Structural Geology
Structural Geology
Structural Geology
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Learning Outcomes
Students should be able to:
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Know the conceptual deformation to create various
kind of structure.
Understand the stress – strain relationship with more
emphasis on ductile and brittle deformation.
Familiarize the mega structure such as fold, fault and
unconformity.
Appreciate the importance and usefulness of
structural geology in petroleum trap.
Structural Geology - Introduction
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Structural Geology is the study of the architecture of the
earth’s crust, its deformational features and their mutual
relations and origin.
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To understand earthquake for example, one must know about
faults.
Appreciating how major mountain belts and the continent have evolved
calls for a comprehension of faulting and folding.
Understanding plate-tectonic theory as a whole also requires a
knowledge of structural geology
In areas of active tectonics, the location of geologic structure is very
important in selection of suitable sites for buildings, dams, highway,
bridge, tunnels, nuclear power plants, etc.
Understanding structural geology can help us more fully appreciate the
problem of finding more of the earth’s natural resources, such as metal
ores, petroleum & gas, rock aggregates, etc.
The knowledge of structural geology is also very important in geohazards
(landslide, earthqukae, tsunami, subsidence, erosions, etc) mitigation
and control measures.
Kinds of rock behavior
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Brittle - material breaks when stressed
Ductile - material changes shape permanently when
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stressed
Elastic - material changes shape temporarily when
stressed, then breaks (at the elastic limit)
Influences on rock behavior
Temperature - enhances ductile behavior
Pressure - enhances ductile behavior
Time (rate at which stress is applied) - enhances ductile
behavior
Rock characteristics – variable
In general, the upper crust is more brittle than the lower crust
because it is cooler and under less pressure.
Forces that cause deformation
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Stresses
• Compressive Stress –
• pushed together or squeezed from opposite directions.
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• common along convergent plate boundaries; typically results in
rocks being deformed by a shortening strain;
• Tensional Stress –
• Forces pulling away from one another in opposite directions;
results in a stretching or extensional strain
• Quite rare in the earth crust
• Shear Stress –
• Due to movement parallel but in opposite directions along a fault
or other boundary
• Results in a shear strain parallel to the direction of the stresses.
• Notable along transform plate boundaries and along other
actively moving faults.
Behaviour of Rocks: Stress & Strain
Rocks behave as elastic, ductile, or brittle materials depending on the amount
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and rate of stress applied, the type of rock, and the temperature and pressure
under which the rock is strained.
Elastic – if a deformed body recovers its original shape after the stress is
reduced or removed (e.g. rubber). Rocks can behave in an elastic way at very
low stresses, however once the stress exceeds the elastic limit the rock will
deform permanently.
Ductile – a rock that behaves in a ductile or plastic manner will bend while
under stress and does not return to its original shape after relaxation of the
stress. Under high pressure & temperature (e.g. during regional
metamorphism) rocks behave in a ductile manner. Ductile behaviour results in
folding or bending or rock layers.
Brittle – a rock exhibiting brittle behaviour will break or fracture at stress
higher that its elastic limit. Rock typically exhibit brittle behaviour at or near
the earth’s surface where pressure & temperatures are low. Faults and joints
are examples of structures that form by brittle behaviour of the crust.
Behaviour of Rocks to Stress & Strain
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at a particular point.
Strain - the change in size
(volume) or shape, or both,
while an object is
undergoing stress.
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Fault
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Crenulation
Folding
Assimilation
Measuring Attitude of
Rocks
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Dip - Angle of bed with
the horizontal
Strike - Bearing
(compass direction) of
line of intersection
between horizontal
plane and the inclined
bed.
Dip Direction – is the
compass direction in
which the angle of dip is
measured.
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Terms
Anticline
Syncline
Limb
Axial plane
Hinge Lines/Fold
axes
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Note:
• Plan view – geological map
• Side view – geologic cross sections
Kinds of fold sets
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Fractures in Rock
If a rock is brittle, it will fracture
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Types of fractures in rock:
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Faults
Joints
JOINTS
• Joints are fracture or crack in bedrock, 18
with no displacement occurs along the
fracture
• Most rock at or near the surface is brittle,
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so nearly all exposed bedrock is jointed to
some extent.
• Joints normally occur in sets, in which the
individual joint planes are oriented nearly
parallel to one another within the same
set.
• Common to rocks exposed at
all surface; the that the
indicates many causes are
• Tectonic
and varied activity; mountain building
(orogenics), plate interactions, etc.
• Non-tectonic stresses - shrinkage due
to cooling or drying; e.g. Columnar
basalt
• Expansion due to release of pressure -
very common at surface; e.g
exfoliation joints
The significant of joints
Valuable ore deposits (e.g.
gold, tin, etc) are often
precipitate within the
mineralised veins that filled
up the joints.
Accurate information about
joints are also very
important in the planning
and construction of large
engineering projects (e.g
highways, dam, reservoirs,
etc.).
Good for fractured
petroleum reservoir.
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FAULT
S along which movement has
Faults are fractures in bedrock
taken place.
The displacement may be only several cms or hundreds of
kms.
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Active faults – movements has taken place during the last
11,000 years. But most faults are no longer active.
Faults are identified from the dislocated beds, broken or highly
fractured or pulverized rock mass sandwiched between the
displaced blocks.
Single breaks Fault Plane
Complex zones of shearing Fault Zone
San Andreas Fault
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Terminology:
Footwall – the underlying surface of an inclined fault plane.
Hanging Wall – the overlying surface of an incline fault plane.
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Types of dip slip faults are distinguished based on the relative
movement of the footwall block and the hanging wall block.
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Horsts and grabens
• In a normal fault, the hanging wall block has moved downward relative to the
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footwall block.
• A normal fault results in extension or lengthening of the crust.
• Extension of the earth crust is compensated by downward movement of
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the
hanging wall block.
• Sometimes a block bounded by normal faults will drop down, creating a graben.
(“graben” is a German word for “ditch”), while the uplifted fault-bounded block is
called horst.
• Rifts are graben associated with diverging plate boundaries, either along mid-
oceanic ridges or on continents.
Horst
Graben
The importance of Geological
Structures
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gas accumulation in the earth’s crust.
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CONCLUSION
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The main geological structures are created by compressive,
tensional and shearing deformations.
The structural pattern appears on the Earth surface could
be
correlated to plate tectonic.
The importance of structural geology for petroleum traps.