1. 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. Force Force – Mass x acceleration (F = ma)
The action that puts stationary objects in motion or
Changes the motion of moving objects.

5. Stress
Stress - Force applied to a given area. Determines the concentration of force.
Differential Stress – Unequal in different directions.
3 major types of differential stress
Compressional stress
Tensional stress
Shear stress

6. Compressional Stress “Push-together” stress.
Shortens and thickens crust.
Associated with orogenesis (mtn. building).

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

8. Shear Stress Slippage of one rock mass past another.
In shallow crust, shear is often accommodated by bedding planes.

10. 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.

11. How Rocks Deform
Elastic deformation – The rock returns to original size and shape when stress removed.
When the (strength) of a rock is surpassed, it either flows (ductile deformation) or fractures (brittle deformation).
Brittle behavior occurs in
the shallow crust; ductile in
the deeper crust.
Stephen Marshak

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

13. Mapping Geologic Structures
Geologists describe and interpret rock structures.
Structure usually determined from a limited number of outcrops.
Mapping is aided by advances in aerial photography, satellite imagery and Global Positioning Systems (GPS).
The most common and useful technique for geological mapping remains….
FIELD WORK !!

15. The Formation A mappable rock unit.

16. Mapping Geologic Structures
Describing and mapping the orientation of a geologic structure or fault surface involves determining …
Strike (trend)
Dip (inclination)

17. Mapping Geologic Structures
Strike (trend)
The compass direction of the line produced by the intersection of an inclined rock layer or fault with a horizontal plane.
Generally expressed an an angle relative to north.
N37°E
N12°W

18. Mapping Geologic Structures
Dip (inclination)
The angle of inclination of the surface of a rock unit or fault measured from a horizontal plane.
Includes both an angle of inclination and a direction toward which the rock is inclined.
82°SE
17°SW

21. 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

22. Characteristics of Folds
Parts of a fold
Limbs – The two “sides” of a fold.
Fold axis or hinge line – A line connecting points of maximum curvature along a fold.
Axial plane – An imaginary surface that divides a fold symmetrically.

23. Common Types of Folds Anticline – Upfolded or arched rock layers.
Syncline – Downfolds or rock troughs. (Think “sink”)
Depending on their orientation, anticlines and synclines can be described as
Symmetrical
Asymmetrical
Recumbent (an overturned fold)
Plunging

24. Anticline

25. Syncline

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

29. 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.

32. Faults

33. 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
Oblique

34. Faults Faults grind rocks to create fault gouge.
Walls of a fault bear evidence of this grinding as slickensides.
“Slicks” reveal
fault direction.

35. Fault Types Dip-slip faults – Motion is parallel to fault dip.
Strike-slip faults – Motion is parallel to fault strike.

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

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

38. Types of Dip-Slip Faults
Two dominant types
Normal fault
Reverse Fault
Thrust (a low angle reverse fault)

39. Types of Dip-Slip Faults
Normal fault
Hanging wall moves down relative to the footwall.
Accommodate lengthening or extension of the crust.
Exhibit a variety of scales.

40. Normal Faults
Larger scale normal faults are associated with fault-block mountains (Basin and Range of Nevada).
Normal fault bounded valleys are called grabens (Rhine graben).
Normal fault bounded ridges are called horsts.

41. Fig b
W. W. Norton

42. 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

43. 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

44. Fig a
W. W. Norton

46. U.S. Geological Survey

47. Strike-Slip Faults
Dominant displacement is horizontal and parallel to the strike of the fault
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

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

49. Joints Joints are a very common rock structure.
They are fractures with no
offset.
Result from tectonic
stresses on rock mass.
Occur in parallel groups.

50. Significance of Joints
Chemical weathering tends to be concentrated along joints
Many important mineral deposits are emplaced along joint systems
Highly jointed rocks often represent a risk to construction projects

51. QUESTIONS