How and why does subduction occur?

1
How and why does subduction occur?

• Convergence
• One plate sinks beneath the other
• Dense oceanic crust sinks into mantle

2
What and where is the Wadati-Benioff zone?

• Inclined array of earthquakes at subduction zone
• Correspond to upper edge of plate

3
How is magma produced at arcs?

• Water from subducting slab lowers the melting
  point of overlying mantle
• Note water also makes for explosive eruptions

4
Where and how do sediments accumulate in arc
environments?

• Sediments accumulate in accretionary wedges at
  the trench
• Two sources 1) volcanic arc 2) ocean sediments

5
Describe deformation of the sediment in the trench

• Bulldozer action scrapes ocean sediments and
  thrusts material into a wedge

6
Briefly explain the orogenic cycle using India as
an example

• Gondwana rifts into pieces incuding India
• India diverges from antarctica but converges on
  Asia
• India sutures to Asia

7
How can continents accumulate additional terrain
via plate convergence?

• collage tectonics
• Example The berkshires

8
Have the continents always looked like they do
today?

• Continents have grown by lateral accretion
• Average age of continents 2 billion
• Average age of oceans 100 million
• Appalachian not at plate boundary

9
Has the mid-continent always been flat?

• Continental shield have the oldest rocks.
• Composed of sutured belts of mountains, which
  were eroded to platforms long ago
• These platforms have harbored shallow seas

10
What happens to the weak upper layers of crust
during collision?

• Compression results in thrust faulting and
  folding into nappes.
• Deformation intensifies in the core of the orogen
  (higher T and P)

11
What happens to the weak upper layers of crust
during collision?

• Compression results in thrust faulting and
  folding into nappes.
• Deformation intensifies in the core of the orogen
  (higher T and P)