Presidency University, Kolkata

1. Folds: That we see and That we predict
Rohan Roy UG II, Presidency University
Introduction: 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.
Deferenttype 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.
Fig: 1 The Greek island of Crete
Fig:2 Layer parallel shortening Fig:3 Subduction folding

2. 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 meaning about d term.
Deferenttype 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,
Ptygmatic Fold:
Shortening=15%
Rotating>= 0%
Strainrate= high
Shortening=
30%
Rotating= 30%
Strainrate =
moderate
Shortening=70%
Rotating= high
Strainrate = low

3. Cuspate lobate fold:
Folding by Underthursting (at subduction zone):
So, here we discuss about the folding by Under thrusting at plate boundary. To investigate the
process, one should know the mechanical properties of mantle lithosphere.
Equation 19.4 is the Biot’s linear theory,
Shortening=10-
13%
Amplificationrate
= 0
Rotation= 0
Shortening=30%
Amplificationrate
= 0
Rotation= low
Shortening=50%
Amplificationrate =
high
Rotation= high
The spectrumof foldshapesproducedby bucklinga
competent layer in a less competent host material.
The upper ptygmatic fold shows the maximum
viscosity contrast µ1/µ5, the lowest cuspate-lobate
foldthe minimumcontrastµ4/µ5andthe twomiddle
folds have intermediate viscosity ratios µ2/µ5 and
µ3/µ5. (Ramsayand Huber) (Fig:4)
Fig:4 alternatingcompetence and
incompetence layerof varying
thickness
Source: Ramsay and Huber

4. Biot’s linear theory encompasses the cases where compression of the lithosphere takes place in
the beginning and continue through the formation of highs and basins.
Firstly, Burov’s experiments shows a mechanical decoupling between the crust and the upper
mantle.
So, here is the overview of the strain accumulation in the Moho line where the decoupling
occurred,
To describe the mechanical properties of mantle lithosphere Burov and Daiment (1995) have been able to
show that a model in which a weak lower crust is sandwiched between a strong brittle elastic upper crust
and an elastic ductile mantle.
Recently in 2002 Jackson proposed two type of model to describe the mechanical properties of
mantle lithosphere and its stability. That’s are,
Crème Brulee model: The mantle is weaker than the crust and the crust at the Moho depth is also
weak and the system is decoupled.
Jelly sandwich model: The mantle is stronger than the crust and the crust at the Moho depth is also
strong and the system is mechanically coupled.
Fig:Crème Brulee Model Fig:JellySandwichModel
Fig:5 Stressaccumulationduringlayerparallel shortening

5. The physical considerationof mantle lithosphere:
The crème brulee and jelly sandwich model represented the mechanical properties of mantle
lithosphere.
If the crème brulee model is consider in that system, so the mantle lithosphere is mechanically
indistinguishable from asthenosphere, which suggest low viscosity. The mean heat flow in the Archean
cratons is 40mW/m2. A significant part of heat flow is coming from the radiogenic isotope. If the Moho
is relatively low viscous material, the heat flow would increase >150mW/m2. But it is not happening.
(Which would be the case in an actively extending rift)
So the mantle must maintain a fixed, restively high, viscosity that prevent the convective heat
advection to the Moho.
Since this type of heat flow is not observe in the orogeny and cratons, a thick, cool, stable mantle
layer should prevent the heat convection between the crustal part of the lithosphere and the convective
upper mantle.
This picture shows the deformation after 300 km of shortening, which at 60mm/yr takes 5 myr. The jelly
sandwich model is stable, and subduction occurs by the under thrusting of a continental slab that, with or
without the crust, maintain its overall shape.
The crème brulee model on the other the hand, is unstable. So, there is no subduction. The crème
brulee model is there for unable to explain the collision systems and metamorphism.
On the other hand, the jelly sandwich model can explain the metamorphism as well as explain the
structure formed by the collision.
CONCLUSION:
As we discuss earlier, folding is a natural process that happens due to variation of
stress. Here I discuss about the process of folding and their classification that maybe help us to start
gathering the knowledge and it may help to grow the interest about folding mechanism. Here I discus about
the mechanical properties of mantle lithosphere which helps us for further research. Here I discuss about
the crème brulee and jelly sandwich model and why jelly sandwich model should be accepted. The
discussion and the minor explanation about the subduction zone folding is totally theoretical. But we should
start from somewhere to reveal the suspense which is still unknown to us. This article atleast grow the
interest about structural geology as well as it can be a starting point for further explanation of this same
topic in future.
Hence Albert Einstein said,
“IMAGINATION IS MORE IMPORTANT THAN KNOWLADGE.
KNOWLADGE IS LIMITED BUT IMAGINATION ARTICLES THE WORLD.”