Download Geodynamics of congested subduction zones

Geodynamics of congested subduction zones implications for evolution of the Tasmanides
Pete Betts - School of Earth, Atmosphere, and environment, Monash University
Louis Moresi – Department of Earth Sciences, University of Melbourne
Ross Cayley – Geological Survey of Victoria
Meghan Miller – University of Southern California
Robin Armit
David Willis
New Perspectives 11th September 2014
Congested subduction zones
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Mason et al, 2008; Betts et al, 2012; 2014; Moresi et al, 2014
showed how trench motion and slab
configuration are influenced by buoyant
material colliding with a subduction zone.
raised question: how does subduction
continue after accretion of one terrane ?
How does a microcontinent / plateau
switch to the over-riding plate ?
This is important at many different scales
Can we understand this process better and can
we apply this to understand places where
accretion is complete?
What does this mean for the evolution of the
Tasmanides?
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Continental accretion & subduction
zone dynamics
Accretion is a vital component in
understanding how continents grow ...
A number of poorly understood aspects of
accretion attacked using Underworld and
tested against the Tasmanides
How does a microcontinent / plateau switch to
the over-riding plate?
How does subduction step back & recover
(accretion v. collision)?
What is the signature in the over-riding plate
(i.e. in the geological record)?
2D v 3D interpretations
Accretionary
Orogens
Mechanism for Phanerozoic
continental growth and transfer
of crust.
Altiads – Central Asian Orogen
(closing an internal ocean)
Tasmanides – Facing an
external Ocean (paleo-Pacific
ocean).
By comparison the Tasmanides
is relatively simple from a
geodynamics and rock record
perspective.
After Collins et al. 2011
Nature Geoscience
The Tasmanides
Recent Game changes – in my opinion
1. Recognition of the Selwyn Block
(Cayley and Taylor, 2002).
2. Hot (and extensional) Orogens
(Collins, 2002).
3. VanDieland micro-continent concept
(Cayley, 2011).
4. High resolution geophysical data
under-cover.
5. “Cayley model” in all its intricacies
(Cayley in prep).
The Tasmanides
• Adelaide Fold Belt (Neoproterozoic-Ordovician).
• Lachlan Orogen (Neoproterozoic-Carboniferous).
• Thomson Orogen (Neoproterozoic – Triassic)
North Queensland
(Mossman)
• New England Orogen (Cambrian-Triassic).
• North Queensland (Neoproterozoic-Triassic).
Thomson
Orogen
New England
Orogen
Turbidites
Lachlan Orogen
Granite rocks
Mafic-Ultramafic belts
Cambrian Turbidites
Neoproterozoic-Cambrian Rift and Cambrian back arc
Cambrian Rift and passive margin
The Tasmanides –
is the Murray River the most unusual
tectonic boundary on the planet?
Victoria
After Gray and others (1990’s 2000’s)
NSW
After Glen et al. (2013)
North QLD
After Betts et al. (2012)
The Tasmanides – It’s not really 3D
SW Pacific Margin
After Aitchison and Buckman (2012)
Oroclines are the flavour of the month!
Highlights the 3D problem
• Characterised by several
large oroclinal features
• Silurian to Carboniferous
• Combination of roll-back
and accretion.
Lachlan
Orocline
High resolution geophysical data
Bob Musgrave, 2009
NSW geological survey
VanDieland Micro-continent
VanDieland accretion
Selwyn Block
Modified after Cayley 2011 GR
VanDieland accretion
VanDieland accretion
VanDieland accretion – the Cayley (and Musgrave) model
VanDieland an embedded terrane
•
The Australian accretion of the VanDieland microcontinent resulted in the terrane being deeply
embedded in the over-riding plate and left largely
undisturbed since then.
•
Evidence of rotations in present day structural
grain (from potential fields, paleomagnetism and
other geological indicators).
Moresi, L., Betts, P. G., Miller, M. S., & Cayley, R. A.
(2014). Dynamics of continental accretion. Nature.
doi:10.1038/nature13033
2D thinking is misleading
•
In 3D a small buoyant block is easily
accreted or eaten by the subduction
zone. How about a large one ?
How does the slab recover from
accretion with break-off /
windowing ?
What should we look for in the
superficial geological record ?
What happens for a large
terrane/microcontinent ?
Size does matter!
