Structural geology is the study of rock structures and deformations within the Earth's crust. There are several types of rock structures that provide evidence of past deformation, including folds, faults, joints, and foliations. Folds occur when rock layers are bent, and there are different types such as anticlines, synclines, tight folds, overfolds, recumbent folds, and nappe folds. Understanding rock structures provides insight into the stress fields and tectonic processes that shaped the geological past.

1. Structural Geology

2. What is Structural Geology?
•Structural geology is the
study of the three-dimensional
distribution of rock units with
respect to their deformational
histories.

3. • The primary goal of structural geology is to use
measurements of present-day rock geometries to
uncover information about the history of
deformation (strain) in the rocks, and ultimately,
to understand the stress field that resulted in the
observed strain and geometries.
• This understanding of the dynamics of the stress
field can be linked to important events in the
regional geologic past.

4. Structural fabrics and defects
• Folds
• Joints
• Faults
• Foliations
• These are internal weaknesses of rocks which
may affect the stability of human engineered
structures.

5. Deformation of Rocks
• Within the Earth rocks are continually being
subjected to forces that tend to bend them, twist
them, or fracture them. When rocks bend, twist
or fracture we say that they deform (change
shape or size).
• Deformation common
at plate margins.
• Deformation concepts…
– Force
– Stress
– Strain

6. Stress
• The forces that cause deformation of rock are
referred to as stresses (Force/unit area).
• Differential Stress – Unequal in different
directions.
• A uniform stress is a stress wherein the forces act
equally from all directions.
• 3 major types of differential stress
– Compressional stress
– Tensional stress
– Shear stress

7. Compressional Stress
• Push Together stress.
• Shortens and thickens crust.
• which squeezes rock.

8. Tensional Stress
• “Pull-apart” stress.
• Thins and stretches crust.
• Associated with rifting

9. Shear Stress
• Slippage of one rock mass past another.
• In shallow crust, shear is often accommodated
by bedding planes.

11. Strain
• Changes in the shape or size of a rock body
caused by stress.
• Strain occurs when stresses exceed rock
strength.
• Strained rocks deform by folding, flowing, or
fracturing

12. How Rocks Deforms
• Elastic deformation – The rock returns to original
size and shape when stress removed.
• When the (strength) of a rock is surpassed, it
either flows (ductile deformation) or fractures
(brittle deformation).
• Brittle behavior occurs in
the shallow crust; ductile in
the deeper crust.

13. We can divide materials into two
classes.
• Brittle materials have a small or large region of
elastic behaviour but only a small region of
ductile behaviour before they fracture.
• Ductile materials have a small region of elastic
behaviour and a large region of ductile
behaviour before they fracture.

14. How a material behaves will depend
on several factors
• Temperature - At high temperature molecules and their bonds can stretch and
move, thus materials will behave in more ductile manner. At low Temperature,
materials are brittle.
• Confining Pressure - At high confining pressure materials are less likely to
fracture because the pressure of the surroundings tends to hinder the formation
of fractures. At low confining stress, material will be brittle and tend to fracture
sooner.
• Strain rate -- At high strain rates material tends to fracture. At low strain rates
more time is available for individual atoms to move and therefore ductile
behaviour is favoured.
• Composition -- Some minerals, like quartz, olivine, and feldspars are very
brittle. Others, like clay minerals, micas, and calcite are more ductile This is due
to the chemical bond types that hold them together. Thus, the mineralogical
composition of the rock will be a factor in determining the deformational
behaviour of the rock. Another aspect is presence or absence of water. Water
appears to weaken the chemical bonds and forms films around mineral grains
along which slippage can take place. Thus wet rock tends to behave in ductile
manner, while dry rocks tend to behave in brittle manner.

15. Evidence of Former Deformation
• Evidence of deformation that has occurred in the past
is very evident in crustal rocks.
• For example, sedimentary strata and lava flows
generally follow the law of original horizontality. Thus,
when we see such strata inclined instead of horizontal,
evidence of an episode of deformation.
• In order to uniquely define the orientation of a planar
feature we first need to define two terms –
– Strike (trend)
– Dip (inclination)

16. Dip and strike

17. Dip and strike
• Consider a flat uniform stratum which is tilted out of
the horizontal. On its sloping surface there is one
direction in which a horizontal line can be drawn,
called the strike. It is a direction that can be
measured on beds that are exposed to view and
recorded as a compass bearing. At right angles to the
strike is the direction of maximum slope, or dip.
• The angle of inclination which a line drawn on the
stratum in the dip direction makes with the
horizontal is the angle of dip (or true dip), and can
be measured with a clinometers and recorded to
the nearest degree.

