Fold
Þ Def: undulation or wrinkles present in the rocks at
the Earth surface is called fold.
Terminology of Fold
Þ Attitude of Beds: it
refers to the three dimensional orientation of the some geological feature,
such as a bed, a joint, a hornblende, needle or a fold.
Þ
Strike: The orientation of rock strata with the horizontal surface is
called strike.
Þ
Dip:
The angle of inclination of rock strata with respect to horizontal surface is
called dip.
The angle of between
Þ
Hinge:
the hinge of a fold is the line of maximum curvature in a folded bed; it is
characterized by orientation and Position.
Þ
Axial Plane: it is the surface connecting all hinges. It may be simple plane
or curved surface.
Þ
Axis of Fold: it is a line parallel to the hinges.
Nomenclature of Fold:
Þ
Anticline: it is defined as a fold that is convex upward,
and older rocks in the center, the word is from the Greek word meaning “Opposite
Inclined”. The two limbs dip away from each other.
Þ
Syncline: it is defined as a fold is convex downward. The word from Greek
word meaning “together inclined” the two limbs dip toward each other.
Þ
Symmetrical Fold: a fold is one in which the axial surface is essentially
vertical.
Þ
Asymmetrical: A fold is defined as one in which the axial surface is
inclined.
Þ
Overturned Fold: or over fold. The axial plane is inclined and the both limbs
dip in the same direction usually at different angles.
Þ
Recumbent Fold: it is one in which the axial plane is essentially horizontal.
Þ
Isoclinal Fold: it comes from the Greek word meaning equally inclined, refers
to folds in which the two limbs dip at equal angles in the same direction.
o
A vertical Isoclinal Folds: it is one which the axial plane is vertical.
o
Inclined or overturned isoclinal fold:
it is one in which the axial plane is inclined.
o
A recumbent fold: it is one in which the axial plane is horizontal many recumbent
folds are isoclinical.
Þ
Chevron Fold: it is one in which the hinges are sharp and angular.
Þ Box Fold: it is one in which the crest
is broad and flat, two hinges are present one on either side of the flat crest.
Þ Fan Fold: it is one in which both
Limbs are overturned. In the anticlinal fan folds the two limbs dip towards
each other, in the synclinal fan fold, the two limbs dip away from each other.
Þ
Kinks Bands: they are narrow bands usually only a few inches or few feet
wide, in which the beds assume a dip that is steeper or gentler the in the
adjacent bent beds.
Þ
Closed or Tight Fold:
It
is one in which the deformation has been sufficiently intense to cause flowage
of the more mobile beds, so that these beds thicken and thin.
Þ
Open Fold: it is one in which this flowage has taken place, although the
more extreme causes. These two types may be readily distinguished from each
other; they are intermediate examples that are difficult to classify.
Þ
Drag Fold: it form when a competent ‘Strong’ bed slides past and
incompetent ‘weak’ bed such miner folds may form on the limbs of larger fold.
Þ
Dome:
it is an anticlinal uplift that has no distinct trend.
Þ
Basin:
it is a synclinal depression that has no distinct trend.
Folding
Folding is a process that
embraces all geologic processes by which rocks of the earth surface become
curved during deformation. Since folds are permanent deformation structures
with no or little loss of cohesion of the folded layer, folding refers to the
essentially slow, ductile behavior of relatively soft and/or hot rocks.
In general three principle types of
folding may be recognized, but these are actually ideal limiting cases and
transitions and combinations are common. These types are;
1- Flexure folding:
It also sometimes referred as
true folding. For purpose of analysis the behavior of flat beds under a
compressive force acting parallel to the bedding may be considered. If a sheet
is bent the convex side is subjected to tensional force whereas the concave
subjected to compression. There is an immediate surface of no strain. Usually
the rocks are sufficiently plastic so that rupture does not occur. The convex
side will lengthen and thin, whereas the concave side will shorten and thicken.
The unique feature of sedimentary
rock is the presence of bedding planes, and a very important factor is the
sliding of beds past one another. Some geologists refer to this type of folding
as flexural-slip folding. This concept is of great importance in interpreting
certain types of drag folds.