Model set-up – USING UNDERWORLD
• Layer 1 density accounts for ~7km oceanic crust (but not phases changes during
subduction)
• Layer 1 yield strength is (very) low to account for (unresolved) near-surface
faulting, entrainment of sediments into the plate boundary & crust
• Viscosity is truncated after averaging (to 105 x asthenosphere)
• Layer 3 has significant strength for 80 Myr old lithosophere. In some models this
layer yields too.
• Continental Ribbon material replaces layer 1 and layer 2.
Lithosphere mapped into a layered model (Continental vs. Oceanic)
Micro-continent ribbon accretion
Weak slab-strong over-riding plate
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Large-scale rifting of the upper plate driven by rollback
Slab window created early in collision
Micro-continent ribbon accretion
Weak over-riding plate – older slab
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Small-scale localised deformation of the upper plate driven by rollback
Slab window doesn’t play much of a role
Micro-continent ribbon accretion
Building a mountain belt
Benambran Orogeny?
Motions are driven from below
Moresi, Betts, Miller, Cayley,
Dynamics of continental accretion.
Nature, 2014, doi:
10.1038/nature13033
Older & Stronger
Younger & Weaker
VanDieland accretion
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Stage 1 – in initial accretion of the microcontinent
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Shortening in front of the micro-continent and
extension driven by roll back away from accretion.
Different behaviors along the length of the margin
Stage 2 – transitional phase
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Roll-back and lateral escape of the overriding plate
Trench migration in two directions
Embedding of the micro-continent
Back arc extension - oceanic
Arc migration
Stage 3 – re-establishment of stable
subduction and convergent margin
Moresi et al., 2014
VanDieland accretion
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•
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Convergence direction is not change and
the trajectory of the plates remain the same.
Geometry is driven by trench migration.
Accreted terrane may also be entrained and
become curved in the migration of trench
parallel with the margin.
Slab window influence the rate of retreat
and allows mantle to flow away from the
migrating slab.
Modified after Cayley 2011 GR
Time evolution – lateral retreat is fast!
27
Trench motions – the mechanics of oroclines
A’
Over-riding Plate
A
Oceanic Plate
Margin shortens and
bends
•
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•
Modified after Cayley 2011 GR
The slab rolls back in two different directions (doesn’t need to
stretch tear to do this)
There is no plate boundary along A-A’ so any convergence not
accommodated by rollback results in indentation
Pinning at the end of the indenter supplies the lateral loads that
result in lateral transport of material along the margin and
bending.
What happens to the slab
behind the accreted terrane.
A’
A
Trajectories during lateral
subduction — almost
pure rollback creates
dangling slab
Trench motion
New England Oroclines
Modified after Rosenbaum at al. (2012) Tectonics
New England Oroclines
Modified after Rosenbaum at al. (2012) Tectonics
Oroclines behind the accreted terrane
Some other examples.
Modified after Betts et all., 2014 – Geoscience Frontiers
Oroclines behind the accreted terrane
Some other examples.
Modified after Betts et all., 2014 – Geoscience Frontiers
What we have learned
There is accretion and
embedding – and these are
different.
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There are three stages:
1.
Initial collision of the
micro-continent.
2.
A transitional stage –
trench advance, slab
retreat, and trench
migration.
3.
Re-establishment of
stable linear subduction
Initial
collision
Transition
stage
Re-establishment
Continental accretion and subduction zone dynamics
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How does a micro-continent switch to the over-riding plate ?
• slab tear / windowing / breakoff
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How does subduction step back (accretion v. collision) ?
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Potentially via rapid lateral rollback with highly oblique
subduction
(this depends on compliance of over-riding plate)
What is the signature in the over-riding plate (i.e. in the
geological record) ?
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Indentation accompanied by slab tearing or stretching
& local shortening of the over-riding plate.
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Simultaneous rollback and extension elsewhere in the
ORP.
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Lateral roll-back of subduction zone dragging material
around the accreted micro-continent.
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Oroclines associated with edges of accreted
microcontinents (the pinning point.
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Arc does not automatically restart beneath the microcontinent.
2D v 3D interpretations !!
3
Congested subduction zones – key to the past
• Subduction-zones eat information !
• Steady behaviour leaves a modest imprint in
the geological record but transitions are
exceptional and may dominate in what is left
behind.