BOUDINAGED FOLDS AND FOLDED BOUDINS
We know that the strain ellipse, whether finite or infinitesimal, contains areas of positive extension (with elements getting increased in length, or the boudinage as the naturally occurring phenomenon) and the negative extension (which implies folding or contraction or negative extension). When the incremental strain ellipse is superimposed onto a finite strain ellipse with the principal strains of both not being coincident, four distinct zones or areas of overlap are produced. The diagram given below shows this feature. Top left is the finite strain ellipse or the strain that has occurred while the one at top right shows the incremental strain ellipse or what might happen if this is superimposed on preexisting finite strains on lines or planes oriented differently with respect to both, the principal finite strains and principal incremental strains.Zone I: is bounded by lines of ef=0 and ei=0 or the one of +ve ef and positive ei. This implies that the lines were extended by +ve finite extension and will further extend by +ve incremental extension. Geologic situation is that of planes that are broken by boudinage and will continue to suffer boudinage on superimposition of incremental strain. Further boudinage may occur or the boudins may be separated by further extension. As it may require tremendous amount of mechanical work to further break a layer which is already broken by boudinage, the natural situation might be the separation between the individual boudins. This zone is therefore the one of ACTIVE BOUDINAGE.
Zone II :is of -ve finite extension followed by +ve incremental extension. The lines that were contracted (or planes folded) now fall in the +ve extensional area of the incremental and the lines must therefore extend or planes unfold and then give rise to boudins. As the rocks are very heterogeneous in nature, it is not possible for folded layers to get unfolded; rather they get disrupted by boudinage. This zone is therefore one of BOUDINAGED FOLDS.
Zone III :shows the one f -ve finite and -ve incremental extension. This implies continuous contraction or folding. The gentle folds generated by finite -ve extension get tight and highly amplified by further -ve incremental extension. This zone is the one of ACTIVE FOLDING.
Zone IV: is one of +ve finite extension but -ve incremental extension.
The lines that were extended are now going to contract because of -ve
incremental extension. The result which is geologically realistic is the
development of structures that could best be described as FOLDED BOUDINS.
See the photographs of folded boudins in the garnet staurolite schist rock
at the base of the Delhi Supergroup rocks near Badnore in Central
Rajasthan elsewhere in this collection.