GEOLOGIC STRUCTURES
Structural geology is the branch of geology that pertains to the shapes, arrangement, and interrelationships of
bedrock units and the forces that cause them. Structural geologists study the deformational features of Earth’s
crust and their mutual relations and origins.
I.
Tectonic Forces
(Forces that move and deform rocks, especially along plate boundaries.)
A.
1. Force per unit area
2. Acts on a body and tends to change its shape
B.
1. Change in rock size or shape in response to stress.
2. Rocks become deformed as a result.
C.
1.
a)
b)
c)
d)
Forces from opposite directions, towards each other
Common at convergent plate boundaries and the most common type of stress.
Rocks are deformed by shortening strain parallel to the direction of stress.
Perpendicular to the direction of stress (vertically) deformation is elongated.
2.
a) Forces pulling away from one another in opposite directions.
b) Results in stretching or extensional strain.
c) Bodies elongate or stretch parallel to the applied stress.
3. ___________________________
a) Forces parallel to one another in opposite directions, usually along a fault.
b) Results in shear strain parallel to the stress direction.
c) Occur along transform plate boundaries.
D. Behavior of Rocks to Stress and Strain
1. _______________________
a) A deformed body recovers its original shape with the removal of stress.
b) Most rocks behave elastically at low stresses
c) The elastic limit of a rock defines the amount of stress beyond which the rock will no longer act
elastically and will then deform permanently.
2.
a) During stress the rock body will bend but does not return to its original shape after a reduction of
stress.
b) Folding or bending of rock is the result of plastic behavior.
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Geologic Structures
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3. ___________________________
a) Rock bodies fracture at stresses greater than the elastic limit.
b) Occurs near or at Earth’s surface under conditions of low temperature and pressure.
c) Faults and joints are the result of brittle behavior.
II. Measuring and Representing Rock Units and Geologic Structures
A. Measuring the Attitude of Rock Units
1.
is the orientation of a rock unit or surface.
2.
a. The compass direction of a line formed by the intersection of a horizontal plane with an inclined
stratum, fault, fracture, or other surface.
b. While it’s bidirectional, customarily strike is expressed relative to north with the bearing in
degrees (e.g., N60oW, N35oE).
c. Determining the strike is the same as finding the bearing of the line of strike.
3.
a. ___________________ of Dip: The angle between a horizontal plane and the inclined stratum,
fault, or fracture.
b. ___________________ of Dip:
i
The compass direction in which the dip is measured.
ii
Determined by the direction in which water would run down the inclined surface.
iii Always perpendicular to strike.
4. Brunton Pocket Transit (Compass):
Used for measuring strike and dip as illustrated below.
Diagrams are from: Compton, Robert R., Manual of Field Geology. John W iley & Sons, New York, 1962.
The Brunton Compass
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Geologic Structures
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Measuring Strike and Dip with the Brunton Compass
A. Measuring approximate strike against a bedding plane
B. Measuring the dip of a bedding surface.
B. Representing Rock Structures
1.
a. Uses standardized symbols and patterns to represent rock types and geologic structures.
b. Contour lines representing the topography are also plotted.
2.
a. Drawing of a vertical slice through a portion of Earth.
b. Provides a cutaway view.
c. Constructed from geologic maps by projecting the dip of rock units into the subsurface.
3.
a. A combination of a geologic map and a cross section.
b. Provides a small three-dimensional model of a portion of Earth’s crust.
c. Has the appearance of a solid block
i
The geologic map is on top.
ii
A geologic cross section is shown on each visible side.
III. Types of Geologic Structures
A.
1. Upward or downward bends of rock layers.
2. Origins
a. Ductile Strain
b. Not metamorphic rock
c. Can form at moderate depth under conditions of high confining pressure which results in plastic
behavior.
d. Can form at shallow depths under conditions of a low rate of strain.
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Geologic Structures
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3. Fold Geometry
a.
i
ii
Also called a
Each stratum is bent around this imaginary axis.
b.
