Crustal Deformation
10
Crustal Deformation begins with a brief examination of the processes of crustal deformation, including the
various types of stress, strain, and the factors that affect rock deformation. The mapping of geologic structures
is examined along with the concept of strike and dip. The various types of folds (anticlines, synclines, domes,
and basins) and faults (both dip-slip and strike-slip) are investigated in detail. The chapter closes with a
discussion of the formation and geologic significance of joints.
Learning Objectives
After reading, studying, and discussing the chapter, students should be able to:
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Briefly discuss the significance and role of structural geology in understanding Earth.
Explain the concept of deformation, including force, stress, and strain.
Compare and contrast the various types of stress.
Discuss how temperature, confining pressure, rock type, and time all affect rock deformation.
Distinguish between brittle and ductile deformation.
Explain the concept of strike and dip and how it relates to the mapping of geologic structures.
Discuss the folding of rocks, including the origin, characteristics, and some geographic examples of
folding.
Discuss the faulting of rocks, including the origin and stresses responsible for faults.
Compare and contrast the two major categories of faults, including the terminology and movements
used to define the various types of faults and some geographic examples where faulting occurs.
Briefly discuss the origin and significance of joints.
Chapter Outline___________________________________________________________________
I.
II.
Structural Geology: a study of Earth’s
architecture
A. Structural geologists study the
architecture of Earth’s crust and “how it
got to be that way” insofar as it resulted
from deformation
B. A working knowledge of rock structures
is essential to our modern way of life
B.
Most crustal deformation occurs along
plate margins
C. Deformation involves
1. Force – that which tends to put
stationary objects in motion or
change the motions of moving
objects
2. Stress
a. The amount of force applied to a
given area
b. Types of stress
1. Confining pressure – stress
applied uniformly in all
directions
Deformation
A. Deformation is a general term that refers
to all changes in the original form and/or
size of a rock body
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2. Differential stress – applied
unequally in different
directions
a. Compressional stress –
shortens a rock body
b. Tensional stress – tends to
elongate or pull apart a rock
unit
c. Differential stress can also
cause a rock to shear – a
motion similar to the
slippage that occurs
between individual playing
cards when the top of the
deck is moved relative to
the bottom
d. Strain – irreversible change
in the shape and size of a
rock body caused by stress
D. How rocks deform
1. Rocks subjected to stress greater than
their own strength begin to deform
usually by
a. Folding
b. Flowing, or
c. Fracturing
2. General characteristics of rock
deformation
a. Elastic deformation
1. The rock returns to nearly its
original size and shape when
the stress is removed
2. Once the elastic limit
(strength) of a rock is
surpassed, it either
a. Flows (ductile
deformation), or
b. Fractures (brittle
deformation)
b. Factors that influence the strength
of a rock and how it will deform
1. Temperature
2. Confining pressure
3. Rock type
4. Time
III.
Mapping geologic structures
A. When conducting a study of a region, a
geologist identifies and describes the
dominant rock structures
1. Usually only a limited number of
outcrops, sites where bedrock is
exposed at the surface, are available
2. Work is aided by advances in
a. Aerial photography
b. Satellite imagery, and
c. Global Positioning Systems (GPS)
B. Describing and mapping the orientation
or attitude of a rock layer or fault surface
involves determining the features
1. Strike (trend)
a. The compass direction of the line
produced by the intersection of an
inclined rock layer or fault, with a
horizontal plane
b. Generally expressed as an angle
relative to north (e.g., N10°E)
2. Dip (inclination)
a. The angle of inclination of the
surface of a rock unit or fault
measured from a horizontal plane
b. Includes both
1. An angle of inclination, and
2. A direction toward which the
rock is inclined
3. The direction of dip is always
at a 90° angle to the strike
IV.
Folds
A. During crustal deformation rocks are
often bent into a series of wavelike
undulations called folds
B. Characteristics of folds
1. Most folds result from compressional
stresses which shorten and thicken
the crust
2. Parts of a fold
a. Limbs – refers to the two sides of
a fold
b. Axis – a line drawn down the
points of maximum curvature of
each layer
Crustal Deformation
1.
