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Rock Deformation

Rock Deformation. Geological Structures. Joints Faults Shear Zones Folds Igneous Bodies Unconformities. Geological Structures. Why do rocks deform?. STRESS. (force per unit area). compressive stress. tensional stress. shear stress. Geological Structures.

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Rock Deformation

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  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

  15. 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

  16. Components of Principle Stress

  17. Components of Principle Stress

  18. Fault Structures

  19. Thrust Fault Outcrop Pattern

  20. Brampton BGS Map June 2000

  21. Folds

  22. Fold Geometry

  23. Fold Geometry

  24. Style of Folding

  25. Fold Features

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

  27. Fold Outcrop Patterns

  28. Plunging Folds

  29. Plunging Folds

  30. Plunging Folds

  31. Domes & Basins

  32. Fold Mechanisms

  33. 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

  34. Fold Mechanisms

  35. 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

  36. Unconformities

  37. Unconformity unconformity

  38. 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

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

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

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

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

  43. Geological Structures 5. Describe this fold structure?

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

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