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TECTONIC LANDFORMS
TECTONIC LANDFORMS
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Deformation is a general term that refers to all changes in
the original form and/or size of the rock body
The forces that deform rocks are known as stress
Differential stress is when force is equally applied in
different directions
Types of stress
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Compressional stress
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Tensional stress
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Differential stress that shortens a rock body
Stress that elongates or pulls apart a rock body
Shear
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Stress that causes two adjacent parts of a body to slide past one
another
TECTONIC LANDFORMS
Compressional Stress
Force
Tensional Stress
Force
Force
Force
Rock Body
Rock Body
A force that which tends to
compress an object thereby
changing its shape. These are
dominant at convergent plate
boundaries
An extensional force that tends
to stretch or pull material apart.
Such a stress is capable of
changing the shape of a
material. These are dominant at
divergent plate boundaries
TECTONIC LANDFORMS
Shear Stress
Rock Body
Forces which push two
parts of a rock body in
opposite directions. These
are dominant at
transform boundaries
LANDFORMS ASSOCIATED WITH
COMPRESSIONAL STRESS
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Folds: a bent layer or series of layers that were originally
horizontal and subsequently deformed
Types of Fold
Anticline
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Syncline
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A linear downfold in sedimentary strata, the opposite of an anticline
Monocline
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A fold in sedimentary strata that resembles an arch
Large step-like folds in otherwise horizontal strata
Mountain Ranges
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Collision between two plates cause folding and development of
anticlines and synclines
These tend to occur in groups rather than in isolation
When they are eroded they result in a ridge-and-valley landscape
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Ridge = anticline
Valley = syncline
LANDFORMS ASSOCIATED WITH
COMPRESSIONAL STRESS
(Strahler & Merali, 2008)
Monocline
Formation of a fold (Anticlines and Synclines)
❶ Rocks that are buried deep beneath the Earth’s surface become hot as a result of the escape of heat from the Earth’s interior. Under these conditions the behaviour of the rock changes from being brittle, to more like plastic. (Plastic in this case doesn’t refer to the material itself, but rather to the bendability of the substance)
Original horizontal surface
❷ When compressive stress is applied to these strata, they deform rather than break. This deformation is in the form of folds where the strata buckles under stress (much like a piece of fabric when the ends are pushed towards each other). Note the increase in height as a result of the deformation. This increase in height is why these deformations are associated with mountain ranges like the Cape Fold Mountains.
Anticline
Anticline
Original surface
Syncline
Change in height
Formation of a fold (Monoclines)
❶ Monoclines are fold systems that are intimately associated with faulting. Initially the basement rock is overlain by layers of sedimentary rocks.
Layers of
horizontal
sedimentary rocks
Basement Rock
❷ As large slabs of basement rock were displaced along ancient faults, the comparatively ductile sedimentary strata above responded by folding. This resulted in the formation of a monocline
Strata now has a
pronounced ‘steplike’ appearance
Basement
Rock
Fault in basement rock
ANTICLINE & SYNCLINE
(LAINGSBURG - SOUTH AFRICA)
Syncline
Anticline
(Norman & Whitfield, 2006)
EXAMPLE OF RIDGE VALLEY LANDSCAPE
(CAPE FOLD MOUNTAINS – SOUTH AFRICA)
Image: NASA Earth Observatory
FAULTS
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Faults are fractures in the crust along which appreciable
movement has taken place
Two categories
Dip-Slip Faults
2. Strike-slip Faults
1.
Horizontal plane
DIP
N
Up
E
W
Down
S
Rock
Body
FAULTS ASSOCIATED WITH TENSIONAL
AND COMPRESSIONAL FORCES
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Faults are fractures in the crust along which appreciable
movement has taken place
Dip-Slip Faults
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A fault in which the movement is parallel to the direction of the
dip
Vertical displacements along dip-slip faults can produce long low
cliffs, called fault scarps
Two types
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Normal Faults:
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Reverse Faults
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Normal Fault
Hanging wall block moves down relative to the footwall block
Faults in which the hanging wall moves up relative to the footwall
Reverse Fault
FAULTS ASSOCIATED WITH TENSIONAL
AND COMPRESSIONAL FORCES
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Two Types of DipSlip Faults
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Normal (A): Hanging
wall block (H) moves
down relative to the
footwall block (F)
Reversed (B):
Hanging wall block
(H) moves up
relative to the
footwall block (F)
A
F
H
B
H
F
Formation of a normal fault
❶ A rock body is subjected to tensional stress (the stress that ends to pull the rock body apart). The cause of this stress may be an event such as rifting (e.g. the process that caused the development of the Rift Valley in Africa)
Rock Body
❷ This “pulling apart” of the rock leads to the build‐up of large amounts of stress within the rock body. Failure ultimately occurs at one (or many) points of weakness leading to the development of a normal fault. Note the change in shape from the original structure of the rock body to its current shape.
Original shape
Body
Rock
Normal fault
Formation of a reverse fault
❶ A rock body is subjected to compressional stress (the force that tends to compress the rock body). This type of stress can typically be found where continental masses move into each other (convergent boundaries). Note that the direction of the applied force is different to that of tensile stress.
Rock Body
❷ This compression of the rock body leads to the build‐up of large amounts of stress within the rock body. Failure eventually occurs at one (or many) zones of weakness within the rock leading to deformation and the development of a reverse fault. Original shape
Rock
Body
Reverse fault
FAULTS ASSOCIATED WITH TENSIONAL
AND COMPRESSIONAL FORCES
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Horsts and Grabens
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Normal faulting is prevalent at spreading centres (where plate
divergence occurs)
A central block (graben) is bounded by normal faults and drops as
the plate separates
The grabens produce an elongated valley bounded by relatively
uplifted structures (horsts)
Horst
Horst
FAULT EXAMPLE
(HEBRON FAULT - NAMIBIA)
What type of fault is this?
Hanging Wall
Foot Wall
LANDFORMS ASSOCIATED WITH SHEAR
FORCES
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Strike-Slip Faults
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Dominant displacement is horizontal and parallel to the strike of
the fault surface
Large strike-slip faults can consist of a zone of roughly parallel
fractures (up to several km in width)
Major strike-slip faults cut through the lithosphere and
accommodate movement between crustal plates (transform fault)
Formation of a strike-slip fault
❶ A rock body is subjected to forces which act on it from opposite directions. In the diagram below one force is being exerted from the north while the other is being exerted from the south. This places stress on the rock body.
N
W
E
Rock Body
S
❷ Eventually, failure will occur at one or more lines of weakness (fault) and the separate rock bodies will move in the direction of the applied force.
N
W
E
S
STRIKE-SLIP EXAMPLE
(SAN ANDREAS FAULT – USA)
(Strahler & Merali, 2008)