13. subduction zones william wilcock ocean/ess 410
TRANSCRIPT
13. Subduction ZonesWilliam Wilcock
OCEAN/ESS 410
Lecture/Lab Learning Goals
• Be able to sketch the different kinds of convergent plate margins and label key processes
• Understand the processes in the “subduction zone factory”
• Understand the Wilson cycle• Understand the different forces that drive subduction
and that control the angle of the subducting slab.• Know the different kinds of earthquakes that occur in
subduction zones• Be able to interpret focal mechanisms from
subduction zone settings (LAB)
3 Types of Convergent
Margin (Plate Boundary)
Subduction Zone Processes
• Subduction zones are important because they are the downwelling branches in Earth’s mantle convection.
• Subduction zones are responsible for some of the primary geologic processes on earth:
1. Convergence leads to the growth of continents by • volcanism• accretion of terrains
and loss of the continents by• Tectonic erosion• Sediment subduction
2. Subduction zone processes dominate the development of active geologic structures on the continents
Subduction Factory
Subduction Factory
Peridotite Solidus
Water lowers the melting
temperature of mantle peridotite
Sketch – Not to scale
~1 % water
Wet S
olidus
>>1% w
ater
Mantle Geotherm
Old Plate
Wilson Cycle - Cyclical growth and loss of ocean basins leads to
continental growth
Passive Margin
Convergent (Active) Margin
Stable Continent Bigger than Stage A
Tectonic Erosion
Sediment Trapped Subduction
Forces acting on a subducting slabThe plate sinks under gravity (red arrow) according to its
weight, thus how cold and dense it is.
The slab also drags along adjacent mantle (black arrows). This mantle is pushed up against the subducting slab on the left hand side generating a high pressure region. The mantle is dragged down with the slab on the right hand side generating a low pressure.
This pressure differential tends to lift the slab.
Age
Velocity
Velocity
Balance of Gravitational and Pressure Forces Influences Slab Dip
1. Old (Cold) Plate & Slow Subduction• Large gravitational force, small
pressure force. Steep subduction angle
2. Young (Warm) Plate & Fast Subduction• Small gravitational force, large
pressure force. Shallow subduction angle
Earthquake Maximum Magnitude
Plate Age
Sub
duct
ion
Rat
e, c
m/y
r
Slab Dips
Island Arcs - Back Arc Spreading
Back-Arc Spreading
Two Ideas
1. Subducting slab falls away
2. Mantle flow in wedge creates extension
Thermal Structure
Deep Forces Resulting From Phase Changes
Enhances Subduction
Opposes Subduction
Sometimes but not always
mantle slabs do not
penetrate 670 km
discontinuity
Subduction Zone Earthquakes
Earthquakes
Shallow Earthquakes•Plate Boundary - Megathrust•Surrounding Plates
Deep Earthquakes•Mineral phase changes as pressure increases and loss of water bearing minerals - incompletely understood
Cascadia Locked Zone
Cascadia Subduction Zone
Last earthquake 1700.
Recurrence interval 200-1000 years (average = 500 years)
• Block diagram/cross section of tectonics
Cascadia
Warm slab implies dewatering at shallow depths and weak arc volcanism
Deformation in Subduction Zones
Accretionary Prism forms when sediments are present and scraped off subducting slab
• This shearing also affects the forearc causing rotation of the strong Oregon block.
• Compression of Puget Sound
Oblique Subduction Leads to Shearing