Grotzinger • Jordan
Understanding Earth
Sixth Edition
Chapter 7:
DEFORMATION
Modification of Rocks by
Folding and Fracturing
© 2011 by W. H. Freeman and Company
Chapter 7:
DeformationModification of Rocks
by Folding and
Fracturing
About Deformation
• Deformation mainly occurs near plate
boundaries.
• Field observations of deformation show
us how to reconstruct geologic history.
• Deformation includes faulting of rigid
rocks and folding of rocks that can be
bent.
Lecture Outline
1. Plate tectonic forces
2. Mapping geologic structure
3. How rocks deform
4. Basic deformation structures
5. Unraveling geologic history
1. Plate Tectonic Forces
● Deformation
● tensional forces
● compressive forces
● shearing forces
2. Mapping Geologic Structure
● Outcrop – basic source of
geologic information in the
field
2. Mapping Geologic Structure
2. Mapping Geologic Structure
2. Mapping Geologic Structure
2. Mapping Geologic Structure
● Measuring strike and dip
● strike is the compass
direction of a rock layer as it
intersects a horizontal surface
● dip is the amount of tilting of
the layer and is measured at
right angles to strike
2. Mapping Geologic Structure
N
W
S
45°
Dip
angle
E
N
W
S
45°
Dip angle
E
2. Mapping Geologic Structure
N
W
S
45°
Dip
angle
E
N
W
S
Water trickles down
slope parallel to dip.
45°
Dip angle
E
Dinosaur Ridge, located
west of Denver, Colorado.
2. Mapping Geologic Structure
● Geologic maps
● geologic maps represent
the rock formations
exposed at Earth’s surface
● a common scale for
geologic maps is 1:24,000
2. Mapping Geologic Structure
● Geologic cross sections
● geologic cross sections –
diagrams showing the
features that would be
visible if vertical slices were
made through part of the
crust
Thought questions for this chapter
In what sense is a geologic map a scientific model of the
surface geology? Is it fair to say that geologic cross
sections in combination with a geologic map describe a
scientific model of a three-dimensional geologic
structure?
Why is it correct to say that “large-scale geologic
structures should be represented on small-scale geologic
maps”? How large a piece of paper would be required to
make a map of the entire U.S. Rocky Mountains at 1;24,000
scale?
Can you explain the geologic story in Exercise 6 (p. 188) in
terms of plate tectonics?
3. How Rocks Deform
● Rock behavior in the laboratory
● brittle
● ductile
3. How Rocks Deform
An undeformed sample
Under conditions
representative of the
shallow crust, the
marble is brittle.
An undeformed sample
Under conditions
representative of the
shallow crust, the
marble is brittle.
Under conditions
representative of
the deeper crust,
marble is ductile.
An undeformed sample
3. How Rocks Deform
● Rock behavior in the Earth’s crust
● depths affect brittle v. ductile
● rock type affects way rocks
deform
● rate of deformation is a factor
4. Basic Deformation Structures
● Types of faults
● dip-slip – normal, reverse,
and thrust
● strike-slip – right- and leftlateral
● oblique-slip
4. Basic Deformation Structures
Example
of a strikeslip fault
4. Basic Deformation Structures
● Types of folds
● symmetrical folds anticlines and synclines
● asymmetrical folds
● overturned folds
● plunging folds
4. Basic Deformation Structures
4. Basic Deformation Structures
4. Basic Deformation Structures
● Circular structures
● dome
● basin
Example
of a dome
Example
of a basin
4. Basic Deformation Structures
● Other features
● joints
● deformation (cataclastic) textures
● fault breccia
● mylonite
joints
fault
breccia
mylonite
5. Styles of Continental Deformation
● Tensional tectonics
● Compressive tectonics
● Shearing tectonics
5. Styles of Continental Deformation
5. Styles of Continental Deformation
5. Styles of Continental Deformation
Example of
tensional
tectonics
Example of
compressive
tectonics
Example of
shearing
tectonics
Thought questions for this chapter
The submerged margin of a continent has a thick layer of
sediments overlying metamorphic basement rocks. That
continental margin collides with another continental mass,
and the compressive forces deform it into a fold and
thrust belt. During the deformation, which of the following
geologic formations would be likely to behave as brittle
materials and which as ductile materials?
(a) sedimentary formations in the upper few km
(b) metamorphic basement rocks at depths of 5-15
km
(c) lower crustal rocks below 20 km
In which of these layers would you expect earthquakes?
6. Unraveling Geologic History
● Geologic history is a
succession of episodes
of deformation and other
geologic processes.
● Can be described in time
steps 1, 2, 3, …
TIME 1
Sediments are deposited
on the seafloor.
TIME 2
Compressive forces cause
folding and faulting.
compressive
forces
faults
TIME 3
Uplift is followed by erosion, which
creates new horizontal surface.
TIME 4
Volcanic eruptions cover
the surface with lava flows.
lava flows
TIME 5
Tensional forces cause
normal faults, creating
down-dropped
blocks and
breaking up
earlier features.
tensional forces
normal faults
Thought questions for this chapter
In his epic narrative about a geologic traverse across
North America, Annals of a Former World, writer John
McPhee called geologic maps “textbooks on a piece of
paper.” Can you locate a passage in this textbook that
describes a geologic structure and sketch a geologic map
consistent with McPhee’s description?
Key terms and concepts
Anticline
Basin
Brittle
Compressive force
Deformation
Dip
Dip-slip fault
Dome
Ductile
Fault
Fold
Foot wall
Formation
Geologic cross section
Geologic map
Key terms and concepts
Joint
Normal fault
Shearing force
Strike
Strike-slip fault
Tensional force
Thrust fault