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Subduction Zones Flashcards

1
Q

Signatures at Subduction Zones

A
  • Bathymetry/morphology
  • Gravity
  • Heat flow (thermal structure)
  • Seismicity (and seismic structure)
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2
Q

Subduction zone morphology

A
  • Outer arc bulge
  • Trench
  • Accretionary prism
  • Deformation front
  • Forearc basin
  • Volcanic Arc
  • Back-arc basin
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3
Q

Outer arc bulge

A
  • Flexure (plate bends to subduct)

- Oceanic plate, seaward of trench

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4
Q

Trench

A

Depth depends on:

  • Plate age, older is denser and cooler, negative buoyancy and slab pull
  • Thickness
  • Sediment filling trench
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5
Q

Accretionary prism

A
  • Trench-fill and off-scraped sediments

- Landward of trench

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6
Q

Deformation front

A
  • First fault or fold in prism

- may or may not line up exactly with prism

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7
Q

Forearc Basin

A
  • Flat-bedded seds

- Not all subduction zones

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8
Q

Volcanic Arc

A
  • Subducted plate at 100km depth

- Approximately 200km landward from trench

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9
Q

Back-arc Basin

A
  • Extension
  • Spreading axis
  • Not all subduction zones
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10
Q

Gravity variations over subduction zones

A
  • Flexural bulge: slight positive gravity anomaly
  • Trench and prism: large negative anomaly
  • Forarc basin: Second gravity low
  • Island arc/continent: Large positive anomaly, near volcanic arc, over thickest part of continental crust
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11
Q

Heat flow

A
  • Ocean crust has high heat flow, depending on age
  • Landward of deformation front, surface heat flow decreases slowly to 40mW/m^2
  • Near volcanic arc, abrupt increase to 75-100mW/m^2
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12
Q

Why does surface heat flow decrease landward of the deformation front?

A
  • Thicker sediments, insulating

- Warm ocean crust gets deeper below surface

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13
Q

Thermal structure of the slab

A
  • Cold ocean litho carries down isotherms (cold compared to normal mantle material at depth)
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14
Q

Length of slab Benioff zone

A
  • Depends how deep the slab’s central core remains relatively cold
  • Down-dip length of slab benioff zone is proportional to subduction rate times age (faster and older has longer seismic zone)
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15
Q

Controls on temperature of slab

A
  • Slab age and thickness
  • Subduction rate
  • Slab dip
  • Thermal conductivity of slab and adjacent mantle
  • Radioactive heat production, mainly in continent
  • Frictional heating, is small
  • For large depths: Adiabatic heating, latent heat of phase change
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16
Q

What are controls on slab temperature for large depths?

A
  • Adiabatic heating due to compression

- Latent heat associated with phase changes at 410 and 660km

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17
Q

What happens at the 410 km discontinuity?

A
  • Olive to spinel

- High-P phase of olivine

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18
Q

What happens at the 660km discontinuity?

A
  • Spinel to post-spinel (perovskite etc.)

- High-P phases

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19
Q

Within a slab, is olivine/spinel phase change boundary at a depth less than or greater than 410km?

A
  • Less than 410km

- Shallower in slab than adjacent mantle

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20
Q

Within a slab, is spinel/post-spinel phase change boundary at a depth less than or greater than 660km?

A
  • Greater than 660km

- Deeper in slab than adjacent mantle

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21
Q

Seismicity associated with slab, top to bottom

A

1- Extension: Bending
2- Compression: Thrusting on interface
3- Extension or compression: Driving vs. resisting forces
4- Compression: Slab resistance- higher strength below 410km

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22
Q

Double Benioff zone

A
  • Assignment 1
  • Lower plane seismic zone in upper mantle
  • May be due to dehydration embrittlement of serpentinized hydrated minerals
  • Water infiltrates mantle through deep ruptures in outer-arc bulge
  • EQ’s correspond to antigorite dehydration reactions under similar temp/pressure pathways
  • Brittle fracture as hydrated minerals dehydrate at depth
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23
Q

Accretionary prism

A
  • Centre of mass of thrust wedge moves up decollement slope
  • Seds overlying subducting oceanic plate are ‘scraped’ off
  • Deformation front with faults coming from a decollement throughout seds
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24
Q

