3.Geo.3.pptx

CHAPTER 4
GEOLOGICAL STRUCTURES

GEOLOGIC STRUCTURES
Folds
Faults
Deformations
Chapter Content:

 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

Deformation of Rocks…
Geologic
Structures

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).

 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.

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

 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.
Deformation

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

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

 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

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.

 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

 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

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

 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

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

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

 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

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

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.

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

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

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

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

FAULT-RELATED STRUCTURES
Graben Rifit

 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

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

 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

3.Geo.3.pptx

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