds of class IA and 3 indicate differential compression in their evolution, class IB and IC suggest a flexure-slip mechanism and class 2 suggests a slip mechanism in the fold formation.

I guess that the aim of the classification is to find out the ancient strain direstions (as CK Singh mentioned above) and the type of the in-situ deformation conditions. The shape, dimensions and repetition of the fold structure are the key features to poin out the tectonic environment of the geological media during his folding procedure.

In order to understand the genesis of a sedimentary fold structure, on must know the mode of deformation – a plastic bending or a viscous flow.

By applying magnetic methods, we were able to discriminate between those two modes and demonstrate that assumed liquefaction structure, indeed, were formed by fluidized sediments (Mörner & Sun, 2008; EPSL, 267: 495-502).

A fold classification is useful for systematic description and study of folds, but the conclusions we draw from them on the mechanisms of their formation are only going to be accurate, depending on how successful we are in making judgments on (a) the ductility contrasts between layers that have been folded, (b) the stresses and the strain history (including strain rates) the rocks have suffered during the deformations concerned. Perhaps, theory + self-testing of our proposed mechanisms through simulations may enable us to close the gaps between the reality and interpretations.

Thanks for all your answers so far! I think that many of the features mentioned can be deduced from folds without any attempt at classification: E.g. strain directions from the orientation of fold axes and other features; the existence of a penetrative strain from systematic variations in layer thickness (but there you will often have cleavage or foliation as an additional indicator anyway). Competence contrasts are probably the parameters you could hope most to deduce from fold shape variation across different layers. Identifying shear folds on the basis of class 2 geometry alone would seem hazardous to me.

The classification gives you the systematic and scientific description of the geometry of folds, which helps to separate folds with different geometry and that in turn may be useful for many a purposes such as to assess intensity of deformation, use for shear sense indicator, change of orientation of stress during folding process, to understand rheology of folded layers effective during folding, etc. I suggests to go through review papers in JSG by Huddleston and Treagus (32) 2010.

Study of a given fold may not tell you the truth if its profile, directions of extension, compression and shearing in the volume of deformation the fold is residing have changed during later deformations. Then one will have to subtract the later (overprinted) effects if the folds' original features and its mechanism of formation are to be inferred.

“Classification” of an entity is done with some purposeful intentions. Classifications can be on genetic, geometric, compositional and so forth. In cases of folds attempts have been made to geometrically classify the fold forms and relate them to their causal mechanisms, as has already been discussed. It is geometry that we visually appreciate, numerically model and encounter foremost and hence using geometric constraints has been the first choice for such a classification.

But interestingly the discussion has swayed from the topic of interest.

Superposed folds are imprint of one fold form onto another, and it is possible to filter out the geometry of each episode separately

I agree with Jonas. There is sometimes an unfathomable need to classify folds, which is sometimes not useful at all. Different classes of folds can occur in the same rocks and simply imply that strain is heterogeneous - something we all know. In a pragmatic sense, many people use fold geometry without going to effort of the Class 1B - Class 3 classification and still get very meaningful information e.g. economic geologists working in fold-hosted deposits. In all honesty, there are a lot of structural geologists but only a small percentage who actively classify folds. So, what is the use of fold classification? Sometimes there is none. It has to be taken in the context of the research being undertaken and should not be considered a common necessity. And, on a digression from this topic, I note that a lot of people seem to want to over-classify all sorts of things e.g. boudin shapes, porphyroblast geometries etc. Although this can be useful it often represents documentation of rare, ideal examples and only serves to get the author another obscure publication.

Systematic classification is an essential part of any science. Proper detailed classification allow for re-understanding of (the same) facts when new knowledge is at hand, which in turn leads to contradictions and paradigm changes - I consider this, by itself, enough to systematically classify folds.

That useful information can be extracted without fold classification does not overrule the need of it. Fold classification is key to understand the mechanics behind their formation. Strain heterogeneity is precisely what makes the classification useful. As (classified) data population increases, so does our understanding on the distribution of strain and the quality of the inferred conclusions on stress drivers.

Hi David - thanks for your input. Can you give some real-life examples where fold classification has been useful for you?

Hi David. I can't say that I agree entirely with your comments. Perhaps I am reading your answer incorrectly but the reasoning seems a bit circular. Are you saying that we should classify folds to get information on the strain so that we can classify the folds? And why deal with folds for the strain analysis? You have to make many assumptions, such as uniform thickness of pre-folded layers, presence of heterogeneities, non-variation of pre- and post-folding heterogeneities in the 3rd dimension etc. Outside of the realm of pure structural geology, can you give any examples of the pragmatic, useful application of fold classification in the strict sense of Class 1A to Class 3?

Hi Collegues,

I have follow the discussion about fold classification system and I saw that mainly the structural geologists are interested with this topic. I have read the question again and I think that this question includes more curiocity about some other possible outputs of such kind of classifications like economic or social point of view. This question can also evaluate as "what are the outputs of the fold classification in reservoir geology or seismic-risk analysis?", etc...

Sincerely all...

I agree Yahya. Although I suspect that the majority of Class 1A to Class 3 fold classification has been undertaken on hand-specimen- to outcrop-scale folds and rarely at greater scales. At least geologists have a chance to examine fold attributes in 3D at these scales. Folds of larger scales are simply open to containing many more heterogeneities and controls on their development and the classification will show folds belonging to more than one class. In this sense, the usefulness of the classic fold classification is somewhat limited, especially if we are talking about reservoir geology, poly-deformed folded base metal deposits, folds that have been reactivated (see Bell, 1985 for the original definition) etc. I prompted David Fernández-Blanco a couple of times for some useful applications of fold classification but he seems to have declined to reply.

Love the answer by B.K. Davis! I used to follow classic textbook and teach those classifications in my first years teaching; I now barely mention them in a couple of quick slides, and devote a lot more time on things like map reading, microstructures (calcite veins at an angle to bedding to deduce sens of shear in the flanck of a fold, obliquity of shistosity on bedding... Which are usefull to deduce a vertical strata polarity and sens of mouvement), and observations of folds with axis perpendicular to stretching lineation vs axis parallel to lineation. These are usefull for any geologist on the field whereas I have never seen a geologist wondering wether the fold at hand was type 1C, 2 or 3...(many students have difficulties deciphering between the axis and the axial plane for god's sake! So why waste my time on isogones?!)

I suppose this classification participated to a trying effort of quantification in earlier times but it has not proved to be very usefull in the long haul. I notice this need to define and arrange any structural feature in well separated shoeboxes is very pregnant in British textbooks, but a lot less in books from other countries (particularly in France in books from the like of Maurice Mattauer or Adolphe Nicolas).

As good music which most times encompass several genre (pop, rock, reggae, all in the same song), many usual and real-life examples of geological structures do not fit in a single category (unlike idealized drawings).

Good question… Please check references below.. hope it will be helpful for you…

Regards…

http://www.soest.hawaii.edu/martel/Courses/GG303/Lecture_28

http://shodhganga.inflibnet.ac.in/jspui/bitstream/10603/152653/15/14%20chapter%207.pdf