Accretionary Forearc

A
  • Contains a sediment prism
  • Thick forearc basin
  • Trench fill
  • Fluid vents
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25
Non-Accretionary Forearc
- No sediment prism - Thin forearc basin - Serpentinite mud volcanoes - Exposed basement instead of prism - Subducting seamounts
26
Accretionary wedge prism structure
- Deformation front - Decollement (detachment surface) - Deformation, imbricate listric faults (dipping toward arc, youngest at deformation front)
27
Accretionary wedge prism structure: Incoming Sediments
- Turbidites from continent | - Hemipelagics/pelagics
28
Hemipelagics/pelagics
- 'ocean rain' - 200-600m thick - Ash, organisms, or fine far-travelled continental material
29
Accretionary prism reflection
- From deformation front towards arc the reflectors become less coherent - Implies pervasive deformation (ie at grain level)
30
Frontal accretion
- New thrust wedges added at toe of prism - Sediments added above decollement (off scraping) - Older wedges move upwards, rotated towards arc
31
Fate of sediments at a subduction zone
- Accretion - Subduction - Subduction erosion
32
Accretion, fate of seds at sub zone
- Frontal accretion - Basal accretion/Underplating - Some seamounts or notches may get caught and eroded and become part of accretionary prism
33
Basal accretion
- Underplating - Material initially subducted, but decollement may jump to deeper level - May include ocean crust (deep - high-P metamorphism)
34
Subduction fate of sed at sub zone
Seds carried below decollement | - Carried down subduction channel
35
Subduction Erosion
- Fate of sed at sub zone - Frontal/basal - Transfer of trench/continental slope material from upper to subducting plate - Chunks of upper plate eroded by (added to) lower plate
36
Subduction Erosion occurs due to?
- Seamount subduction | - Rough ocean plate surface (horsts and grabens, grabens fill w/ upper plate seds)
37
Seamount subduction
- Subduction erosion - Uplift ahead of seamount, collapse features behind - Ex. Costa Rica, Tonga (Louisville ridge)
38
What does the slope of the accretion wedge depend on?
- Type of sed - Moisture - Dip angle of plate
39
Blueschist
- High-P, Low-T at subduction zone | - Uplifted if found on surface, didn't keep going down subduction channel
40
Costa Rica
- Seamount subduction - Seamount chain lines up with bumps on upper plate - Bumps likely subducted seamounts - Parallel fabric to trench on subducting plate, could be normal faulting on bend?
41
Tonga Trench and Louisville Ridge
- Subducting seamount chain ridge - Seismic gap exists at subduction zone - Gap could be from seamounts splitting subduction zone into multiparts - EQ's rupture to seamount but do not propagate beyond
42
Accretionary vs Erosional subduction margins
- Megathrusts may only occur on smooth sediment laden margins b/c smoothing propagates EQ's further than a rough zone (seamounts etc.) - Exception is Japan 2011 megathrust on erosional margin
43
Angle of repose vs. sed grain size
- Angle of repose increases w/ increasing grain size
44
Critical wedge theory
- Backstop: like bulldozer scraping off soil - Slope angle alpha of top of soil builds to critical angle, then slumping occurs to maintain critical value - Weak Material has a small critical taper angle - Presence of fluids reduces strength, acting as lubricating layer (e.g. along decollement) - Accretionary wedge: base dips at angle beta (w/ plate) - Critical angle is alpha plus beta
45
Critical angle
= Alpha plus beta - Where alpha is slope angle of top of soil - Where beta is base dip of accretionary wedge (w/ plate)
46
Controls on taper angle
- Sediment strength, from pore fluid pressure | - Also pore fluid pressure and basal friction, complicated theory
47
What happens to the wedge angle when the plate dips more steeply?
- Steeper the plate dips, the less angle can be maintained on top of the sed. wedge
48
Sediment composition
- Steeper alpha, angle on top of wedge when section is dominated by clay/mud - Shallower when less percent of incoming section is clay/mud
49
Well-drained permeability
- Rapid fluid escape - Low pore pressure - Wedge steepens - Steep stable geometry - Strong base
50
Poorly-drained permeability
- Retarded fluid escape - Elevated pore pressures - Wedge remains shallow - Shallow stable geometry - Weak base
51
Sediment (stratigraphic) thickness (m) vs. taper angle
- Variable taper angle, alpha, with sediment thickness | - Generally the thicker the sediment, the harder it is to build a steep wedge
52
Controls on pore fluid pressure
- Sediment properties like Permeability (composition, e.g. fine grained pelagic vs. coarse turbidites) and Stratigraphic thickness - Convergence rate (burial rate)
53
Forearc basin often forms where?
- On continental shelf, accreted sediments
54
Accretion increases sediment volume, so why does subsidence occur to form a forearc basin?
- When beta > critical angle and alpha < 0 (negative slope) the top of the wedge subsides
55
Dip of subducting slab increases...?
Towards the arc
56
Dip of subducting slab increases...?
- Beta increases further down subduction zone towards the arc
57
When slab dip beta > critical angle, what is alpha?
Alpha < 0, negative slope | - Top of wedge subsides
58
What is the general max critical angle?
11 degrees
59
What happens when slab dip beta is greater than 11 degrees?
Surface slope alpha is negative - Subsidence occurs - Can form forearc basin
60
Dips around Vancouver Island
- Deformation front off van isle slab dip is 4 degrees | - Tofino basin slab dip greater than 11 degrees, creates toxin basin
61
Dips around Olympic mountains
- Olympic mountains slab dip less than 11 degrees, creates uplift - Clallam syncline/ Puget sound slab dip is greater than 11 degrees, subsidence

EOS410PltTec (12 decks)

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