Subduction Zones Flashcards
Signatures at Subduction Zones
- Bathymetry/morphology
- Gravity
- Heat flow (thermal structure)
- Seismicity (and seismic structure)
Subduction zone morphology
- Outer arc bulge
- Trench
- Accretionary prism
- Deformation front
- Forearc basin
- Volcanic Arc
- Back-arc basin
Outer arc bulge
- Flexure (plate bends to subduct)
- Oceanic plate, seaward of trench
Trench
Depth depends on:
- Plate age, older is denser and cooler, negative buoyancy and slab pull
- Thickness
- Sediment filling trench
Accretionary prism
- Trench-fill and off-scraped sediments
- Landward of trench
Deformation front
- First fault or fold in prism
- may or may not line up exactly with prism
Forearc Basin
- Flat-bedded seds
- Not all subduction zones
Volcanic Arc
- Subducted plate at 100km depth
- Approximately 200km landward from trench
Back-arc Basin
- Extension
- Spreading axis
- Not all subduction zones
Gravity variations over subduction zones
- 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
Heat flow
- 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
Why does surface heat flow decrease landward of the deformation front?
- Thicker sediments, insulating
- Warm ocean crust gets deeper below surface
Thermal structure of the slab
- Cold ocean litho carries down isotherms (cold compared to normal mantle material at depth)
Length of slab Benioff zone
- 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)
Controls on temperature of slab
- 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
What are controls on slab temperature for large depths?
- Adiabatic heating due to compression
- Latent heat associated with phase changes at 410 and 660km
What happens at the 410 km discontinuity?
- Olive to spinel
- High-P phase of olivine
What happens at the 660km discontinuity?
- Spinel to post-spinel (perovskite etc.)
- High-P phases
Within a slab, is olivine/spinel phase change boundary at a depth less than or greater than 410km?
- Less than 410km
- Shallower in slab than adjacent mantle
Within a slab, is spinel/post-spinel phase change boundary at a depth less than or greater than 660km?
- Greater than 660km
- Deeper in slab than adjacent mantle
Seismicity associated with slab, top to bottom
1- Extension: Bending
2- Compression: Thrusting on interface
3- Extension or compression: Driving vs. resisting forces
4- Compression: Slab resistance- higher strength below 410km
Double Benioff zone
- 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
Accretionary prism
- 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
Accretionary Forearc
- Contains a sediment prism
- Thick forearc basin
- Trench fill
- Fluid vents
Non-Accretionary Forearc
- No sediment prism
- Thin forearc basin
- Serpentinite mud volcanoes
- Exposed basement instead of prism
- Subducting seamounts
Accretionary wedge prism structure
- Deformation front
- Decollement (detachment surface)
- Deformation, imbricate listric faults (dipping toward arc, youngest at deformation front)
Accretionary wedge prism structure: Incoming Sediments
- Turbidites from continent
- Hemipelagics/pelagics
Hemipelagics/pelagics
- ‘ocean rain’
- 200-600m thick
- Ash, organisms, or fine far-travelled continental material
Accretionary prism reflection
- From deformation front towards arc the reflectors become less coherent
- Implies pervasive deformation (ie at grain level)
Frontal accretion
- New thrust wedges added at toe of prism
- Sediments added above decollement (off scraping)
- Older wedges move upwards, rotated towards arc
Fate of sediments at a subduction zone
- Accretion
- Subduction
- Subduction erosion
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
Basal accretion
- Underplating
- Material initially subducted, but decollement may jump to deeper level
- May include ocean crust (deep - high-P metamorphism)
Subduction fate of sed at sub zone
Seds carried below decollement
- Carried down subduction channel
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
Subduction Erosion occurs due to?
- Seamount subduction
- Rough ocean plate surface (horsts and grabens, grabens fill w/ upper plate seds)
Seamount subduction
- Subduction erosion
- Uplift ahead of seamount, collapse features behind
- Ex. Costa Rica, Tonga (Louisville ridge)
What does the slope of the accretion wedge depend on?
- Type of sed
- Moisture
- Dip angle of plate
Blueschist
- High-P, Low-T at subduction zone
- Uplifted if found on surface, didn’t keep going down subduction channel
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?
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
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
Angle of repose vs. sed grain size
- Angle of repose increases w/ increasing grain size
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
Critical angle
= Alpha plus beta
- Where alpha is slope angle of top of soil
- Where beta is base dip of accretionary wedge (w/ plate)
Controls on taper angle
- Sediment strength, from pore fluid pressure
- Also pore fluid pressure and basal friction, complicated theory
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
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
Well-drained permeability
- Rapid fluid escape
- Low pore pressure
- Wedge steepens
- Steep stable geometry
- Strong base
Poorly-drained permeability
- Retarded fluid escape
- Elevated pore pressures
- Wedge remains shallow
- Shallow stable geometry
- Weak base
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
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)
Forearc basin often forms where?
- On continental shelf, accreted sediments
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
Dip of subducting slab increases…?
Towards the arc
Dip of subducting slab increases…?
- Beta increases further down subduction zone towards the arc
When slab dip beta > critical angle, what is alpha?
Alpha < 0, negative slope
- Top of wedge subsides
What is the general max critical angle?
11 degrees
What happens when slab dip beta is greater than 11 degrees?
Surface slope alpha is negative
- Subsidence occurs
- Can form forearc basin
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
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
