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Geol 104: Geologic Structures
Tectonic Forces and
Geologic Structures
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What are geologic structures?
Why do geologic structures form?
How do rocks respond to tectonic forces?
How do different geologic structures form?
How do geologic structures relate to plate
tectonic setting?
• Why care about geologic structures?
What are Geologic Structures?
• Shapes, arrangements and interrelationships
of bedrock units.
• Manifestations of tectonic forces at work.
Why do Geologic Structures Form?
• Geologic structures represent a rock’s
response to tectonic forces.
• Stress
– Force on an object over a specified area.
• Strain
– Change in volume or shape of an object in
response to stress. Often referred to as
deformation.
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Geol 104: Geologic Structures
Types of Stress
• Compressive
– Forces directed toward each other.
• Tensional
– Forces directed away from each other.
• Shear
– Forces acting parallel to a plane.
• Hydrostatic pressure
– Forces directed towards each other in all
directions.
Stresses in the Earth
• Hydrostatic Confining Pressure
– Due to weight of overlying column of rock.
• Tectonic Compression
– Convergent plate margins
• Tectonic Extension
– Divergent plate margins
• Shear
– Transform plate margins
Response of Rocks to Stress
• Elastic
– Reversible, no permanent change in volume or
shape.
• Ductile (Plastic)
– Permanent change in volume or shape.
• Brittle (fracture)
– Irreversible change involving rapid rupture.
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Geol 104: Geologic Structures
Rock’s Response to Stress
• Determined by:
– Temperature
• Higher temperature promotes ductile behavior.
– Confining pressure
• Higher pressure promotes ductile behavior.
– Strain rate and time
• Faster strain rate promotes brittle behavior.
– Material properties (composition)
• Silicates more brittle than carbonates, sulfates, etc.
– Water
• Presence of water promotes ductile behavior.
Geologic Structures
• Grain-scale structures
– Pores
– Solution cavities
• Outcrop-scale structures
– Folds
– Fractures
• Joints
• Faults
– Unconformities
Mapping Geologic Structures
• Strike
– Compass direction of planar feature.
• Dip
– Angle of dip - angle between planar feature
and the horizontal.
– Direction of dip - compass direction in which
angle of dip is measured.
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Geol 104: Geologic Structures
Terminology of Folds
• Folds are bends or wavelike features in
layered rocks.
• Folds are classified by their form:
– Anticline
• Convex upward arch, oldest rocks in center.
– Syncline
• Convex downward arch, youngest rocks in center.
– Plunging fold
• Fold axis inclined relative to horizontal.
Interpreting Folds
• Folds are further classified with regard to
the intensity and nature of stresses operative
(‘bending vs buckling’).
– Open folds
– Isoclinal
– Overturned
– Recumbent
Fractures
• Joints
– Fracture in rock along which there has been no
offset.
• Faults
– Fracture along a planar surface across which
rocks have moved relative to each other.
– Slip is the distance of offset across a fault.
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Geol 104: Geologic Structures
Types of Faults & Tectonic Setting
• Normal (dip-slip) faults
– Form due to tension forces as in a divergent
tectonic setting.
• Reverse or Thrust (dip-slip) faults
– Form due to compression forces as in a
convergent tectonic setting
• Strike-slip faults
– Form due to shear forces as in a transform
tectonic setting.
– Can be left or right-lateral in offset.
Why Care about Geologic Structures?
• Hydrocarbons [Energy] in Rocks
– Source rock
• geologic formation in which hydrocarbon originates.
– Hydrocarbon trap
• any rock barrier that accumulates hydrocarbons by
preventing upward migration.
– Cap rock
• impermeable rock that prevents upward migration of
hydrocarbons.
– Reservoir rock
• porous, permeable rock in which hydrocarbons
accumulate.
Why Care about Geologic Structures?
• Structural hydrocarbon traps
– Anticlines/Domes
– Faults
– Unconformities
– Salt domes
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Geol 104: Geologic Structures
Review - Tectonic Forces and
Geologic Structures
– What are geologic structures?
– Types of stress and stresses in the Earth
– Response of rock to stress
– Role of temperature, pressure, strain rate,
mineralogy and water on rock deformation
– Folds (terminology and interpretation)
– Faults (types and tectonic setting)
– Structure and Resources
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