1. 13. Subduction Zones William Wilcock
OCEAN/ESS 410
13. Subduction Zones William Wilcock

2. 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. 3 Types of Convergent Margin (Plate Boundary)

4. 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:
Convergence leads to the growth of continents by
volcanism
accretion of terrains
and loss of the continents by
Tectonic erosion
Sediment subduction
Subduction zone processes dominate the development of active geologic structures on the continents

5. Subduction Factory

7. Subduction Factory

8. Mantle
Geotherm
Old Plate
Peridotite Solidus Water lowers the melting temperature of mantle peridotite Sketch – Not to scale
Wet Solidus
>>1% water
~1 % water

9. Wilson Cycle - Cyclical growth and loss of ocean basins leads to continental growth

12. Passive Margin

15. Convergent (Active) Margin

17. Stable Continent Bigger than Stage A

18. Tectonic Erosion

19. Sediment Trapped Subduction

20. 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

21. Balance of Gravitational and Pressure Forces Influences Slab Dip
Old (Cold) Plate & Slow Subduction
Large gravitational force, small pressure force. Steep subduction angle
Young (Warm) Plate & Fast Subduction
Small gravitational force, large pressure force. Shallow subduction angle

23. Earthquake Maximum Magnitude
Subduction Rate, cm/yr
Plate Age

24. Slab Dips

26. Island Arcs - Back Arc Spreading

27. Back-Arc Spreading Two Ideas Subducting slab falls away
Mantle flow in wedge creates extension

28. Thermal Structure

29. Deep Forces Resulting From Phase Changes
Enhances Subduction
Opposes Subduction

30. Sometimes but not always mantle slabs do not penetrate 670 km discontinuity

31. Subduction Zone Earthquakes

32. Earthquakes Shallow Earthquakes Plate Boundary - Megathrust
Surrounding Plates
Deep Earthquakes
Mineral phase changes as pressure increases and loss of water bearing minerals - incompletely understood

33. Cascadia Locked Zone

34. Cascadia Subduction Zone Last earthquake 1700
Cascadia Subduction Zone Last earthquake Recurrence interval years (average = 500 years)

35. Block diagram/cross section of tectonics

36. Cascadia
Warm slab implies dewatering at shallow depths and weak arc volcanism

37. Deformation in Subduction Zones
Accretionary Prism forms when sediments are present and scraped off subducting slab

38. Oblique Subduction Leads to Shearing
This shearing also affects the forearc causing rotation of the strong Oregon block.
Compression of Puget Sound