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EOS410PltTec > Subduction Zones > Flashcards

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
Q

Non-Accretionary Forearc

A
  • No sediment prism
  • Thin forearc basin
  • Serpentinite mud volcanoes
  • Exposed basement instead of prism
  • Subducting seamounts
26
Q

Accretionary wedge prism structure

A
  • Deformation front
  • Decollement (detachment surface)
  • Deformation, imbricate listric faults (dipping toward arc, youngest at deformation front)
27
Q

Accretionary wedge prism structure: Incoming Sediments

A
  • Turbidites from continent

- Hemipelagics/pelagics

28
Q

Hemipelagics/pelagics

A
  • ‘ocean rain’
  • 200-600m thick
  • Ash, organisms, or fine far-travelled continental material
29
Q

Accretionary prism reflection

A
  • From deformation front towards arc the reflectors become less coherent
  • Implies pervasive deformation (ie at grain level)
30
Q

Frontal accretion

A
  • New thrust wedges added at toe of prism
  • Sediments added above decollement (off scraping)
  • Older wedges move upwards, rotated towards arc
31
Q

Fate of sediments at a subduction zone

A
  • Accretion
  • Subduction
  • Subduction erosion
32
Q

Accretion, fate of seds at sub zone

A
  • Frontal accretion
  • Basal accretion/Underplating
  • Some seamounts or notches may get caught and eroded and become part of accretionary prism
33
Q

Basal accretion

A
  • Underplating
  • Material initially subducted, but decollement may jump to deeper level
  • May include ocean crust (deep - high-P metamorphism)
34
Q

Subduction fate of sed at sub zone

A

Seds carried below decollement

- Carried down subduction channel

35
Q

Subduction Erosion

A
  • 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
Q

Subduction Erosion occurs due to?

A
  • Seamount subduction

- Rough ocean plate surface (horsts and grabens, grabens fill w/ upper plate seds)

37
Q

Seamount subduction

A
  • Subduction erosion
  • Uplift ahead of seamount, collapse features behind
  • Ex. Costa Rica, Tonga (Louisville ridge)
38
Q

What does the slope of the accretion wedge depend on?

A
  • Type of sed
  • Moisture
  • Dip angle of plate
39
Q

Blueschist

A
  • High-P, Low-T at subduction zone

- Uplifted if found on surface, didn’t keep going down subduction channel

40
Q

Costa Rica

A
  • 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
Q

Tonga Trench and Louisville Ridge

A
  • 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
Q

Accretionary vs Erosional subduction margins

A
  • 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
Q

Angle of repose vs. sed grain size

A
  • Angle of repose increases w/ increasing grain size
44
Q

Critical wedge theory

A
  • 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
Q

Critical angle

A

= Alpha plus beta

  • Where alpha is slope angle of top of soil
  • Where beta is base dip of accretionary wedge (w/ plate)
46
Q

Controls on taper angle

A
  • Sediment strength, from pore fluid pressure

- Also pore fluid pressure and basal friction, complicated theory

47
Q

What happens to the wedge angle when the plate dips more steeply?

A
  • Steeper the plate dips, the less angle can be maintained on top of the sed. wedge
48
Q

Sediment composition

A
  • 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
Q

Well-drained permeability

A
  • Rapid fluid escape
  • Low pore pressure
  • Wedge steepens
  • Steep stable geometry
  • Strong base
50
Q

Poorly-drained permeability

A
  • Retarded fluid escape
  • Elevated pore pressures
  • Wedge remains shallow
  • Shallow stable geometry
  • Weak base
51
Q

Sediment (stratigraphic) thickness (m) vs. taper angle

A
  • Variable taper angle, alpha, with sediment thickness

- Generally the thicker the sediment, the harder it is to build a steep wedge

52
Q

Controls on pore fluid pressure

A
  • Sediment properties like Permeability (composition, e.g. fine grained pelagic vs. coarse turbidites) and Stratigraphic thickness
  • Convergence rate (burial rate)
53
Q

Forearc basin often forms where?

A
  • On continental shelf, accreted sediments
54
Q

Accretion increases sediment volume, so why does subsidence occur to form a forearc basin?

A
  • When beta > critical angle and alpha < 0 (negative slope) the top of the wedge subsides
55
Q

Dip of subducting slab increases…?

A

Towards the arc

56
Q

Dip of subducting slab increases…?

A
  • Beta increases further down subduction zone towards the arc
57
Q

When slab dip beta > critical angle, what is alpha?

A

Alpha < 0, negative slope

- Top of wedge subsides

58
Q

What is the general max critical angle?

A

11 degrees

59
Q

What happens when slab dip beta is greater than 11 degrees?

A

Surface slope alpha is negative

  • Subsidence occurs
  • Can form forearc basin
60
Q

Dips around Vancouver Island

A
  • Deformation front off van isle slab dip is 4 degrees

- Tofino basin slab dip greater than 11 degrees, creates toxin basin

61
Q

Dips around Olympic mountains

A
  • 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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