Subduction Zones

Question 1

Subduction zone =

Answer 1

Convergent boundaries of tectonic plates where one plate moves under another and is forced or sinks due to gravity into the mantle

Question 2

Benioff-Wadati zones

Answer 2

Reach base of upper mantle, 660km

Typically dip 45’ but ranges:

• young, hot slabs are more buoyant = smaller angle
• features on the subducting slab e.g. oceanic plateau broadly in equilibrium will rest subduction

Question 3

Earthquake mechanisms

Answer 3

Normal faulting near the surface due to slab bends

Thrusting deeper when plates collide

Within the slab = extension at intermediate depths due to:

• slab pull
• compression at depth

Question 4

Melt composition

Answer 4

Much more diverse than at MOR

Vary in silica content and in alkali content

Question 5

Variation in silica content

Answer 5

Due to:
1. Crustal contamination

2. Sediments subducted with slab, silica rich components may melt
3. Fractional crystallisation (Si last to crystallise) causing everything else in the melt to be enriched with Si in comparison to everything else

Question 6

Variation in alkali content

Answer 6

I.e. variation in K2O

More alkali basalts further away from a subduction zone

• further = subducting slab is deeper
• = less degree of partial melting (fewer volatiles due to dehydration)

K+ ion has a large radius and is incompatible

• does not fit easily into a silicate mineral lattice
• = one of the first elements to release into the melt

Therefore lower degree of partial melting = higher % of K2O in melt

Question 7

Oceanic plateau =

Answer 7

Anonymously thick oceanic crust

Question 8

Why does flat, amagmatic subduction occur?

Answer 8

1. Absence of a mantle wedge i.e. subduction too shallow
2. Cooling effects of the shallow slab “underplated” onto the base of the overlying slab
   e. g. Central Chile

N.B. Also delays eclogisation due to lack of dehydration

Question 9

Fore-arc =

Answer 9

Area between arc and trench

Question 10

Fore-arc behaviour

Answer 10

If the subducting oceanic plate has large sediment cover it can be scraped off onto the overriding plate

= ACCRETIONARY WEDGE

Main process by which continents grow

Question 11

Back-arc behaviour: shallow subduction

Answer 11

Friction “coupling” between the overlying and subducting plates

= compression

• large earthquakes
• forms new continents

Question 12

Back-arc behaviour: steep subduction

Answer 12

Slab pull and slab suction = “roll back” of subducting plate

= extension

• back arc basin forms
• often where MOR initiated

N.B. Can also cause subduction zones to migrate opposite to direction of plate movement

Question 13

Free air anomaly

Answer 13

Large positive anomaly over volcanic arc due to subducting slab

Large negative anomaly over trench due to water filled trench

NOT IN ISOSTATIC EQUILIBRIUM

Question 14

Which mechanisms are in play to form the topography of a subduction zone?

Answer 14

FLEXURAL FORCES
- within the slab causing the trench to form

SLAB PULL MECHANISMS

ASTHENOSPHERIC UPWELLING
- within the mantle wedge causing uplift of the volcanic arc

All dynamic forces
- imagine if the slab broke off
- subduction would cease
- lithosphere and topography would adjust to achieve isostatic equilibrium
= arc subsides and trench rebounds

Question 15

Focal mechanisms

Answer 15

Two possible fault planes

More curved/nearer edge of circle = shallower dip

Use map to help

Question 16

How many of the world’s subduction zones are flat slab?

Answer 16

10%

Question 17

Examples of flat subduction

Answer 17

Andean, Peruvian, Chilean
- bathymetric highs
- all slabs until 50 ma
BUT few in the Western Pacific with same correlation

Andean flat slabs like the Peruvian - largest in the world

• extends 700km inboard from trench axis
• 30’ dip then flattens at 100km depth