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OCEAN/ESS 410
13. Subduction Zones
William Wilcock
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
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 slab
The 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.
Velocity
Age
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
Subduction Rate, cm/yr
Earthquake Maximum Magnitude
Plate Age
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 2001000 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
Oblique Subduction
Leads to Shearing
• This shearing also
affects the forearc
causing rotation of the
strong Oregon block.
• Compression of Puget
Sound