Gel 201 lecture 13

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Gel 201 lecture 13

  1. 1. Precambrian Geology
  2. 2. Step 1: Earth, Sun, and planets form out of collapsing, spinning, gassy nebula
  3. 3. Step 2: Solar wind “blows” away close most gases; protoplanets form by accretion
  4. 4. Step 3: Planets grow, collapse under own gravity producing heat
  5. 5. Step 4: Contraction, plus impacts and fission melt planet; materials differentiate
  6. 6. Step 4: Contraction, plus impacts and fission melt planet; materials Mantle & core by 4.5 Ga differentiate
  7. 7. Step 5: Outgassing: 2O, etc. H released producing primitive atmosphere/ ocean
  8. 8. Zircon Step 5: Outgassing: 2O, etc. H released producing primitive atmosphere/ ocean
  9. 9. Water by 4.4 Ga Step 5: Outgassing: 2O, etc. H released producing primitive atmosphere/ ocean
  10. 10. Cooling not linear • Earth cools: rapid loss of short half-life isotopes, slowing of impacts, etc.
  11. 11. When does plate tectonics start?
  12. 12. When does plate tectonics start? What is plate tectonics?
  13. 13. What is plate tectonics?
  14. 14. What is plate tectonics? 1) Rigid plates (lithosphere) 2) Sea floor spreading (move apart & create new crust) 3) Subduction (slide beneath, get recycled)
  15. 15. What is plate tectonics? 1) Rigid plates (lithosphere) 2) Sea floor spreading (move apart, create new crust) 3) Subduction (slide beneath, get recycled)
  16. 16. Precambrian explorers circa 1885
  17. 17. Ages worked out through Stenonian principles C B E D A Cross-section: North Shore Lake Superior
  18. 18. Ages worked out through Stenonian principles C B E D A Cross-section: North Shore Lake Superior
  19. 19. Radiometric dating allowed correlation within NA/world Cross-section: North Shore Lake Superior
  20. 20. Orogeny = Mountain building (‘oros’ = ‘mountain’ + ‘genesis’ = ‘creation’)
  21. 21. Orogeny = Mountain building Orogenic belt = Linear/arcshaped zones of deformation/ volcanism
  22. 22. Orogeny = Mountain building Orogenic belt = Collision zones! (Mountain ranges)
  23. 23. Stable interior of NA = craton
  24. 24. Geological provinces (defined by radiometric dates)
  25. 25. Dates = Archean to late Protereozoic; Concentrically arranged by age
  26. 26. OLDEST Dates = Archean to late Protereozoic; Concentrically arranged by age
  27. 27. What are the provinces made of?
  28. 28. X-bedded sandstone
  29. 29. X-bedded sandstone Graded, immature seds (i.e., volcanics eroded/deposited near source)
  30. 30. X-bedded sandstone Also: LOTS of granite, volcanics, etc., often very metamorphosed
  31. 31. Plus… Thick, widespread flood basalt: ca. 1.2 Ga
  32. 32. Oldest rocks = two kinds: Greenstones and Gneiss belts
  33. 33. Greenstone belts: named for green minerals produced when ultra mafic volcanic rocks metamorphosed Komatiite
  34. 34. Suggest oceanic volcanism, with associated sediments Komatiite
  35. 35. Suggest oceanic volcanism, with associated sediments Komatiite: Only form in Archean— hot process
  36. 36. Gneiss belts: granites, volcanics, & seds
  37. 37. Continental-style material Oldest zircons (4.4 Ga) and oldest rocks (>4.03 Ga)
  38. 38. A collage Story? Joining of independent continental & oceanic blocks
  39. 39. Greenstone belts = “captured” basins
  40. 40. Greenstone belts = “captured” basins
  41. 41. Greenstone belts = “captured” basins
  42. 42. A Timeline
  43. 43. Approx. 2 billion years ago, Archean terranes collide (Superior, etc., stitched by Trans-Hudson Belt)
  44. 44. Proterozoic Process continues: Approx. 1.5 Ga, Archean frags & Proterzoic terranes arrive Proterozoic
  45. 45. Then… approx. 1.2 Ga the craton rifts, forms flood basalts
  46. 46. Finally: Approx. 1 Ga Grenville arrives, Laurentia assembled
  47. 47. Voila! Ancient North America! Simple! Finally: Approx. 1 Ga Grenville arrives, Laurentia assembled
  48. 48. More realistic depiction Whitmeyer and Karlstrom, 2007
  49. 49. So, do we have tectonics?
  50. 50. So, do we have tectonics? When?
  51. 51. What is plate tectonics? 1) Rigid plates (lithosphere) 2) Sea floor spreading (move apart & create new crust) 3) Subduction (slide beneath, get recycled)
  52. 52. What is plate tectonics? 1) Rigid plates (lithosphere) 2) Sea floor spreading (move apart & create new crust) 3) Subduction (slide beneath, get recycled)
  53. 53. What is plate tectonics? 1) Rigid plates (lithosphere) 2) Sea floor spreading (move apart & create new crust) 3) Subduction (slide beneath, get recycled)
  54. 54. What is plate tectonics? 1) Rigid plates (lithosphere) 2) Sea floor spreading (move apart & create new crust) 3) Subduction (slide beneath, get recycled) Are you sure?
  55. 55. All the smart and famous scientists June, 2006: Wyoming
  56. 56. Estimates? June, 2006: Wyoming
  57. 57. Estimates? 4 Ga to 1 Ga June, 2006: Wyoming
  58. 58. 4 Ga to 1 Ga—Why?
  59. 59. 4 Ga to 1 Ga—Why? Conclusive proof: Ophiolite suites (scooped up oceanic crust) and blueschist (metamorphic rocks assoc. with high pressures). None till ~1 Ga
  60. 60. 3 Ga to 4 Ga End of meeting
  61. 61. Final resolution?
  62. 62. Diamonds!
  63. 63. Kimberlites: Mantle derived
  64. 64. Kimberlites: 125-175 km
  65. 65. Mineral inclusions
  66. 66. >3 Ga peridotite minerals
  67. 67. >3 Ga peridotite minerals <3 Ga eclogite minerals
  68. 68. >3 Ga peridotite minerals <3 Ga eclogite minerals <100 km
  69. 69. Big picture
  70. 70. Earth forms
  71. 71. Differentiation
  72. 72. Water and initial crust
  73. 73. Pseudotectonics and accretion
  74. 74. Pseudotectonics and accretion
  75. 75. • ~3 Ga modernstyle plate tectonics • ~2.5-2 Ga large, stable cratons
  76. 76. Microplates, plumes, direct down-going >3 Ga
  77. 77. Analogy for early Archean
  78. 78. Analogy for early Archean
  79. 79. Analogy for early Archean
  80. 80. Microplates, plumes, direct downgoing <3 Ga
  81. 81. 2.5-2 Ga continents formed (Wilson cycle begun)
  82. 82. Next class: Proterozoic

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