OCEANESS 410 13 Subduction Zones William Wilcock LectureLab

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 Ma n Ge tle o Ol the d P rm lat e Water lowers the melting temperature of mantle peridotite s lidu t So ater We %w >>1 ~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