EVOLUTION OF THE CONTINENTS

Evolution of the Continents

• Continents are made up of stable interior, orogenic belts, and coastal plain
  • orogeny - mountain building
  • Orogenic belts consist of deformed sedimentary, igneous and metamorphic rock (Fig. 21-1)
• Stable interior - Craton
  • no tectonic activity for a long time
  • Composed of:
    • Shield
      • Precambrian rocks at or near surface
      • form nuclei of continents (see transparency)
    • Platform
      • veneer of relatively undeformed sedimentary rocks covering shield
  • Platform deposits record sea level change
    • flooding of continent - transgression
      • marine sandstones, limestones, shales
      • epicontinental seas
    • retreat of sea - regression
      • fluvial (river) sandstones, shales; coal
• Orogenic Belts
  • younger deformed rock nearer to margins of continent
  • form from continental collision
  • folding, faulting
    • stacking of thrust sheets (see transparency)

Major Structural Features of North America

• Canadian Shield (Fig. 21-1)
  • Canadian shield dominated by granitic and high-grade metamorphic rocks
  • evidence for ancient orogeny
  • composite of many smaller blocks (former plates) (see transparency)
• Central stable region
  • platform covered with thick accumulations of relatively undeformed Paleozoic sedimentary rock
  • underlain by shield rocks
  • sedimentary rocks - cycles of shales, limestones, sandstones, coal
  • rocks reflect transgression and regression
• Appalachian mountain belt
  • Orogenic belt
  • Late Precambrian and Paleozoic mountain-building associated with subduction and continental collision
• Cordilleran mountain belt (Fig. 21-10)
  • extends from Alaska to Guatemala
  • includes mountain belts: Coast Ranges, Sierra Nevadas, Rocky Mountains
  • Includes Basin-Range Province (Nevada)
    • tension; normal faulting
  • Later Mesozoic and early Tertiary orogeny plus later rejuvenation
• Colorado Plateau
  • "Island" of central stable region relatively unaffected by Cordilleran mountain-building except for uplift
  • includes Grand Canyon
• Gulf and Atlantic Coastal Plain
  • Gently dipping platform
  • Relatively thick accumulations of later Mesozoic and Tertiary sediments
  • Alternating shales and sandstones with some lignite