By: S.Qutub Ali Shah
Structural geology is the study of architecture and
geometry of the earth surface, developed as result of deformation.
Structure
The word “structure” is derived from Latin word “Struere”,
which means - to build. Structure is any shape or form having dimensions.
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.
Most problems confronting the structural geologists may be
analyzed by assuming balance forces because the velocity of rock bodies is so
small that acceleration is negligible. Along faults, however, the motion
causing earthquakes may be so rapid that acceleration is important.
Units of measurements:
Various systems are adopted to measure the physical
quantities, 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 unbalanced force
that will give a mass of 1 kilogram an acceleration of 1 meter per second per
second.
The unit of measurement of force in CGS system is DYNE.
1 Dyne is defined as when a force
accelerates the object of mass 1 gram, 1 cm per second per second is called 1
dyne.
The unit of measurement of force in BES system is POUNDAL.
1 Poundal is defined as when a mass of 1
pound accelerated 1 foot per second per second, the force that accelerate the
body is said to be 1 poundal.
Composition and resolution of force
Force may be represented by a vector that is a line
oriented in the direction in which the force is operating and proportional in
the length to the intensity of the force. Two or more forces can operate in different direction at a point as
shown in figure, where OX and OY act at O. the same result would be produced by
the force OZ acting in the direction indicated; OZ is the resultant of OZ and
OY. A resultant is the single force that produces the same result as two or
more forces, and it may be represented by the diagonal of the parallelogram
constructed on two arrows that represent the two forces. The equilibrant is the
force necessary to balance the two or more forces. OW is the force necessary to
balance OX and OY or OZ. it is equal to the resultant of the two forces, but
acts in the opposite direction. The process of finding the resultant of two or
more forces is called the composition of force.
The effect of a single force may be considered in terms of
two or more forces that would produce the same result. In the figure shown
below, OY and OZ would produce the same result as OX. A single force may thus
be resolved into two components, acting in defined directions by constructing a
parallelogram, the diagonal of which have the directions of the components. The
process of finding the components of a single force is called the resolution of
force.
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.
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.
Compressive Stress: The stress which tends to push
together the material on opposite sides of the plane is called the compressive
stress.
Tensile Stress: The stress which tends to pull
apart the material on opposite sides of the plane.
Stress Difference: The algebraic difference between
the greatest stress and the least stress at any point is called the stress
difference.
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 and distortion.
Dilation: It is the type of strain, occurs
when the volume of the body changed but not the shape. 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.
Distortion: It is the change in shape of the
body or both the volume and shape, because of applied stress. Under directed stresses
distortion occurs, which tends to change the shape or size or both of the solid
body.
Stages of deformation
If any object is subjected to some force, it will respond
in three ways to the applied forces that are called the stages of deformation.
These three stages of deformation are described below.
Elastic Deformation: If the applied forces are
removed, the body will return to its original size and shape. This type of
deformation is called the elastic deformation.
Plastic Deformation: Any object has its own strength
to resist deformation and overcome the effect of applied force which is called
elastic limit. If the applied force exceeds the elastic limit, the object
starts to deform plastically. A plastic deformation is one in which the
specimen does not return to its original size and shape after the removal of
force but to an extent it tries to return in its original state.
Rupture: When there is a continuous
increase in the stress, the fractures developed in the object and it fails by
rupture. Rupture is the permanent deformation.
Mohr’s circle of stress:
The relation between stress and rupture may be determined
graphically by Mohr’s circle of stress.
Consider a cylinder, the ends of which are subjected under
confining pressure hence compressed. Thus the greatest principle stress axis is
σ1 is parallel to the axis of the cylinder, whereas σ2 and σ3 are equal.
The figure is the plot of Mohr’s circle of stress. The
origin is at O, the vertical axis is shearing stress. The value of the
confining pressure σ3 and compressive stress σ1 are plotted on the horizontal axis. The stress difference
is (σ1 - σ3). A circle is drawn through σ3 and σ1 with the centre on horizontal
axis, the centre of the circle is (σ1 + σ3 /2), and the radius is (σ1 - σ3 / 2).