:
An imaginary plane through the fold axis.
: The rock strata on either side of the fold axis (sides of the fold)
c.
4. Types of Folds
a.
i
ii
iii
arching folds
Limbs dip away from the axial plane
Oldest rocks are in the middle
i
ii
iii
Downward-arching fold
Limbs dip towards the axial plane
Youngest rocks are in the middle of the fold
b.
c.
Folds that have two axial planes that separate two nearly horizontal
limbs from a more steeply inclined limb.
d. _______________________ Folds
i
The axial plane is inclined, not vertical
ii
Folds with vertical axial planes are symmetrical.
e.
i
ii
iii
iv
f.
i
ii
iii
The fold axis is not horizontal but, rather, plunges into Earth.
Plunge is the angle between the fold axis and the horizontal.
The trend of the plunge is the compass direction, measured in the direction that the axis is
inclined downward.
On surfaces leveled by erosion the exposed strata of plunging fold resemble V’s or
horseshoes while the exposed strata of a non-plunging fold exhibits a striped pattern.
Folds (“Hairpin” folds)
Have limbs that are parallel to one another
Implies intense compressive or shear stress.
Axial plane is vertical (upright fold)
Folds
g.
i
ii
iii
Unequal compressive stresses
Fold limbs dip in the same direction
Older rock strata on top of younger strata
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Geologic Structures
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h.
i
ii
iii
Folds
Overturned to such a degree that the limbs are essentially horizontal.
Found in cores of Canadian Rockies, Alps, and Himalayas.
Indicate compressive and/or shear stress was greater in one direction.
5.
a.
b.
c.
d.
e.
Large, somewhat circular structure in which strata have been warped upwards.
Strata dip away from a central point
The cross section resembles an anticline.
Large-scale features (often more than 100 km across)
Strata are oldest in the center of the dome.
a.
b.
c.
d.
e.
Large, somewhat circular structure in which strata have been warped downwards.
Strata dip towards a central region.
The cross section resemble a syncline.
Also a large-scale feature.
Strata are youngest in the center of the basin.
6.
B. Fractures in Rocks
1.
a. Breaks in rocks with essentially no displacement on either side of the crack.
b. Joint Sets have joints oriented approximately parallel to one another.
c. Joints can have non-tectonic origins (i.e., columnar jointing in basalts, sheet jointing as a result
of weathering).
2.
a. Faults are fractures in bedrock along which
b.
i
ii
iii
has occurred
Faults
Slippage has occurred parallel to the direction of dip.
Classified on the basis of relative movement of the
(a) Footwall: underlying surface of an inclined fault plane
(b) Hanging Wall: Overlying surface of an incline fault plane
Types
(a) ________________ Fault:
1
The hanging wall block has moved downward in relation to the hanging wall
block.
2
Caused by tensional stress.
3
Named because it’s a normal response to gravity (can also be called a gravity
fault.
4
If a block bounded by normal faults drops down a graben is formed (from the
German word for “ditch”. This occurs at rift valleys.
5
If a block bounded by normal faults is uplifted it becomes a horst (from German
for mountain) and a fault-block mountain range has formed.
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Geologic Structures
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(b)
c.
_______________________ Faults
1
The hanging wall block has moved upward relative to the footwall block.
2
Caused by compressive stress.
3
Thrust faults are reverse faults that develop at a very low angle to the horizontal.
Faults
i
ii
iii
Movement is horizontal, parallel to the direction of strike.
Caused by shear stress.
(a) Right lateral fault:
1
Rocks on opposite side appear to have moved to the right.
2
If a stream or other feature has been displaced, it would appear to the right
across the fault.
(b) Left lateral fault:
1
Rocks on opposite side appear to have moved to the left.
2
If a stream or other feature has been displaced, it would appear to the left across
the fault.
The San Andreas fault in California is a large strike-slip fault.
d. _______________Faults: Have displacement components parallel to the fault’s strike and the
dip
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Geologic Structures
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