deformation

1. Grotzinger • Jordan
Understanding Earth
Sixth Edition
Chapter 7:
DEFORMATION
Modification of Rocks by
Folding and Fracturing
© 2011 by W. H. Freeman and Company

2. Chapter 7:
DeformationModification of Rocks
by Folding and
Fracturing

3. 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.

4. Lecture Outline
1. Plate tectonic forces
2. Mapping geologic structure
3. How rocks deform
4. Basic deformation structures
5. Unraveling geologic history

5. 1. Plate Tectonic Forces
● Deformation
● tensional forces
● compressive forces
● shearing forces

6. 2. Mapping Geologic Structure
● Outcrop – basic source of
geologic information in the
field

7. 2. Mapping Geologic Structure

8. 2. Mapping Geologic Structure

9. 2. Mapping Geologic Structure

10. 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

11. 2. Mapping Geologic Structure
N
W
S
ion Strike d
ect
irecti
dir
on
Dip 45°
Dip
angle
E
N
W
S
45°
Dip angle
E

12. 2. Mapping Geologic Structure
N
W
S
ion Strike d
ect
irecti
dir
on
Dip 45°
Dip
angle
E
N
W
S
Water trickles down
slope parallel to dip.
45°
Dip angle
E

13. Dinosaur Ridge, located
west of Denver, Colorado.

14. 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

15. 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

17. 3. How Rocks Deform
● Rock behavior in the laboratory
● brittle
● ductile

18. 3. How Rocks Deform
An undeformed sample

19. Under conditions
representative of the
shallow crust, the
marble is brittle.
An undeformed sample

20. Under conditions
representative of the
shallow crust, the
marble is brittle.
Under conditions
representative of
the deeper crust,
marble is ductile.
An undeformed sample

21. 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

22. 4. Basic Deformation Structures
● Types of faults
● dip-slip – normal, reverse,
and thrust
● strike-slip – right- and leftlateral
● oblique-slip

23. 4. Basic Deformation Structures

25. Example
of a strikeslip fault

27. 4. Basic Deformation Structures
● Types of folds
● symmetrical folds anticlines and synclines
● asymmetrical folds
● overturned folds
● plunging folds

28. 4. Basic Deformation Structures

30. 4. Basic Deformation Structures

32. 4. Basic Deformation Structures
● Circular structures
● dome
● basin

33. Example
of a dome

34. Example
of a basin

35. 4. Basic Deformation Structures
● Other features
● joints
● deformation (cataclastic) textures
● fault breccia
● mylonite

36. joints

37. fault
breccia

38. mylonite

39. 5. Styles of Continental Deformation
● Tensional tectonics
● Compressive tectonics
● Shearing tectonics

40. 5. Styles of Continental Deformation

41. 5. Styles of Continental Deformation

42. 5. Styles of Continental Deformation

44. Example of
tensional
tectonics

45. Example of
compressive
tectonics

48. Example of
shearing
tectonics

49. 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?

50. 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, …

51. TIME 1
Sediments are deposited
on the seafloor.

52. TIME 2
Compressive forces cause
folding and faulting.
compressive
forces
faults

53. TIME 3
Uplift is followed by erosion, which
creates new horizontal surface.

54. TIME 4
Volcanic eruptions cover
the surface with lava flows.
lava flows

55. TIME 5
Tensional forces cause
normal faults, creating
down-dropped
blocks and
breaking up
earlier features.
tensional forces
normal faults

56. 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

57. Key terms and concepts
Joint
Normal fault
Shearing force
Strike
Strike-slip fault
Tensional force
Thrust fault