# Deformation of rocks

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```					Crustal Deformation

Stress and Strain

Types of Deformation

Geologic Structures
Folds and Faults

Geologic Structures
on Maps
Orientation of deformed rocks
We need some way to describe the
distribution of geologic structures.
Strike: bearing of a line defined by the
intersection of the plane in question and
the horizontal
Dip: angle between the plane and the
horizontal, measured perpendicular to
strike.
Fig. 10.4
Arrows indicates the                The length of the arrow
force on 1 side of the              indicates how strong the
block                               force is

Confining stress - equal     Differential stress - unequal
forces on all sides of the   forces on the sides of the
block (yellow arrows)        block (red extension of arrow)
STRESS
(force per unit area)
•Compression
•Tension
•Shear
STRENGTH
. the ability of an object to resist
deformation

• compressive strength

• tensile strength

• shear strength
STRAIN
Change in shape or size of an object in
response to an applied stress.
= Deformation
Three Types of Strain
Elastic
Ductile (Plastic)
Brittle (Rupture)
Elastic Deformation

a temporary change in shape
or size that is recovered when
the applied stress is removed.
Ductile (Plastic) Deformation
• A permanent change in shape
or size that is not recovered
when the stress is removed.

• i.e. it flows or bends
Brittle Deformation
(rupture)
• the loss of cohesion of a body
under the influence of deforming
stress.

• i.e. “it breaks”
Factors Determining the
Style (ductile or brittle) of Deformation

• Pressure
• Temperature
• Strain rate
• Rock composition
• Pressure:
–High P - Ductile Deformation
–Low P - Brittle Deformation
• Temperature:
–High Temp - Ductile Deformation
–Low Temp - Brittle Deformation
• Strain Rate:
–High Strain Rate - Brittle Deformation
–Low Strain Rate - Ductile Deformation
• Composition:
–“Hard”(e.g. granite) - Brittle Deformation
–“Soft” (e.g. marble) - Ductile Deformation
Types of folds
(bent planar structures)
anticline: Flanks bent downwards, older
rocks on the inside
syncline: Flanks bent upwardsolder rocks
on the outside

(scale - from mm to tens of km)
Fold terms
• Axial plane: the plane of mirror symmetry
dividing the fold into two limbs
• Axis: line formed by the intersection of
the axial plane and a bedding plane
• Horizontal fold: where the fold axis is
horizontal
• Plunging fold: where the fold axis is not
horizontal
Strike and Dip Symbols
you should know.

Inclined beds             Horizontal beds

30
N
Vertical beds
More Symbols you should know.

Horizontal Folds          Plunging Folds
N
Horizontal Anticline   Plunging Anticline

Horizontal Syncline     Plunging Syncline
Fault Symbols you should know.
U
D
Normal Fault

D      Reverse Fault
U

Left-lateral   Right-lateral
Why is understanding deformation
of the Earth important?

• Major part of earth history
• Explains origins of mountain belts
• Trapping hydro-carbons and the
concentration of ore deposits
• Faulting causes earthquakes
• In Houston, growth faults and subsidence
Faults may be "reactivated"
History of a fault may be very long
Previously developed weakness is the
most likely place to break
reactivation may have opposite sense as
before
'Active' = 10,000 to 100,000 years
very important for dams and reactors
Dip-Slip Faults

Faults where the movement
blocks was in the dip
direction.
Types of Dip-Slip Faults:

Normal Faults: a dip-slip fault
where the hanging wall moved
down relative to the footwall.
FAULT
A fracture along
which some
movement has
occurred.
An example of
brittle failure.
Types of Dip-Slip Faults:

Reverse Faults: a dip-slip fault
where the hanging wall block
moved up relative to the
footwall block.
Strike-Slip Faults

Faults where the movement
blocks was in the strike
direction.
Types of Strike-Slip Faults:
• Left-lateral: where the opposite
block moved to the left, relative to
the observer.
• Right-lateral: where the opposite
block moved to the right, relative
to the observer.
Fault Scarp
A scarp formed by the offset
fault. It is like an exposed
fault plane which may be
modified by erosion.

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