Horizontal, or parallel to the
surface, or
2. Inclined at an angle to the
surface known as the plunge
c. Axial plane – an imaginary
surface that divides a fold as
symmetrically as possible
C. Common types of folds
1. Anticline – upfolded, or arched, rock
layers
2. Syncline – downfolds, or troughs, of
rock layers
3. Depending on their orientation,
anticlines and synclines can be
a. Symmetrical – limbs are mirror
images of each other
b. Asymmetrical – limbs are not
mirror images of each other
c. Recumbent – an overturned fold
d. Where the ends of folds die out,
they are said to be plunging
4. Monoclines – large, step-like folds in
otherwise horizontal sedimentary
strata
D. Other types of folds
1. Dome
a. Upwarped displacement of rocks
b. Circular, or slightly elongated
structure
c. Oldest rocks in center, younger
rocks on the flanks
d. May contain hogbacks –
prominent angular ridges formed
by outcroppings of resistant strata
that have resulted from
differential erosion
2. Basin
a. Circular, or slightly elongated
structures
b. Downwarped displacement of
rocks
c. Youngest rocks are found near the
center, oldest rocks on the flanks
V.
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Faults
A. Faults are fractures (breaks) in rocks
along which appreciable displacement
has taken place
B. Sudden movements along faults are the
cause of most earthquakes
C. Classified by their relative movement
which can be
1. Horizontal
2. Vertical, or
3. Oblique
D. Types of faults
1. Dip-slip fault
a. Movement is primarily parallel to
the dip (or inclination) of fault
surface
b. May produce long low cliffs
called fault scarps
c. Parts of a dip-slip fault
1. Hanging wall – the rock
surface that is immediately
above the fault
2. Footwall – the rock surface
below the fault
d. Types of dip-slip faults
1. Normal fault
a. Hanging wall block moves
down relative to the
footwall block
b. Accommodate
lengthening, or extension,
of the crust
c. Most are small, with
displacements of only a
meter or so
d. Large scale normal faults
1. Associated with
structures called faultblock mountains
2. e.g., Teton Range of
Wyoming and Sierra
Nevada of California
e. Prevalent at spreading
centers where plate
divergence occurs
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Forms a central block,
called a graben,
bounded by normal
faults as the plates
separate
2. Grabens produce an
elongated valley
bounded by uplifted
structures called horsts
2. Reverse and thrust faults
a. Hanging wall block moves
up relative to the footwall
block
1. Reverse faults have dips
greater than 45°
2. Thrust faults have dips
less than 45°
b. Accommodate shortening
of the crust
c. Strong compressional
forces
2. Strike-slip fault
a. Dominant displacement is
horizontal, and parallel to the
strike of the fault
b. Types of strike-slip faults
1. Right-lateral – as you face the
fault, the crustal block on the
opposite side of the fault
moves to the right
Left-lateral – as you face the
fault, the crustal block on the
opposite side of the fault
moves to the left
3. Transform fault
a. Large strike-slip fault that
cuts through the
lithosphere
b. Accommodates motion
between two large crustal
plates
1.
2.
VI.
Joints
A. Fractures called joints are among the
most common rock structure
B. Most occur in roughly parallel groups
C. With the exception of columnar joints
and curved joints associated with
sheeting, most joints are produced when
rocks in the outermost crust are
deformed.
D. Significance of joints
1. Chemical weathering tends to be
concentrated along joints
2. Most important mineral deposits are
emplaced along joint systems
3. Highly jointed rocks often represent a
risk to construction projects
Answers to the Review Questions
1. Rock deformation describes how the shape and volume of a rock change in response to stress. Think of a
small, reference cube or sphere embedded in an undeformed rock. With the application of stress, the rock
deforms (undergoes strain) and any changes in the volume and dimensions of the reference object are
recorded by the strain. Depending on the magnitude and type of stress involved, deformation may also
produce changes in the location and orientation of a rock.
2. Five geologic structures associated with deformation are anticlines, synclines, normal faults, strike-slip
faults, and joints. The type of geologic structure(s) formed in a given area is dependent upon several
factors including the type of applied stress, rock type, temperature, confining pressure, and time.
Crustal Deformation
85
3. Stress is closely related to force although the two are technically not the same. Force refers to any action
that tends to put stationary objects into motion or change the motion of moving objects. Stress refers to
the amount of force applied to a given area. Therefore, stress is force that is specified over a defined area.
4. Compressional stress is a type of differential stress (applied unequally in different directions) that tends to
shorten or squeeze a rock body. Shortening occurs in the direction parallel to the direction of the
compressional stress and elongation or stretching occurs in the direction perpendicular to the
compressional stress. Compressional stresses are associated with tectonic plate collisions where Earth’s
crust is shortened and thickened by folding, flowing, and faulting. Tensional stress works in the opposite
of compressional stress in that it tends to stretch or elongate a rock body. Tensional stresses occur where
tectonic plates are pulled apart at divergent plate boundaries.
5. Shearing refers to the unequal or step-wise slippage that occurs to parallel layers or surfaces when
differential stresses are applied. It is similar to the movements between individual playing cards when the
top of the deck is moved relative to the bottom. In a near-surface environment, where temperatures and
confining pressures are low, shearing often occurs on closely spaced surfaces of weakness such as
bedding planes, foliation, and joints. At greater depths, temperature and confining pressures are higher
and shearing is accomplished by solid state flow.