2- Shear folding:
It is also known as slip
folding, results from minute displacements along closely spaced fractures. Each
fracture is actually a minute fault. If, however, the fractures are only a
fraction of an inch apart, and the beds because friction, tend to parallel the
fractures, the resulting structure is a major fold with many associated minor
folds. In shear folding the beds are thinned but never thickened.
3- Flow folding:
In this process the deformation
is analogues to the lamellar flow of liquid.
Stress and Stain
Stress:
The mutual action and
reaction along a surface or plane between two bodies is called stress.
Uniform stress describes
the situation where the stress is equal in all direction, such as stress on a
body immersed in a liquid or gas.
Uniform stress in rock is also called confining stress because
any rock body in the lithosphere is confined by the rocks around it and is
uniformly stressed by the weight of surrounding rocks.
Confining stress cause a
body to change the size but not shape, this type of deformation is called dilation.
Differential stress is stress that is not equal in all
directions. This type of stress causes the rock to change its shape.
The deformation in which size and shape of body changed is
called distortion.
If the stress tends to
push together the material on opposite sides of the plane, it is referred as
compressive stress.
The tensional stress tends to pull apart the material on
opposite sides of the plane.
Shear stress causes slippage and translation. Shear stress is
parallel to the y-axis and vertical to x-axis.
Differential stress
results from the tectonic forces.
Strain
Strain is defined as the
change in size or shape or both in a solid as a result of stress. Strain may be
dilation, which is a change in volume, or distortion, which is change in form
or both. When there is a change in the confining pressure, an isotropic body
will change in volume, but not in shape. With increasing confining pressure
volume of the body decreases and the dilation is negative. With decrease of the
confining pressure, volume of the body increases and dilation is positive.
Under directed stresses distortion occurs, which tends to change the shape or
size or both of the solid body
Types of strain:
è Dilation change in volume is
called Dilation.
è Distortion is the change in the
shape of the body.
Stages of Deformation:
1.
Elastic
Deformation: An elastic deformation is one that Elastic substance will return
to the original shape after removal the stress.
2.
Plastic
Deformation: it is a deformation in which material does not return to its
original shape is called Plastic Shape.
§
Brittle:
A deformation that rupture before any significant plastic deformation.
§
Ductile:
a deformation that undergoes a large plastic deformation before rupture is
called Ductile Deformation.
Differential stress: if stress is not equal from all direction then we say that the
stress is a differential stress.
Three
kinds of differential stress occur.
Tensional stress: Which stretches rock ß[]à.
Compression Stress: which is compress or squeezer rock à[]ß.
Shear Stress: twisting of a rock or which result in slippage and translation.
The
stress which is parallel to y axis is called shear stress.
Force
Force:
it is a vector quantity
that changes or tends to produce the change in state of the body. Force is
defined by its magnitude and direction. An unbalanced force is one that causes
a change in the motion of the body. Balanced forces exist where no change in
motion occurs
Units of measurements:
Various systems are adopted to measure the force, such as MKS,
CGS, BES and SI systems.
The unit of
measurement of force in MKS and SI system is same which called NEWTON.
1 NEWTON is defined as when a mass of 1kg travels 1 meter in 1
second, the force that moves the body is said to be 1 Newton.
The unit of
measurement of force in CGS system is dyne. 1 dyne is defined as when a mass of
1g travels 1 cm in 1 second, the force that moves the body is said to be 1
dyne.
The unit of
measurement of force in BES system is poundal. 1 poundal is defined as when a
mass of 1 pound travels 1 foot in 1 second, the force that moves the body is
said to be 1 poundal.
Litho static or
confining pressure:
Rocks in the lithosphere, because of the weight of rocks lie
above them are subjected to a kind of pressure. This type of pressure is called
litho static pressure. In experimental works, the equal, all sided pressure on
solids is called the confining pressure. The litho static pressure increase
with depth, increase in litho static pressure causes a decrease in the volume
of rock but increase in the density.
Differential forces: In many cases the forces acting on a body are not equal from all
sides. A body is said to be under tension when it is subjected to external
forces that tend to pull it apart. A body is said to be under compression when
it is subjected to external forces that tend to compress it. A couple consists
of two equal forces that act in opposite directions in the same plane, but not
along the same line.
Torsional Force: the force which twists the body is called the Torsional force.
Couple Force: a couple consists to equal forces that act in opposite
directions along the different line.
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