1. Folds, Faults, and Geologic Maps
Chapter 9
Geology Today
Barbara W. Murck & Brian J. Skinner
Folds, South Georgia Island
N. Lindsley-Griffin, 1999

2. Stress - the force acting on a surface, per unit area
Fig. 9.2, p. 243
N. Lindsley-Griffin, 1999

3. Stress and Strain
Strain - the change in shape or volume of a rock in response to stress (Fig. 9.3, p. 244)
N. Lindsley-Griffin, 1999

4. Types of Deformation
Elastic - nonpermanent; rock returns to original shape when stress is removed
Ductile (plastic) - permanent; rock flows or bends when stressed beyond its elastic limit
Brittle - rock breaks or cracks
Fig. 9.5 A, p. 245
N. Lindsley-Griffin, 1999

5. Controlling Factors Factors that influence how rocks deform:
Temperature - increase in T makes solids more ductile
Confining Pressure - increase in P inhibits formation of fractures; solids resist breaking
(Fig. 9.6, p. 246)
Undeformed Ductile Brittle
N. Lindsley-Griffin, 1999

6. Controlling Factors Factors that influence how rocks deform:
Rate of deformation - stress applied quickly enhances brittle behavior. Stress applied slowly over a long period of time promotes ductile behavior.
Controlling Factors
(Fig. 9.6, p. 246)
Undeformed Ductile Brittle
N. Lindsley-Griffin, 1999

7. Controlling Factors Factors that influence how rocks deform
Composition - some minerals are very brittle (quartz, garnet, olivine) as are rocks that contain them.
Minerals that behave ductilely: mica, clay, calcite, gypsum
Rocks that behave ductilely: shale, slate, limestone, marble
Controlling Factors
(Fig. 9.5, p. 245)
N. Lindsley-Griffin, 1999

8. Strike and Dip
Strike - orientation of a horizontal line in a plane. (Expressed as a compass direction)
Dip - angle between a tilted surface and a horizontal plane.
(Expressed as an angle and direction)
N. Lindsley-Griffin, 1999

9. Faults Fault - a fracture in
the crust along which
movement has occurred.
Normal Fault (Fig. 9.4, p. 245)
Faults are classified by how steeply they dip and relative movement of the blocks:
Hanging wall - block on top of the fault
Footwall - block below the fault
Hanging Wall
Footwall
Footwall
N. Lindsley-Griffin, 1999

10. Faults Normal Fault - hanging wall down relative to footwall.
Tensional stress
(Fig. 9.9, p. 250)
N. Lindsley-Griffin, 1999

11. Faults
Horst - two normal faults dip away from each other, block between them is elevated.
Graben - two normal faults dip towards each other, block between them drops down.
Tensional stress
(Fig. 9.10, p. 250)
N. Lindsley-Griffin, 1999

12. Faults Reverse Fault - hanging wall moves up relative to footwall
Dip is steep
Compressional stress
(Fig. 9.11, p. 251)
N. Lindsley-Griffin, 1999

13. Faults Thrust Fault - hanging wall moves up relative to footwall
Reverse fault for comparison
Thrust Fault - hanging wall moves up relative to footwall
Dip is < 30 degrees
(Fig. 9.12, p. 251)
N. Lindsley-Griffin, 1999

14. Faults
Strike-slip fault - movement is mostly horizontal and parallel to strike of fault.
Left-lateral - block opposite you moves left.
Right-lateral - block opposite you moves right.
(Fig. 9.13, p. 251)
N. Lindsley-Griffin, 1999

15. How do you describe something as complex as a fold?
Folds
Fold axis - the line along which the axial plane intersects a single rock layer
Axial plane (axial surface) - the plane that divides the fold in half as symmetrically as possible
Fig B, p. 255
N. Lindsley-Griffin, 1999

16. How do you describe something as complex as a fold?
Folds
Fold limbs - the two sides of the fold that are separated by the axial plane
Anticline - limbs point down
Syncline - limbs point up
Fig B, p. 255
N. Lindsley-Griffin, 1999

17. How do you describe something as complex as a fold?
Folds
Orientation of both axial plane and fold axis are used.
Horizontal fold axis with vertical axial plane = upright fold
Plunging fold -
fold axis is not horizontal
Fig B, p. 255
N. Lindsley-Griffin, 1999

18. Five common types of fold geometries (Fig. 9.20, p. 257)
Folds
N. Lindsley-Griffin, 1999

19. To describe a fold, first determine orientation of axial planes, limbs, axes
Folds
Here, axial planes are not quite vertical, axes point straight out, limbs dip at different angles:
open, asymmetrical, anticline - syncline
Fig A, p. 255
N. Lindsley-Griffin, 1999

20. To describe a fold, first determine orientation of axial planes, limbs, axes
Folds
Here, axial planes are not quite vertical, axes point straight out, limbs dip at different angles:
open, asymmetrical, anticline - syncline
Fig A, p. 255
N. Lindsley-Griffin, 1999

21. overturned, symmetrical folds
Here, both limbs dip in the same direction so that one limb is actually upside down:
overturned, symmetrical folds
N. Lindsley-Griffin, 1999
Fig. 9.1, p. 242

22. overturned, symmetrical folds
Here, both limbs dip in the same direction so that one limb is actually upside down:
overturned, symmetrical folds
N. Lindsley-Griffin, 1999
Fig. 9.1, p. 242

23. Open, asymmetrical folds (Fig. 9.4 A, p. 245)
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24. Open, asymmetrical folds (Fig. 9.4 A, p. 245)
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25. Domes and Basins (Fig. 9.18, p. 256)
Folds
Special fold types
Domes and Basins (Fig. 9.18, p. 256)
N. Lindsley-Griffin, 1999

26. Folds Special fold types
Monocline - upper and lower limbs are horizontal, only the central limb is inclined (Fig. 9.16, p. 254)
N. Lindsley-Griffin, 1999

27. Geologic Maps Showing strata on maps (Fig. 9.21, p. 258)
Contacts - boundaries between distinct rock types
Block diagram shows eroded strata, cross section
Geologic map shows contacts with strike and dip symbols.
N. Lindsley-Griffin, 1999

28. Showing folds on maps (Fig. 9.19, p. 256)
Plunging folds make horseshoe patterns on surface
Block diagram shows pattern projected to horizontal surface, as if strata were eroded flat
Map shows how folds are depicted on geologic maps.
N. Lindsley-Griffin, 1999

29. Cross Sections
Cross sections show how structures and rock layers behave at depth.
Appalachian Mtns.
(Fig. 9.26, p. 266)
N. Lindsley-Griffin, 1999

30. Geologic Maps
Geologic maps - show topograpic contours, major folds and faults, contacts between rock units, age and type of rock. (Fig. 9.22, p. 259)
N. Lindsley-Griffin, 1999

31. Topographic Maps
Topographic maps use contour lines to depict topography. (Fig. 9.23, p. 263)
N. Lindsley-Griffin, 1999