6. As discussed above, stress is force applied over a certain area. The irreversible changes in the size and
shape of an object produced by stress is known as strain. Therefore, strain can be thought of as the
permanent deformation that results from stress.
7. Brittle deformation describes material failure by cracking and rupture. Faults and joints in rocks are good
examples. Brittle deformation is favored by shallow depths, low rock temperatures, and massive rigid
rocks. Ductile deformation describes material failure by internal flowage; recrystallization is usually
involved, especially at elevated temperatures. Ductile deformation is enhanced by elevated temperatures
and confining pressures. Folding at great depths and elevated temperatures is accomplished by ductile
(plastic) flowage without rupture. At shallow depths, layered sedimentary rocks can readily fold (be
shortened in a horizontal direction by crumpling and buckling) because the layers bend internally and
slide past one another along the bedding surfaces.
8. Rocks fail by brittle and ductile deformation when applied stresses exceed their elastic limit (strength).
Temperature, confining pressure, and mineral composition exert important influences on rock strength
and rock deformation.
High temperatures and high confining pressures favor plastic deformation over brittle fracturing; brittle
cracking and fracturing are favored at low temperatures and low confining pressures. Mineral
composition, texture, and other bulk-rock characteristics (stratification, compositional heterogeneity,
porosity, cementation, etc.) have important effects on rock strength and on ways that rocks deform.
Minerals such as halite and gypsum readily recrystallize and flow at low temperatures; at elevated
temperatures, granitic (felsic) rocks recrystallize and flow at lower temperatures than do mafic rocks. The
effect of mineralogy on rock strength is well illustrated by comparing deformation in different,
monomineralic rocks such as limestone (marble) and quartz sandstone under similar conditions. At low
temperatures and low confining pressures, both fail by brittle fracturing. Calcite recrystallizes at lower
temperatures than quartz; thus, calcite-rich rocks deform by recrystallization and flowage at lower
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temperatures and confining pressures than do quartzites. At elevated temperatures and confining
pressures, both rocks deform by ductile flowage.
9. Outcrops are surface exposures of the local, subsurface, lithological material or bedrock. As such, they
provide the basic information and data utilized in geologic mapping. Outcrops provide samples of the
bedrock and exhibit those structures and features (such as stratification, cross bedding, mineral veinlets,
faults, cleavage, etc.) that help a geologist interpret the geologic history of an area. In areas with extensive
coverings of soil or other surficial materials (regolith, glacial deposits, landslide debris, sand dunes, etc.),
information about the bedrock and subsurface lithologies may be available only through drilling.
10. The measurements are strike and dip (Figs. 10.7 and 10.8). Strike and dip defines the orientation of
geologic surfaces, such as stratification planes, contacts between different rock units, faults, and joints.
Strike is the compass direction (with respect to geographic north) of any horizontal line lying in the
geologic surface. Dip is the angle between the geologic surface and a horizontal plane; it is visualized and
measured in a vertical plane aligned at right angles to the strike line. On geologic maps, a strike and dip
symbol is shown as a longer, straight line drawn parallel to the strike; the shorter line of the symbol
(drawn at right angles to the strike) points in the dip direction. The numerical value of the dip angle is
generally printed with the symbol (Fig. 10.8).
11. Anticlines are folds with two, well-defined limbs dipping in opposite directions away from a long, linear,
fold axis. Strata are raised or buckled upward along the axial part of the fold relative to their elevations
farther out on the limbs; thus after erosion, older strata are exposed along the axial part of the fold.
Synclines are folds with two, well-defined limbs that dip inward toward a long, linear, fold axis. Strata are
lowered or buckled down in the axial region; thus after erosion, younger strata are exposed in the axial
portions of synclines.
Domes are more or less circular zones of upraised rocks in which the beds follow the geometry of a dome
and dip away in all directions from a high point or apex. Unlike an anticline, the dome structure does not
have an axis. Geometrically, a basin may be thought of as an inverted dome. The strata dip inward in all
directions toward the central, most downbuckled point in the structure. Anticlines and synclines have
long, roughly parallel limbs and linear axes. Limbs of domes and basins make circular outcrop patterns,
and the crests of domes and the lowest parts of basins are points, not axes. It is also very important to
distinguish between structural and topographic basins and domes (Figs. 10.15, 10.16, and 10.17).
12. Both are types of folds; they typically form in layered, sedimentary strata. Monoclines have only one limb
(Fig. 10.14) and strata that are steeply inclined within the structure are sub-horizontal and relatively
undeformed laterally. Monoclines of the Colorado Plateau, such as the Waterpocket fold in Utah, formed
in Phanerozoic sedimentary strata above reverse faults in the crystalline, Proterozoic basement rocks.