Assume a plane making an angle ϴ with the greatest
principle stress axis. The line cl is plotted to make an angle 2ϴ with the
horizontal axis, 2ϴ is plotted clockwise from the horizontal axis. This line
cuts the circle at l. The co-ordinate of l, which are Ʈ/ and n/,
give the shearing and normal stresses on the plane.
On a plane parallel to the greatest principle stress axis
(2ϴ=0), the normal stress across the plane is σ3 and the shearing stress is 0. If
the plane makes an angle of 45o with the greatest principle stress
axis (2ϴ = 90o) the shearing stress is at maximum and the normal
stress is (σ1 + σ3 /2). If the plane makes an angle
of 90o with the greatest principle stress axis (2ϴ = 180o),
the shearing stress is 0 and the normal stress is σ1.
Normal stress n/ = σ1 + σ3 /2 - σ1 - σ3 / 2 Cos 2ϴ
Shearing stress Ʈ/ = σ1 - σ3 / 2 sin 2ϴ
FOLDS
Undulations or wrinkles present in stratified rocks on the
surface of earth are called folds. Some folds are miles of across, the width of
others is to be measured in feet or inches or even fractions of inch. The size
of exposure determines the size of folds. Folds of many thousands of feet may
be observed in the areas of high relief, whereas the exposures are small, folds
of few feet or tens of feet across may be observed.
Parts of fold
Hinge:
The hinge of a fold is the line of maximum curvature in a
folded bed. It is characterized by orientation and position. There is a hinge
for each bed. The hinge may be horizontal, inclined or vertical.
Axial Plane:
The axial plane is the surface connecting all the hinges.
It may be simple plane or curved plane. It also may be horizontal, inclined or
vertical.
Axis:
The axis is a line parallel to the hinge. It is the
straight line moving parallel to itself that generate the fold.
Limbs:
The sides of folds are called limbs of fold. A limb extends
from the axial in one fold to the axial plane in the next.
Crest:
The line connecting the highest points on the same bed in
an infinite number of cross sections is called crest. There is a separate crest
for each bed. The plane or surface formed by all the crests is called the
crestal plane.
Trough:
The trough is the line occupying the lowest part of the
fold or the line connecting the lowest parts of the same bed in an infinite number
of cross section. The plane connecting
such lines may be called the trough plane.
Nomenclature of folds
Anticline:
The word is driven from Greek word which means “opposite
inclined” referring to the fact that in the simplest anticline the two limbs dip
away from each other. It may be defined as a fold that is convex upward, or the
fold that has older rock in the centre or core. But the term has also been
extended to folds such as that where two limbs dip in the same direction at
different angles, but the older rocks are at the centre.
Syncline:
The word is driven from Greek word which means “together
inclined” referring the fact that in the simplest synclines the dip towards
each other. In general the syncline is defined as a fold that is concave downward,
with the younger rocks lies at the centre.
Symmetrical Folds:
The fold whose axial plane is vertical with horizontal
surface is called the symmetrical fold.
Asymmetrical Fold:
In the asymmetrical fold the axial plane is inclined, makes
an angle with the horizontal surface.
Overturned Fold:
In these types of folds the axial plane is inclined and
both limbs dip in the same direction usually at different angles. The
overturned, inverted or reversed limb is the one that has been rotated through
more than 90 to attain its present attitude. The normal limb is the one that is
right side up.
Recumbent Fold:
A recumbent fold is that in which the axial plane is
essentially horizontal. The strata in the inverted limb are usually much
thinner than the corresponding beds in the normal limb. The term arch-bend has
been used for the curved part of the fold between the normal and inverted
limbs. The recumbent fold is composed of entirely one kind of rock, the term
core and shell may be used to refer the inner and outer parts of the fold
respectively. Many recumbent folds have subsidiary recumbent anticlines
attached to them; these subsidiary folds may be called digitations.
Isoclinals Fold:
As the term self explanatory that a fold whose limbs are
dipping in equal angles in the same direction. The vertical isoclinals fold is
one whose axial plane is vertical. If the axial plane is inclined or
horizontal, the fold said to be inclined isoclinals fold of horizontal
isoclinals fold.