18. Dip and strike
Strike (illustrated by line s-
t below) is the compass
direction of a line marking
the intersection of an
inclined plane with a
horizontal plane such as
the Earth’s surface
Dip (line d-p above) is the
maximum angle between
the inclined plane and the
horizontal plane. Dip is
always perpendicular to
strike, and has both a
compass direction and an
angle.

19. Folds
• It is frequently seen that strata in many parts of the
Earth's crust have been bent or buckled into folds;
dipping beds, mentioned above, are often parts of
such structures.
• An arched fold in which the two limbs dip away from
one another is called an antiform, or an anticline.
• A fold in which the limbs dip towards one another is
a synform, or a syncline.

20. Fold geometry
• In the cross-section of an upright fold the highest
point on the anticline is the crest and the lowest point
of the syncline the trough; the length of the fold
extends parallel to the strike of the beds, i.e. in a
direction at right angles to the section.
• The line along a particular bed where the curvature is
greatest is called the hinge or hinge-line of the fold ;
and the part of a folded surface between one hinge
and the next is a fold limb.
• The surface which bisects the angle between the fold
limbs is the axial surface, and is defined as the locus
of the hinges of all beds forming the fold. This
definition allows for the curvature which is frequently
found in an axial surface.

21. Folds

23. Fold

24. Fold

25. Fold groups
• The relative strength of strata during folding
is reflected by the relationship between
folds. Those shown in Figures are called
harmonic folds as adjacent strata have
deformed in harmony. Disharmonic folds
occur when adjacent beds have different
wavelengths the smaller folds being termed
parasitic folds , or 'Z 4M' and 'S' folds. Box-
folds and kink bands are examples of other
groups that may occur.

26. Fold Group

27. Fracture-cleavage
• This is mechanical in origin and consists of
parallel fractures in a deformed rock. A band of
shale lies between two harder sandstone beds,
and the relative movement of the two
sandstones has fractured the shale, which has
acquired cleavage surfaces oblique to the
bedding. The cleavage is parallel to one of the
two conjugate shear directions in the rock. The
spacing of such shear fractures varies with the
nature of the material, and is closer in
incompetent rocks such as shale than in harder
(competent) rocks

28. Fracture-cleavage

29. Boudinage in Fold
• When a competent layer of rock is subjected
to tension in the plane of the layer,
deformation by extension may result in
fracturing of the layer to give rod-like pieces,
or boudins (rather like 'sausages'), with small
gaps between.They are often located on the
limbs of folds; softer material above and
below the boudin layer is squeezed into the
gaps.

30. Boudinage in Fold

31. Gravity folds
• Gravity folds, which may develop in a
comparatively short space of time, are due to
the sliding of rock masses down a slope under
the influence of gravity.

32. Folds
A fold is when the earth’s crust is pushed up from
its sides. There are six types of folds that may
occur:
• Anticline
• Syncline
• Tight Fold
• Overfold
• Recumbent Fold
• Nappe Fold

33. Anticline
• An anticline occurs when a
tectonic plate is compressed by
movement of other plates. This
causes the center of the
compressed plate to bend in an
upwards motion.
• Fold mountains are formed when
the crust is pushed up as tectonic
plates collide. When formed,
these mountains are usually
enormous like the newly formed
Rocky Mountains in Western
Canada and the United States
• To the top right is a picture of an
anticline. Beneath is a picture of
the Rocky Mountains.

34. Syncline
• A syncline is similar to an
anticline, in that it is formed by
the compression of a tectonic
plate. However, a syncline
occurs when the plate bends in
a downward motion.
• The lowest part of the syncline
is known as the trough.
• To the top right is a diagram of
a syncline fold (The bottom of
the fold center is the trough).
Beneath, is an example of a
syncline in California. Can you
distinguish the trough in this
picture?

35. Tight Fold
• A tight fold is a sharp peaked
anticline or syncline.
• It is just a regular anticline or
syncline, but was compressed
with a greater force causing the
angle to be much smaller.
• Folds such as these occur to form
steep mountain slopes like those
in Whistler, British Columbia.
• To the left is a photo of a tight
fold formed by extreme pressure
on these rocks.

36. Overfold
• An overfold takes place when folding rock becomes bent or
warped.
• Sometimes the folds can become so disfigured that they may
even overlap each other.
• An example of overfolding is shown in the diagram below.

37. Recumbent Fold
• This type of fold is
compressed so much that it
is no longer vertical.
• There is a large extent of
overlapping and it can take
the form of an “s”.
• To the right is a diagram
that shows the process of
recumbent folding.

38. Nappe Folding
• This fold is similar to a
recumbent fold because of
the extent of folding and
overlapping. However,
nappe folding becomes so
overturned that rock layers
become fractured.
• To the right is a picture of
someone standing under a
fractured fold.