Fault offsets in the crystalline rocks at depth were gradually accommodated upward by bending of the
sedimentary strata.
Anticlines and synclines form as layered strata are squeezed and crumpled by unidirectional, horizontal,
compressive stresses. In simple anticlines, strata dip in opposite directions (Fig. 10.9), and older strata are
elevated in the axial part of the fold. Extensive areas or regions of folded strata (Figs. 10.10), such as the
Valley and Ridge province of the Appalachian region in the eastern United States, consist of numerous,
laterally connected anticlines and synclines.
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87
13. The Black Hills (Fig. 10.16) are a late Cretaceous-early Tertiary, elliptically shaped, domal uplift of older
crystalline rocks flanked by Paleozoic and Mesozoic sedimentary rocks that dip away from the elevated
core of Precambrian rocks.
14. Both are dip-slip movements in which one block moves up and the other down along the fault surface.
Assume that dip-slip faults with vertical dips (the fault surface is vertical) are normal faults. For dip-slip
faults with inclinations or dips other than vertical, the hanging wall-footwall designation is very useful.
The hanging wall block is the block that is entirely above the fault surface, and the footwall block is
entirely below. In normal fault movement, the hanging wall block slides down along the fault surface with
respect to the footwall block. Horizontal distances between points in the blocks are increased (stretched)
and the stresses are tensional. In reverse fault movement, the hanging wall block slides upward along the
fault surface with respect to the footwall block. Horizontal distances between points in the two blocks are
decreased (shortened) and the stresses are compressional.
15. Based on the relations shown in the photo (Fig. 10.18), the sense of displacement is normal. The hanging
wall block (left) slipped down with respect to the footwall block (right). Even without the arrows on the
photograph showing relative motion, the stratigraphic sequence can be matched across the fault to
determine the sense of movement along the fault.
16. A horst (Fig. 10.22) is an uplifted, fault block bounded by two normal faults. A graben (Fig. 10.22) is a
down dropped, fault block bounded by two normal faults. A graben valley is the down dropped surface of
an active or recently active graben. The valley is bounded by uplifted fault blocks, which may or may not
be horsts. Death Valley in southeastern California is a good example of a graben valley.
17. Fault-block mountains (Fig. 10.22) are associated with geologically young, high-angle normal faults that
flatten or merge with a regional-scale, low-angle fault at depth. Uplifted blocks form the mountain ranges
and down dropped blocks form the valleys. The topography replicates active or recently active fault
movements. Long, linear, fault-block ranges and valleys are horsts and grabens.
18. Both are brittle failure, dip-slip faults caused by lateral compression. The hanging wall block moves up
and over the footwall block, and overall, horizontal distance perpendicular to the fault trace is shortened.
The main distinction is based on the dip angle or inclination of the fault. Reverse faults are high-angle,
dip-slip faults and thrusts are low-angle, dip-slip faults. In sub-horizontal sedimentary strata, thrusts can
propagate along weak bedding plane zones, resulting in extensive, horizontal displacement, crustal
shortening, and emplacement of older strata over younger strata.
19. The San Andreas fault is a well-known, strike-slip (transform) fault (Fig. 10.27) that forms the boundary
between the North American and Pacific plates between the head of the Gulf of California and the
Mendocino fracture zone north of San Francisco (Box 10.2). Canyons in the hilly terrain to the right of
the fault trace are beheaded and offset, suggesting active faulting with an important, strike-slip
component. A canyon showing right-lateral displacement is clearly evident in the larger-scale photo (Fig.
10.C). Linear valleys, mangled and pulverized rock of the fault zone, sag ponds, seeps and springs, offset
drainages, numerous earthquakes, and juxtaposition of fundamentally different bedrock assemblages are
characteristic features of active, strike-slip faults.
20. Tensional faults (normal faults) dominate at divergent boundaries, and faults due to compression (reverse
faults) dominate at convergent margins. Transform (sliding) plate boundaries are strike-slip faults.
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21. Faults and joints are both fractures in rock. Along faults, the fracture-bounded blocks have been displaced
(offset) from their unfractured positions; the blocks are not significantly displaced along joints. Joints
typically come in sets. A joint set is a group of fractures in a given area that more-or-less exhibit a
common orientation (strike and dip). Multiple joint sets may be present in any given area. Joints usually
exhibit a strong control over differential weathering and erosion. The fractures, being zones of weakness
and accessible to water, weather and erode faster than stronger, unfractured bedrock.
Lecture outline, art-only, and animation PowerPoint presentations for each chapter of Earth,
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