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Structural Geology
Structural Geology
Tectonic collision deforms crustal rocks
producing geologic structures.
Folds
Faults
Joints and Fractures
Deformation
All changes in the original location,
orientation or form of a crustal rock body.
Deformation common
at plate margins.
Deformation concepts…
Force
Stress
Strain
Force
Force – Mass x acceleration (F = ma)
The action that puts stationary objects in
motion or
Changes the motion of moving objects.
Stress
Stress - Force applied to a given area.
Determines the concentration of force.
Differential Stress – Unequal in different
directions.
3 major types of differential stress
Compressional stress
Tensional stress
Shear stress
Compressional Stress
“Push-together” stress.
Shortens and thickens crust.
Associated with orogenesis (mtn. building).
Tensional Stress
“Pull-apart” stress.
Thins and stretches crust.
Associated with rifting.
Stephen Marshak
Shear Stress
Slippage of one rock mass past another.
In shallow crust, shear is often
accommodated by bedding planes.
Strain
Changes in the shape or size of a rock body
caused by stress.
Strain occurs when stresses exceed rock
strength.
Strained rocks deform by folding, flowing, or
fracturing.
How Rocks Deform
Elastic deformation – The rock returns to
original size and shape when stress removed.
When the (strength) of a rock is surpassed, it
either flows (ductile deformation) or fractures
(brittle deformation).
Brittle behavior occurs in
the shallow crust; ductile in
the deeper crust.
Stephen Marshak
How Rocks Deform
Factors controlling rock strength and
deformation style.
Temperature and confining pressure
Low T and P = brittle deformation
High T and P = ductile deformation
Rock type – Mineral composition controls
strength.
Time – Stress applied for a long time generates
change.
Mapping Geologic Structures
Geologists describe and interpret rock structures.
Structure usually determined from a limited number of
outcrops.
Mapping is aided by advances in aerial photography,
satellite imagery and Global Positioning Systems (GPS).
The most common and useful technique for geological
mapping remains….
FIELD WORK !!
The Formation
A mappable rock unit.
Mapping Geologic Structures
Describing and mapping the orientation of a
geologic structure or fault surface involves
determining …
Strike (trend)
Dip (inclination)
Mapping Geologic Structures
Strike (trend)
The compass direction of the line produced by
the intersection of an inclined rock layer or fault
with a horizontal plane.
Generally expressed an an angle relative to
north.
N37°E
N12°W
Mapping Geologic Structures
Dip (inclination)
The angle of inclination of the surface of a rock
unit or fault measured from a horizontal plane.
Includes both an angle of inclination and a
direction toward which the rock is inclined.
82°SE
17°SW
Folds
Rocks are bent by crustal deformation into a
series of wave-like undulations called folds.
Most folds result from compressional stresses
which shorten and thicken the crust.
Stephen Marshak
Characteristics of Folds
Parts of a fold
Limbs – The two “sides” of a fold.
Fold axis or hinge line – A line connecting points
of maximum curvature along a fold.
Axial plane – An imaginary surface that divides
a fold symmetrically.
Common Types of Folds
Anticline – Upfolded or arched rock layers.
Syncline – Downfolds or rock troughs. (Think
“sink”)
Depending on their orientation, anticlines and
synclines can be described as
Symmetrical
Asymmetrical
Recumbent (an overturned fold)
Plunging
Anticline
Syncline
Anticlines and Synclines are common in fold
and thrust belts related to mountain belts.
Common Types of Folds
Monoclines – Large, step-like folds in otherwise
horizontal sedimentary strata.
Domes -Upwarped circular or slightly elongated
structure. Oldest rocks in center, younger rocks
outside.
Basins – Downwarped circular or slightly elongated
structure. Youngest rocks are found near the
center, oldest rocks on the flanks.
Faults
Faults
Breaks in rock that exhibit offset.
Exist at a variety of scales.
Sudden movements along faults are the cause
of most earthquakes.
Classified by movement…
Horizontal
Vertical
Oblique
Faults
Faults grind rocks to create fault gouge.
Walls of a fault bear evidence of this
grinding as slickensides.
“Slicks” reveal
fault direction.
Fault Types
Dip-slip faults – Motion is parallel to fault
dip.
Strike-slip faults – Motion is parallel to fault
strike.
Dip Slip Faults
May produce long, low cliffs called fault
scarps.
Dip Slip Faults
Fault blocks classified as
Footwall (rock
mass
below the fault)
Hanging wall
(rock mass
above the fault)
Types of Dip-Slip Faults
Two dominant types
Normal fault
Reverse Fault
Thrust (a low angle reverse fault)
Types of Dip-Slip Faults
Normal fault
Hanging wall moves down relative to the
footwall.
Accommodate lengthening or extension of the
crust.
Exhibit a variety of scales.
Normal Faults
Larger scale normal faults are associated
with fault-block mountains (Basin and Range
of Nevada).
Normal fault bounded valleys are called
grabens (Rhine graben).
Normal fault bounded ridges are called
horsts.
Fig. 11.17b
W. W. Norton
Types of Dip-Slip Faults
Reverse faults
Hanging wall block moves up relative to the
footwall block
Reverse faults have dips greater than 45o and
thrust faults have dips less then 45o
Accommodate shortening of the crust
Strong compressional forces
Types of Dip-Slip Faults
Thrust faults - A special case of reverse fault.
Hanging wall block moves up relative to the
footwall block
Thrust faults are characterized by a low dip
angle (less then 45o).
Accommodate shortening of the crust
Strong compressional forces
Fig. 11.17a
W. W. Norton
U.S. Geological Survey
Strike-Slip Faults
Dominant displacement is horizontal and
parallel to the strike of the fault
Types of strike-slip faults
Right-lateral – as you face the fault, the block on
the opposite side of the fault moves to the right
Left-lateral – as you face the fault, the block on
the opposite side of the fault moves to the left
Strike-Slip Faults
Strike-slip fault
Transform fault
– Large strike-slip fault that cuts through the
lithosphere
– Accommodates motion between two large
crustal plates
Joints
Joints are a very common
rock structure.
They are fractures with no
offset.
Result from tectonic
stresses on rock mass.
Occur in parallel groups.
Significance of Joints
Chemical weathering tends to be
concentrated along joints
Many important mineral deposits are
emplaced along joint systems
Highly jointed rocks often represent a risk to
construction projects
QUESTIONS