Chevron Folds:
The fold in which the hinges are sharp and angular is
called chevron fold.
Box Fold:
The folds in which the crest is broad and flat, two hinges
are present, one on the either side of the crest, are called box fold.
Fan Folds:
A fan fold is one in which both limbs are overturned. In
the anticlinal fan fold, the two limbs dip towards each other; in the synclinal
fan fold the two limbs dip away from each other.
Monocline Fold:
The term monocline fold refers to the folds that dip
uniformly in one direction.
Closed Fold:
It is also called tight fold. In these folds the
deformation has been sufficiently intense to cause flowage of the more mobile
beds so that these beds thicken and thin. Oppositely the open folds are those
where flowage has not taken place.
Drag Folds:
Drag folds form when a competent or strong bed slides past
an incompetent or weak bed. Such minor folds may form on the limbs of large
folds because of the slipping of beds past each other, or they may develop
beneath overthrust blocks. The axial plane of the drag folds are not
perpendicular to the bedding of the competent strata, but are inclined at some
angle. Such structural features may develop during sedimentation, when a sheet
of sediment slides over a weaker bed.
Plunging Folds:
The folds whose limbs plunge at a certain length are called
plunging folds.
Dome: It is the type of fold in which
limbs dip away from each other in all direction at the same amount.
Basin: It is a structural feature in
which all the limbs dip towards each other from all directions at the same
amount.
Dynamics of the folding
Dynamics of the folding concerned with the problem of
temperature and pressure at which folding occurs, as well as the stress and
time involved. Folding takes place under a wide range of temperature and
pressure. Folded moraines on mountain glaciers demonstrate that folding may
occur at low temperature and pressure.
During the fold dynamics, the initial material supposed to
be consists of one or more horizontal layers with mechanical properties
differing from those of the medium in which it is embedded. At a first
approximation we consider folding to be an elastic phenomenon. For a plate
embedded in an elastic medium of infinite thickness,
λ= 2πh √E/6Eo ---------------------------
(1)
σ= 3/2 √E Eo 1/6 --------------------------- (2)
Where λ is the wavelength of the fold, h is the thickness
of the plate, E is young’s modulus of the plate, Eo is young’s
modulus of the medium, σ is the stress to cause the folding.
Similar equations consider cases where the enclosing medium
is of finite thickness and other cases where several competent layers are
present. Equation (1) can be also written as:
λ/h = 2π √E/6Eo
This is the ratio of the wavelength of the fold to the
thickness of the beds, is constant if the ratio of the module of elasticity is
a constant.
The basic assumption that folding can be treated as an
elastic phenomenon is generally unjustified. Folding is the result of permanent
deformation.
Faults
Faults are ruptures in the rocks, along which the opposite
walls have moved past each other.
The essential feature is differential movement parallel to
the surface of the fracture. Some faults are only a few inches long, and the
total displacement is measured in fraction of inches, and other faults are
hundreds of miles long with a displacement measured in miles and even tens of
miles.
Geometrical classification of the faults
Classification Based On Fault
Pattern:
Parallel Fault: The faults have essentially the same dip and strike; belong
to the set of parallel faults. If the strikes are the same and dips are
different, the faults are assigned to two or more sets of parallel faults.
En Echelon Faults: The faults that are relatively short and overlap each
other, called as en echelon faults.
Peripheral Faults: The circular or arcuate faults
that bound a area or part of the circular area.
Radial Faults: The faults that belong to a
system of faults that radiate out from a point.
Genetic classification of the faults
The most satisfactory genetic classification is based on
the nature of the relative movement along the fault.
Thrust Fault:
These are the faults along which the hanging wall has moved
up relative to the foot wall. Thrust faults indicate the shortening of the
rocks involved. Three categories are recognized depending upon the angle at
which fault took place.
Reverse Fault: The thrust fault that dips at
the angle of more than 45o.
Thrust Fault: The fault that dips at the angle
of less than 45o.
Overthrust Fault: The fault that dip at the angle
less than 10o.
