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Geologic Structure
Structural geology
Structural geologists study the
architecture and processes responsible for
deformation of Earth’s crust
Rock structures matter! Oilfields are
contained in structures as are ore deposits
Deformation
Deformation refers to all changes in the
original form and/or size of a rock body
Most crustal deformation occurs along
plate margins
Occurs from tectonic forces
Deformation
Deformation involves
• Stress - force divided by area
• Types of stress
– Compressional stress – shortens rocks
– Tensional stress – extends rocks
– Shear stress – pushing in different
directions.
Deformation
Strain – changes in the shape or size of a rock
body from stress
How rocks deform
• Rocks subjected to stresses greater than
their own strength deform by fracturing,
folding or flowing.
Deformation
How rocks deform
• General characteristics
– Elastic deformation – the rock returns to
nearly its original size and shape when the
stress is removed
– When a rock breaks, it is called brittle
deformation. Any material that breaks into
pieces exhibits brittle behavior
– Low temperature and pressure conditions
– When rocks bend or flow, like clay, it is called
ductile deformation
– High temperature and pressure conditions
– Earthquakes release elastic energy
Deformation
How rocks deform
• General characteristics of rock
deformation
– Factors that influence the strength of a rock
and how it will deform
– Temperature
– Confining pressure
– Rock type
– Time (strain rate)
Folds formed under high grade metamorphic conditions
Mapping geologic structures
When conducting a study of a region, a
geologist identifies and describes the
dominant rock structures
• Work is aided by advances in aerial
photography, satellite imagery, digital
topography and Global Positioning
Systems (GPS)
Strike and Dip
Strike: Direction of the line of intersection between a tilted plane and a
horizontal plane.
• 2-directional line
Strike and Dip
Dip: Angle between a tilted plane and a horizontal plane that is
perpendicular to strike.
• Maximum angle ( ≤ 90º )
• Notation:
____ ° N, S, E, W, NE, NW, SE, SW
W
E
Horizontal
Dip Angle
(E. McBride)
Cross-Section: Dipping Strata
W
E
Horizontal
Dip Angle
(E. McBride)
Cross-Section: Dipping Strata
Map Symbols
strike and dip
45º
strike and dip (vertical)
strike and dip (horizontal)
Faults
• Types of dip-slip faults
– Reverse and thrust faults
– Hanging wall moves up relative to
footwall
– Accommodate shortening of the crust
– Results from compressional forces
– Reverse- dip of fault plane is above 45
degrees
– Thrust- dip of fault plane is less than 20
degrees.
On a reverse fault, the hanging wall
moves up relative to the footwall
Thrust faults formed by crustal shortening
Faults
Strike-slip fault
• Displacement is horizontal and parallel to strike of
fault
• Types of strike-slip faults
– Right-lateral – as you face the fault, the block on the
opposite side moves right
– Left-lateral – as you face the fault, the block on the
opposite side moves left
• Because of their large size and linear nature, many
strike-slip faults produce a trace that is visible over
a great distance
• Crushed and broken rocks produced during
faulting are more easily eroded, often producing
linear valleys or troughs
A block diagram showing the
features along a strike-slip fault
Fault
Strike-slip fault
• Transform fault
– Large strike-slip fault that cuts through the
lithosphere
– Accommodates motion between two large
crustal plates
The San Andreas fault system
is a major transform fault