Ch10 structural geology_fall2007


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Ch10 structural geology_fall2007

  1. 1. CRUSTAL DEFORMATION and Geologic Structures
  2. 2. Deformation • Deformation involves: – Stress – the amount of force applied to a given area. – Types of Stress: –Confining Stress – stress applied equally in all directions. –Differential Stress – stress applied unequally in different directions.
  3. 3. Deformational Stress • Types of Differential Stress: (1) Compressional Stress – shortens and thickens a rock body (associated with convergent plate boundaries). (2) Tensional Stress – tends to elongate and thin or pull apart a rock unit (associated with divergent plate boundaries). (3) Shear Stress – produces a motion similar to slippage that occurs between individual playing cards when the top of the stack is moved relative to the bottom (associated with transform plate boundaries).
  4. 4. Deformation of the Earth’s Crust Caused by Tectonic Forces and Associated Differential Stresses
  5. 5. Deformation • Differential stress applied to rocks during tectonic activity causes rocks to respond via deformation. • Strain – changes in the shape or size of a rock body caused by stress. • Strained rock bodies do not retain their original configuration during deformation.
  6. 6. How Do Rocks Deform? • Rocks subjected to stresses greater than their own strength begin to deform usually by folding, flowing, or fracturing. – General Characteristics of Rock Deformation: • Elastic deformation – the rock returns to nearly its original size and shape when the stress is removed. • Once the elastic limit (strength) of a rock is surpassed, it either flows (ductile deformation) or fractures (brittle deformation).
  7. 7. • Factors that influence the strength of a rock and how it will deform: • Depth • Temperature • Confining Pressure • Rock Type • Availability of Fluids • Time How Do Rocks Deform?
  8. 8. • Rocks near the surface, where confining pressures and temperatures are low, will behave as a brittle solid and fracture once their strength is exceeded. • Rocks at depth, where confining pressures and temperatures are high, will exhibit ductile behavior or solid-state flow, in which changes occur without fracturing. How Do Rocks Deform?
  9. 9. Crustal Structures • Folds – During crustal deformation rocks are often bent into a series of wave-like undulations. – Anticlines and Synclines – Domes and Basins – Monoclines • Characteristics of Folds: • Most folds result from compressional stresses which shorten and thicken the crust. • Most of them occur in a series.
  10. 10. Anatomy of a Fold • Limbs – Refers to the two sides of a fold. • Axis (or Hinge) – A line drawn down the points of maximum curvature of each layer.• Axial Plane – An imaginary surface that divides a fold symmetrically. • Plunge – In complex folding, the axis is often inclined at an angle called plunge.
  11. 11. (A) Horizontal Anticline and (B) Plunging Anticline
  12. 12. Common Types of Folds • Anticline – upfolded or arched rock layers. • Syncline – downfolds or troughs of rock layers. Photo courtesy of J. T. Daniels Photo courtesy of Brennan T. Jordan, Department of Earth Sciences, University of South Dakota
  13. 13. Common Types of Folds • Depending on their orientation, anticlines and synclines can be described as… • Symmetrical, asymmetrical, overturned, recumbent (a type of overturned fold – “lying on its side”), or plunging.
  14. 14. Formation of Folds Insert Animation #30: Folds
  15. 15. Name the Folds Below
  16. 16. Plunging Anticlines and Synclines (Note: the outcrop pattern of an anticline points in the direction it is plunging, whereas the opposite is true for a syncline)
  17. 17. Sheep Mountain, A Plunging Anticline
  18. 18. Formation of Folds Insert Animation #30: Plunging Folds
  19. 19. Other Types of Folds • Monoclines • Large, step-like folds in otherwise horizontal sedimentary strata. • Closely associated with faulting.
  20. 20. Other Types of Folds • Dome • Upwarped displacement of rocks. • Circular or slightly elongated structure. • Oldest rocks in center, younger rocks on the flanks.
  21. 21. Other Types of Folds • Basin • Circular or slightly elongated structure. • Downwarped displacement of rocks. • Youngest rocks are found near the center, oldest rocks on the flanks.