Normal Fault:
The type of the fault in which hanging wall block has moved
downward relative to the foot wall is called the normal fault. A detachment
fault is a special category of low angle normal faults.
Strike Slip Fault:
These are also called wrench faults, in which displacement
has been essentially parallel to the strike of the fault.
Criteria for faults
There are five main criteria on which faults are identified
in the field. They are:
Slickenside or Striations:
Linear impressions produced by movement of fault blocks are
called slickenside or striations. They also show the direction of hanging wall.
Breccias:
The angular and of different sized rock fragments present
in any field are also the sign of faults. They may be produced during the
faulting.
Gauge:
This is the powder like material of rocks produced during the
movement of fault blocks.
Repetition or Omission of Beds:
Fault blocks move upward or downward. During the field if
repetition of beds observed, or some beds are missing, that means the fault is
present here.
Joints
The rocks are broken by relatively smooth fractures along
which no any displacement occurred, these fractures are called joints.
Joints are of different sizes and shapes, some joints are
measured in inches and other in several feet. Although most joints are planes
but some are curved surfaces. Initially all the joints are tight fractures, but
because of the weathering the joints enlarged into an open fissure. Joints may
have any attitude, some joints are vertical, other horizontal and still others
inclined.
Characteristically a large number of joints are parallel
consists of joint set. A joint system consists of two or more joint sets.
Geometrical classification of joints
In a geometrical classification, the joints may be
classified on the basis of their attitude relative to the bedding or some
similar structure in the rocks that they cut.
Strike Joints: The joints that strike parallel
or essentially parallel to the strike of the bedding of sedimentary rocks, or
metamorphic structures in metamorphic rocks are called the strike joints.
Dip Joints: The joints that strike parallel
or essentially parallel to the direction in which the bedding dips are called
dip joints.
Oblique Or Diagonal
Joints: The joints that
striking in a direction that lies between the strike and dip of the associated
rock are called the oblique or diagonal joints.
Bedding Joints: The joints that are parallel to
the bedding of the concerned rock are called bedding joints.
Unconformities
It is defined as “the surface of erosion or non-deposition
that separates the younger strata from older rocks is called unconformity”.
Unconformity developed as a result of uplifting and
sub-aerial erosion of rocks that exist before followed by the deposition of new
sediments after a temporal break of time.
The relief of unconformities is different in different
areas, depending upon the time between uplifting and erosion and re-deposition.
In some localities the older rocks were reduced to an extensive amount because
of long time of exposition. In other localities, only a mature stage in the
erosion cycle was reached before the younger rocks began to deposit.
Kinds of unconformities:
Depending upon the attitude of the unconformities, rocks
involved and tectonic history, there are four types of unconformities.
Angular Unconformity:
As the term implies that there is an angle between the
older rock and younger strata. Or angular unconformity is defined as the “type
of unconformity in which younger strata deposits over the tilted older rocks”.
Disconformity:
It is the type of unconformity in which the formations on
opposite sides of the unconformity are parallel. A Disconformity covers a large
area and represents a considerable interval of time.
Local Unconformity:
As the name implies, it is distinctly local in extent and
time involved is short.
Non-Conformity:
The type of unconformity in which younger strata deposited
at older igneous or metamorphic rocks is called non-conformity.
Identification of unconformities
Unconformities can be identified on the basis of two main
evidences. These are:
Lateritic Bed or Oxidized
Layer:
It is the red or brown in color bed produced during the
unconformable period. As the oceanic water regretted and rock beds exposed to
terrestrial environment, the atmospheric oxygen started to react with rock
minerals in the presence of sunlight and several other environmental factors.
Hence looks as red or red to brown in color.
Fossils or Paleontological
Evidences:
If we could not see any lateritic bed then we study the
fossils of rock beds. If we found a sudden change in the fossils between two
beds it means unconformity is present.
Change in Color:
If there is a sharp contrast in color between the rocks
above and below the contact, it shows the unconformity.
Presence of Surface of Erosion between
Rocks:
Unconformities are much more evident from presence of
eroded and uneven surface between two rock units, one is older and other is
younger.
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