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