Geologic structures form due to stresses deforming rocks over time. There are three main types of structures: folds, faults, and fractures. Folds are bent layers caused by compression, and include anticlines and synclines. Faults are fractures with displacement, and can be normal, reverse, strike-slip, or oblique. Fractures include joints with no displacement and faults. Geologic structures are important because they can trap oil and gas, act as pathways for groundwater and minerals, and help date the geologic timescale.

1. CHAPTER 4
GEOLOGICAL STRUCTURES

2. GEOLOGIC STRUCTURES
Folds
Faults
Deformations
Chapter Content:

3. DEFORMATION OF ROCKS

4.  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).
 The forces that cause deformation of rock are
referred to as stresses (Force/unit area):
tensional, compressional or shear stresses
Deformation of Rocks
Geologic
Structures

5. Deformation of Rocks…
Geologic
Structures

6. Deformation Stress and Strain
 Stress: Uniform stress (Confining stress/pressure),
differential stress.
 Three kinds of differential stress occur.
1. Tensional stress (or extensional stress), which
stretches rock.
2. Compressional stress, which squeezes rock, &
3. Shearstress, which result in slippage & translation.
 A strain is a response of material to stress (: change in
size, shape, or volume).

7. Stress and Strain

8.  When a rock is subjected to increasing stress it
passes through 3 successive stages of deformation.
Stages of Deformation
Deformation
 Elastic Deformation:
reversible strain.
 Ductile Deformation:
irreversible strain.
 Fracture: irreversible
strain wherein the
material breaks.

9. Stages of Deformation
 Depending on their relative behavior under stress
materials can be divide into two classes:
 Ductile materials
Deformation

10.  Brittle materials have a small or large region of
elastic behavior but only a small region of ductile
behavior before they fracture.
 the rocks will be fractured or broken and
displaced along fracture planes.
Stages of Deformation
Deformation

11. GEOLOGIC STRUCTURES

12. Folded Structures
 Folds are bends or buckles in layered bedrocks
caused by compressive forces applied parallel to
the bedding planes.
 Folds are wave-like in shape and vary
enormously in size.
 Form when rocks are deformed plastically.
 Rock folding is influenced by the type of rock
and the compressive forces.
Geologic
Structures

13. Types
of
Folds FOLD GEOMETRY

14.  Based on theirs geometry they can be define as:
 Antiform/Anticline
 Synform/Syncline
 Monocline
Geologic
Structures Types of Folds

15.  an up-arched fold in which the two limbs
diverge (dip) away from each other.
 the layers are symmetrical (look alike) to
either side of its center.
 An anticline is an antiform where the rocks
forming the core are the oldest.
 the beds are convex Upwards.
Oldest rock
Types
of
Folds Antiform/Anticline

16. Types
of
Folds Synform/Syncline
 a down-arched fold in which the two limbs
converge (dip) towards each other.
 A syncline is a synform where the rocks
forming the core are the youngest.
 they are concave upwards.

17.  a flexure/bend that has two parallel, gently
dipping (or horizontal) limbs with a steeper
middle part in between.
 there is only one direction of dip.
Types
of
Folds Monocline

18. FOLD TYPES
Recumbent fold.

19.  Fractures are cracks within rocks formed due to
shearing of brittle materials .
 There are two types of fractures:
 Joints: fractures with no displacement
 Faults: fractures with horizontal, vertical
or oblique displacement.
Geologic
Structures FRACTURES

20. Fault Structures
 Faults are well-defined cracks along which
appreciable amount of displacement (movement)
has taken place relative to each other where the
movement can be in any direction (vertically up or
down; laterally to the right or to the left;
obliquely either rotationally or translationally).
Fractures

21.  Faults result from tensional as well as
compressional forces.
 The magnitude of the movement varies
between wide limits from a few cm to hundreds
of metres.
Fault Structures…
Fractures

22. Faults Fault Geometry

23. Fault Terminology
 Hanging Wall - the surface of block
that is on top of the plane of the
fault.
 Footwall - the surface or block that
lies below the plane of the fault.
 Strike - the direction in which the
fault runs.
 Dip - the dip direction is
perpendicular to the strike
direction.
Faults

24. Formation of fault
Fault Plane ‐ the fracture
surface along which relative
movement has taken place.

25.  Based on the stress applied and the directions of
the relative movements along the fault plane  3
types.
 Dip-slip/Vertical fault: Normal fault;
Reverse fault (Thrust fault);
 A strike-slip fault: Left-lateral (sinistral)
fault; Right-lateral (dextral) fault
 Oblique-slip fault.
Types of Faults
Faults

26. Types
of
Faults Dip-Slip Faults
 Dip-slip fault is a fault where the major
movement is along the dip-direction .

27. Types
of
Dip-Slip
Faults Normal Fault
 Normal fault / gravity fault is a dip-slip fault
where the hanging wall moves downward relative to
the footwall
 results from vertical compressive stresses or
horizontal tensional stresses applied to brittle rock
masses.
 Gravity causes the hanging wall to slip down.

28. Types
of
Dip-Slip
Faults Reverse Fault
 Reverse fault is a dip-slip fault where the hanging
wall moves upward relative to the footwall.
 Reverse faults result from horizontal compressive
stresses.
 Thrust fault - reverse
fault with a very
small dip angle that
is close to horizontal

29. Types
of
Faults Strike-Slip Faults
 the major movement is along the horizontal
direction (along the strike direction).
 results from shearing of brittle materials.
 It has two classes:
 Left‐lateral (sinistral) fault - one block is
moved to the left with respect to the other.
 Right‐ lateral (dextral) fault it is the - one
block move to the right with respect to the
other

30. Strike-Slip Faults

31. Types
of
Faults Oblique Faults
 The movement (displacement) take place both
along dip direction and strike direction.
 are the most common faults in nature.

32. Fault-related Structures
 Graben: a block-depressed between two
normal faults which are either parallel or
dipping towards each other.
 Rift valley: a large-scale structure formed by
successive faults forming successive grabens,
like the East African Rift valley or the Red sea Rift.
 Horst: a block rose between two normal faults
that are parallel or dipping away from each other.
Faults

33. FAULT-RELATED STRUCTURES
Graben Rifit

34.  Joints are fractures along which little or no
displacement has occurred and are present within
all types of rocks.
 Joints are formed through failure of rock masses in
tension, in shear or through some combination of
both.
Joint structures
Fractures
Definition of joints

35.  Unloading or sheeting effects (see
Weathering).
 Stresses in a cooling magma.
 wet sediments when they dry out
Joint structures…
Causes of joints
Fractures

36.  Oil and natural gas are formed and found
trapped in subsurface folds.
 Faults, joints, and fractures can act as a
passageway for groundwater and a host for
valuable mineral deposits .
 Unconformities can be used to mark geologic
time boundaries for eras, periods, and epochs.
Importance of geologic structures
Geologic
Structures