  22. 22. Crustal Structures • Faults – Fractures in rocks along which appreciable displacement has taken place. • Fault Zone – Displacements along multiple interconnected faults. • Sudden movements along faults are the cause of most earthquakes.
  23. 23. Types of Faults • Classified by their relative movement which can be Horizontal, Vertical, or Oblique.
  24. 24. Summary of Fault Types • Dip-Slip Faults: • Normal (gravity) – associated with divergent plate boundaries. • Reverse and Thrust – associated with convergent plate boundaries. • Strike-Slip Faults: • Lateral (right and left) – associated with transform plate boundaries.
  25. 25. Dip-Slip Faults • Movement is mainly parallel to the dip of the fault surface. • Parts of a dip-slip fault include the hanging wall (rock surface above the fault) and the footwall (rock surface below the fault).
  26. 26. Dip-Slip Faults • Normal Fault (gravity) Dip-Slip Faults – Hanging wall block moves down relative to the footwall block. – Tensional stress – Accommodate lengthening or extension and thinning of the crust. – Associated with divergent plate boundaries. – Most are small with displacements of a meter or so. – Larger scale normal faults are associated with structures called fault-block mountains (Teton Range in Wyoming, Basin and Range Province in Nevada).
  27. 27. Formation of Normal Faults Insert Animation #29: Faults – Normal
  28. 28. Normal Faulting – Fault Block Mountains • Fault-Block Mountains – Basin and Range Province in Nevada – topography generated by a system of roughly north to south trending normal faults. • Movements along these faults have produced alternating uplifted blocks called horsts (form elevated ranges) and down-dropped blocks called grabens (form basins). • Half-Grabens – a tilted fault block in which the higher side forms mountainous topography and the lower side forms a basin that fills with sediment. • Detachment Fault – nearly horizontal fault extending up to hundreds of kilometers into the subsurface. Smaller faults are connected to this larger fault. Boundary between ductile and brittle deformation.
  29. 29. Dip-Slip Faults • Reverse and Thrust Dip-Slip Faults – Hanging wall block moves up relative to the footwall block. – Reverse faults have dips greater than 45o – Thrust faults have dips less than 45o . • Strong compressional stress. • Accommodate shortening and thickening of the crust. • Associated with convergent plate boundaries.
  30. 30. Formation of Reverse Faults Insert Animation #29: Faults – Reverse
  31. 31. Idealized Development of Lewis Overthrust Fault near Glacier National Park
  32. 32. Strike-Slip Faults • Dominant displacement is horizontal and parallel to the strike of the fault. • May produce broad zones of roughly parallel fractures up too several kilometers in width. • Shear stress. • Associated with transform plate boundaries.
  33. 33. Formation of Strike-Slip Faults Insert Animation #29: Faults – Strike-Slip
  34. 34. Types of Strike-Slip Faults • Right-Lateral – as you face the fault, the opposite side of the fault moves to the right. • Left-Lateral – as you face the fault, the opposite side of the fault moves to the left. Animations: Right-Lateral Strike-Slip Fault
  35. 35. Types of Strike-Slip Faults • Transform Fault – Large strike-slip fault that cuts through accommodates motion between two large crustal plates. – Example: San Andreas Fault System
  36. 36. Name the Type of Fault Below
  37. 37. Name the Type of Fault Below
  38. 38. Name the Type of Fault Below Insert Animation #28: Exposing Metamorphic Rock
  39. 39. Name the Type of Fault Below
  40. 40. Name the Type of Fault Below
  41. 41. Name the Structure
  42. 42. Name the Structure
  43. 43. Mapping Geologic Structures • Geologists measure the orientation or attitude of a rock layers or fault/fracture surfaces in order to describe and map geologic structures that result from deformation.
  44. 44. 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. • Example: N10ºE
  45. 45. 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 inclination and a direction toward which the rock is inclined. • Example: 30ºSE
  46. 46. A Geologic Map Showing Strike and Dip of Structures By knowing the strike and dip, geologists can predict the nature of rock structures hidden beneath the surface.
  47. 47. Geologist Measuring the Dip of Strata in a Roadcut
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