Presidency university, Kolkata

1. Classification of Folds and Folding
presented by
Ritanuka Ghosh and Rohan Roy
Presidency University, Kolkata
So what is fold or folding?
Fold is a curvature on a planner surface. The fold formations are classified in some
ways. Those are, active folding or buckling, passive folding or bending, shear
folding and flextural slip with buckling. We can find fold in different condition. For
example, if we hit on a cushion, a curvature is produced due to the pressure. A fold
can develop in lithosphere due to the vertical pressure of the overriding mountain or
island arc. So that's how we find lots of fold in the crust and the geoscientists are
eager to know the geometry and the mechanism behind these.Fold is also important
for its economical value. Because fold can be the reservoir of fossil fuels. Also, fold
is the evidence of earth's ductile behavior and from the structure of fold we can find
the rock rheology.
The tectonic shortening process further classify into three classes.
1. Volumetric shortening by distributed or localized thickening of lithosphere due
to compression.
2. Folding when shortening in accommodate by unstable, subparallel, sub
periodical, vertical upward and downward bending of the lithosphere.
3. Underthursting of mantle lithosphere at subduction zone.
Deferent type of folding due to single competent layer buckling:
We consider a competent layer surrounded by two thicker incompetent layers.
Wi is the initial wavelength and Wa is the arc length,
Also, µ1 is a competent layer and µ2 is a less competent layer,
Folding by layer parallel shortening:
Homogeneous shortening of the lithosphere under horizontal compression
requires a large amount of work than that required by an equivalent amount of
shortening by folding.
Here we consider the medium as a Newtonian medium,
If µ1/µ2 ratio is 1 then Wi/d = 3.46 and if µ1 = µ2 there is no
Bending fold :
Another important type of folding is bending. It is mainly occure due to
vertical forces which is driven by such tectonic activity. In this type of
fold the stress direction and the flow direction are parallel. Bending fold
can occure in various places i.e. fault duplexes, exhumation of pluton
give a fold in the overlain strata. Bending fold is mainly localised rather
than regional.
At thrust zone the upper lithological unit get folded and form duplex model,
this can be hinterland dipping duplex of foreland dipping duplex. For example,
as Himalaya form due to the collision of indian plate with the eurasian plate
and as indian plate is heavier so it subducted beneath the eurasian plate and
form several thrust plane and shear zone. Here we see several duplex fold
due to thrusting. Mainly the thrust zones are divided into four distinct category
due to their metamorphic facies or fromation history. Those are MFT, MBT,
MCT and ITSZ. Folding due to thrust fault is common phenomenan at several
orogenic belts. The model of duplex is presented underneath,
Passive Folding:
This type of folding mainly form due to shearing wheather that is simple shear
or pure shear.This type of folding is common in orogenic belt. Class 2 type of
fold can be formed by this mechanism. There is no precise direction of applied
stress rather the material can flow in a ductile manner. Passive foldind forms
harmonic folds that have no mechanical role so no influence of certain fold
shape. Passive folding occure where the passive layer come into the shear
zone or affected by the heterogeneous strain. Passive folds are frequently
found in the mylonite zone or in some monomineralic rock i.e. quartzite, maeble
etc.
Polyhermonic folding:
Rather than discussing about hermonic and dishermonic
folding I would like to intoduce polyhermonic folding where the
zone of contract strength of layers of different thickness
overlap each other and the layer of minimum thickness shows
low amplitude and layer of maximum thickness shows highest
amplitude.
If that larger fold and it’s assosiated minor folds have the
parallel axial plane so that theye are called Congruous fold.
That means they are formed by the same crustal deformation.
Pumpelly’s Rule:
1. The median surface of the
higher order folds defines the lower
order folds.
2. The asymmetry of the higher
order fold changes across the axial
surface of the lower order fold.
We find Z type asymmetric fold at
lest limb, S type asymmetric fold at
right limb and M type symmetric
fold at hinge zone commonly.
Flexural Slip:
This type of fold occure when
interlayers of beds slip at the
contact and form fold with a
overall same thick i.e. Class
1B fold. The intensity of folding
is high at the inflection point
and getting smaller at the
hinge zone.This type of fold is
common in the upper crustal
brittle regime. Slicken lines are
observed at the contact
surface or at the slip surface.
Flexural Flow:
This type of fold occure at the
regime of plastic
deformation.At this type of
folding the incompetent mass
act as a welding material
between two competent layer.
As this type of foldind occure
due to buckling, the
compression creats a reverse
fault in inner arc and the
assosiated extension makes
normal fault at outer arc.
Ackmowladgement:
A special thanks to Prof. Santanu Bose for suggesting
such a topic also thanks to Prof. Gautam Deb for the
information and also thanks to HOD Prof. Gautam
Ghosh for support and encouragement.
References:
1.Modern Structural Geology by Ramsay and Huber
Volume: 2
2. Structural Geology by Haakon_Fossen
3. Structural Geology by Twiss and Moores