Rock Deformation

1. Rock Deformation

2. Geological Structures • Joints • Faults • Shear Zones • Folds • Igneous Bodies • Unconformities

3. Geological Structures Why do rocks deform? STRESS (force per unit area) • compressive stress • tensional stress • shear stress

4. Geological Structures What is the result of stress? STRAIN (deformation) • brittle deformation e.g. (faults/joints) • plastic/ductile deformation e.g. (folds/shear zones) • elastic deformation

5. Geological Structures What factors affect deformation? Deforming everyday materials exercise! Brittle Ductile Elastic

6. Geological Structures What factors affect deformation? Deforming everyday materials exercise! elastic brittle brittle ductile elastic brittle brittle ductile Brittle Ductile Elastic elastic brittle ductile ductile brittle brittle brittle brittle

7. Geological Structures What factors affect deformation? Deforming everyday materials exercise! • temperature • rate/speed of deformation (strain rate) • properties of rock (competence)

8. Geological Structures 1. JOINTS • Shrinkage joints • Tectonic joints • Pressure release joints

9. Geological Structures 2. FAULTS • Normal fault Vertical faults • Reverse fault • Thrust fault Horizontal faults • Tear fault

10. Normal Faults NORMAL = lengthening of crust due to tensional stress ii iii Dip of fault i Downthrow side Upthrow side i – throw ii – net slip Fault plane iii – dip slip

11. Vertical Faults NORMAL = lengthening of crust due to tensional stress

12. Reverse Faults REVERSE = shortening of crust due to compressional stress Dip of fault ii i iii Upthrow side Downthrow side i – throw ii – net slip Fault plane iii – dip slip

13. Thrust Faults Dip of fault Upthrow side Downthrow side Fault plane

14. Tear Faults

16. Slickensides along Fault plane Slickensides are polished striated rock surfaces caused by one rock mass moving across another on a fault. Form minute steps facing direction of movement Fault plane

17. Components of Principle Stress

18. Components of Principle Stress

23. Fault Structures

24. Thrust Fault Outcrop Pattern

25. Brampton BGS Map June 2000

26. Folds

27. Fold Geometry

28. Fold Geometry

29. Style of Folding

30. Fold Features

31. Interlimb Angles Open ~ 120° Close ~ 60° Tight ~ 30° Isoclinal ~ 0° (limbs parallel)

32. Fold Outcrop Patterns

33. Plunging Folds

34. Plunging Folds

35. Plunging Folds

36. Domes & Basins

37. Fold Mechanisms

38. Geological Structures Flexural (parallel) Folds • Thickness of individual layers are constant, whether on crest or trough • Impersistent – fold dies out as not enough room to fit in • Movement occurs along the boundaries between layers by flexural movement • Formed in strong/competent layers • Thickness of the most competent layer determines the fold wavelength • Low temperature & pressure • Outer zone of orogenic belt • Slickenside lineations may occur between layers • Helvetic Alps • Strain is low enough not to alter thickness of the folded layer

39. Fold Mechanisms

40. Geological Structures Flow (similar) Folds • Maintains same curvature on the inner and outer surfaces of the folded area • Each layer is thinner on the flanks and thicker on the axis of the fold • Fold extends indefinitely – no space problem • Movement within layer occurs within rock when it is ductile • Develop in highly incompetent beds which behave more as a viscous fluid than a solid rock • In slightly more competent rocks, shear folding occurs, this is produced by differential movement along cleavage planes e.g.. slate • Axial plane cleavage forms parallel to fold axis • Inner zone of orogenic belt • Pennine Alps • High temperatures & pressure

41. Unconformities

42. Unconformity unconformity

44. Bedding Folds Bedding Anticline/syncline Upright/overturned/recumbent Joints Symmetrical/asymmetrical Dip direction/angle Axial plane Strike Trend Faults GEOLOGICAL STRUCTURES Normal Igneous Bodies Reverse Thrust Lava flows Tear Dykes Unconformities Sills Formation Batholiths Recognition

45. Geological Structures 1. What sort of fault is this?

46. Geological Structures 2. What sort of fault is this?

47. Geological Structures 3. What sort of fault is this?

48. Geological Structures 4. What sort of structure is this?

49. Geological Structures 5. Describe this fold structure?

50. Geological Structures 6. What sort of fold is this?