1. Crustal Deformation Structural Geology

2. Structural Geology
Tectonic collision deforms crustal rocks producing geologic structures.
Folds
Faults
Joints and Fractures

3. Deformation
All changes in the original location, orientation or form of a crustal rock body.
Deformation common
at plate margins.
Deformation concepts…
Force
Stress
Strain

4. Stress
Stress - Force applied to a given area. Determines the concentration of force.
3 major types of differential stress
Compressional stress
Tensional stress
Shear stress

5. Compressional Stress “Push-together” stress.
Shortens and thickens crust.
Associated with mountain building

6. Tensional Stress “Pull-apart” stress. Thins and stretches crust.
Associated with rifting.

7. Shear Stress
Slippage of one rock mass past another.

9. Strain Changes in the shape or size of a rock body caused by stress.
Strain occurs when stresses exceed rock strength.
Strained rocks deform by folding, flowing, or fracturing.

10. How Rocks Deform
Factors controlling rock strength and deformation style.
Temperature and confining pressure
Low T and P = faulting deformation
High T and P = folding deformation
Rock type – Mineral composition controls strength.
Time – Stress applied for a long time generates change.

11. The Formation A mappable rock unit.

13. Folds
Rocks are bent by crustal deformation into a series of wave-like undulations called folds.
Most folds result from compressional stresses which shorten and thicken the crust.
Stephen Marshak

14. Common Types of Folds Anticline – Upfolded or arched rock layers.
Syncline – Downfolds or rock troughs. (Think “sink”)

15. Anticline

16. Syncline

17. Anticlines and Synclines are common in fold and thrust belts related to mountain belts.

20. Common Types of Folds
Monoclines – Large, step-like folds in otherwise horizontal sedimentary strata.
Domes -Upwarped circular or slightly elongated structure. Oldest rocks in center, younger rocks outside.
Basins – Downwarped circular or slightly elongated structure. Youngest rocks are found near the center, oldest rocks on the flanks.

23. Faults

24. Faults Breaks in rock that exhibit offset.
Exist at a variety of scales.
Sudden movements along faults are the cause of most earthquakes.
Classified by movement…
Horizontal
Vertical

25. Fault Types Dip-slip faults – Motion is up & down
Strike-slip faults – Motion is left or right

26. Dip Slip Faults
May produce long, low cliffs called fault scarps.

27. Dip Slip Faults Fault blocks classified as Footwall (rock mass
below the fault)
Hanging wall
(rock mass
above the fault)

28. Types of Dip-Slip Faults
Two dominant types
Normal fault
Reverse Fault

29. Types of Dip-Slip Faults
Normal fault
Hanging wall moves down relative to the footwall.
Accommodate lengthening or extension of the crust.

30. Normal Faults
Larger scale normal faults are associated with fault-block mountains (Basin and Range of Nevada).

31. Fig b
W. W. Norton

32. Types of Dip-Slip Faults
Reverse faults
Hanging wall block moves up relative to the footwall block
Reverse faults have dips greater than 45o and thrust faults have dips less then 45o
Accommodate shortening of the crust
Strong compressional forces

33. Types of Dip-Slip Faults
Thrust faults - A special case of reverse fault.
Hanging wall block moves up relative to the footwall block
Thrust faults are characterized by a low dip angle (less then 45o).
Accommodate shortening of the crust
Strong compressional forces

35. U.S. Geological Survey

36. Strike-Slip Faults Displacement is horizontal
Types of strike-slip faults
Right-lateral – as you face the fault, the block on the opposite side of the fault moves to the right
Left-lateral – as you face the fault, the block on the opposite side of the fault moves to the left

37. Strike-Slip Faults Strike-slip fault Transform fault
Large strike-slip fault that cuts through the lithosphere
Accommodates motion between two large crustal plates

38. Joints Joints are a very common rock structure.
They are fractures with no
offset.
Occur in parallel groups.

40. The End