1. Modification of Rocks by Folding and Fracturing
Grotzinger • Jordan
Understanding Earth
Seventh Edition
Chapter 7:
DEFORMATION
Modification of Rocks by Folding and Fracturing
© 2014 by W. H. Freeman and Company

2. Modification of Rocks by Folding and Fracturing
Chapter 7:
Deformation:
Modification 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 Plate tectonic forces 2. Mapping geologic structure
3. How rocks deform
4. Basic deformation structures
5. Styles of continental deformation
6. Unraveling geologic history

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

6. ● Outcrop – basic source of geologic information in the field
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. ● Measuring strike and dip
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

12. 2. Mapping Geologic Structure

13. ● a common scale for geologic maps is 1:24,000
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

14. ● Geologic cross sections
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

16. 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?

17. Thought questions for this chapter
Can you explain the geologic story in Exercise 6 (see textbook, p. 193) in terms of plate tectonics? Where in the U.S. do geologists think this sequence of events has taken place?

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

19. 3. How Rocks Deform

20. ● Rock behavior in Earth’s crust ● depths affect brittle v. ductile
3. How Rocks Deform
● Rock behavior in Earth’s crust
● depths affect brittle v. ductile
● rock type affects way rocks deform
● rate of deformation is a factor

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

22. 4. Basic Deformation Structures

24. Example of a strike-slip fault

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

27. 4. Basic Deformation Structures

29. 4. Basic Deformation Structures

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

32. Example of a dome

33. Example of a basin

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

35. joints

36. fault
breccia

37. mylonite

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

39. 5. Styles of Continental Deformation

40. 5. Styles of Continental Deformation

41. 5. Styles of Continental Deformation

43. Example of tensional tectonics

44. Example of compressive tectonics

46. Example of shearing tectonics

47. 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 km
(c) lower crustal rocks below 20 km
In which of these layers would you expect earthquakes?

48. ● Can be described in time steps 1, 2, 3, …
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, …

49. TIME 1
Sediments are deposited
on the seafloor.

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

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

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

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

54. 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” (p. 378). Can you locate a passage in this textbook that describes a geologic structure and sketch a geologic map consistent with McPhee’s description?

55. Key terms and concepts Basin Brittle Compressive force Deformation Dip
Anticline
Basin
Brittle
Compressive force
Deformation
Dip
Dip-slip fault
Dome
Ductile
Fault
Fold
Foot wall
Formation
Geologic cross section
Geologic map

56. Key terms and concepts Joint Oblique slip fault Shearing force Strike
Hanging wall
Joint
Normal fault
Oblique slip fault
Shearing force
Strike
Strike-slip fault
Tensional force
Thrust fault