Transcript PowerPoint Presentation - How and why does subduction occur?

How and why does
subduction occur?
Convergence
One plate sinks beneath the other
Dense oceanic crust sinks into mantle
What and where is the
Wadati-Benioff zone?
Inclined array of earthquakes at
subduction zone
Correspond to upper edge of plate
How is magma produced at arcs?
Water from subducting slab lowers the
melting point of overlying mantle
Note: water also makes for explosive
eruptions…
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
Describe deformation of the sediment
in the trench
‘Bulldozer action’ scrapes ocean
sediments and thrusts material into a
wedge
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
How can continents accumulate
additional terrain via plate convergence?
‘collage’ tectonics
Example; The berkshires
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
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
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)
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)