Plate tectonics largely based on ocean floor studies

• Will operate even if no continents present
• Cannot see beyond last 165 million years- the age of oldest ocean floor
• Continents as rafts on large lithosphere plates
• Product of bump-grind-stretch tectonics
  • bump - orogenic belt
  • grind - strike slip fault
  • stretch - Rift valleys

Continetal Margins

• Passive continental margins: a margin in the stable the interior of the plate.
  • Divergent margin phenomenon
  • Develop when a new ocean basin forms by rifting
  • E.g., Atlantic ocean margins of Americas, Africa, & Europe
  • Characteristic accumulation of thick sediment sequence, intially clastic nonmarine followed by marine
  • Happening today at Red Sea
  • Triple junction -Red Sea, Gulf of Aden and the African Rifts
• Convergent continental margins: convergent margin of an oceanic plate when subduction coincides with the edge of a continent
  • Continental volcanic arc:
    • e.g. Nazca Plate subduction along the W. Coast of S. America, resulting in Andes Mtns.
    • Juan de Fuca Plate subduction on the W. Coast of N. America leading to formation of the Cascade Range
• Continental Collision Margin: Mountain systems
  • Formation of the Himalyas
    • Older Appalachian Mtns and the Urals
    • Mountain chains of several 1000’s of km.
  • Transform Fault Margins: Margin of a continent coincides with a transform fault boundary of the oceanic crust
    • e.g. San Andreas
  • Accreted Terrane Margin: Former continetal convergent or transform margin, modifgied further by addition of rafted crustal blocks
    • e.g., western margin of N. America from Central California to Alaska

Continents provide evidence for ancient plate tectonics.

Regional Structure of the continents

• Continetal Shields: composed of cratons and orogens
  • Cratons: cores of very ancient rocks (>2.5 b.y.)in the middle of continents that have attained isostatic stability.
  • Orogens: Draped around old cratons, the elongate bands that were once old mountain chains, eroded roots of old mountain chains

North American continent

• Three small cratons: Slave, Wyoming, and Kaminak
• Three large cratons: Churchill, Superior, and North Atlantic
• The large cratons are almost like mini continents
  
• Assembly of these minicontinents at >1.6 b.y.
• Collisions between these mini continents gave rise to ancient orogens.
• Churchill and Superior Craton collision - Trans Hudson Orogen
• Continents show that Plate tectonics were operating as far back as 1.8 b.y.ago