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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
• 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?
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
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
Recent Game changes – in my opinion
The Tasmanides
Turbidites
Granite rocks
Mafic-Ultramafic belts
Cambrian Turbidites
Neoproterozoic-Cambrian Rift and Cambrian back arc
Cambrian Rift and passive margin
New England
Orogen
Lachlan Orogen
North Queensland
(Mossman)
Thomson
Orogen
• Adelaide Fold Belt (Neoproterozoic-Ordovician).
• Lachlan Orogen (Neoproterozoic-Carboniferous).
• Thomson Orogen (Neoproterozoic – Triassic)
• New England Orogen (Cambrian-Triassic).
• North Queensland (Neoproterozoic-Triassic).
The Tasmanides – is the Murray River the most unusual
tectonic boundary on the planet?
After Glen et al. (2013)
After Gray and others (1990’s 2000’s)
Victoria
NSW
After Betts et al. (2012)
North QLD
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.
LachlanOrocline
Bob Musgrave, 2009
NSW geological survey
High resolution geophysical data
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 micro-continent 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
• 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 ?
2D thinking is misleading
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 accretionWeak slab-strong over-riding plate
• Large-scale rifting of the upper plate driven by rollback
• Slab window created early in collision
Micro-continent ribbon accretionWeak over-riding plate – older slab
• 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?
Younger & Weaker
Older & Stronger
Moresi, Betts, Miller, Cayley, Dynamics of continental accretion.
Nature, 2014, doi: 10.1038/nature13033
Motions are driven from below
VanDieland accretion
Moresi et al., 2014
• Stage 1 – in initial accretion of the micro-
continent– 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– 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
VanDieland accretion
Modified after Cayley 2011 GR
• 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.
Time evolution – lateral retreat is fast!
27
Modified after Cayley 2011 GR
A A’Oceanic Plate
Over-rid
ing P
late
• 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.
Margin shortens and bends
Trench motions – the mechanics of oroclines
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
Modified after Betts et all., 2014 – Geoscience Frontiers
Oroclines behind the accreted terrane
Some other examples.
Modified after Betts et all., 2014 – Geoscience Frontiers
Oroclines behind the accreted terrane
Some other examples.
What we have learned
There is accretion and embedding – and these are different.
• 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
– How does a micro-continent switch to the over-riding plate ?
• slab tear / windowing / breakoff
– How does subduction step back (accretion v. collision) ?
• 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) ?
• Indentation accompanied by slab tearing or stretching& local shortening of the over-riding plate.
• Simultaneous rollback and extension elsewhere in the ORP.
• Lateral roll-back of subduction zone dragging materialaround the accreted micro-continent.
• Oroclines associated with edges of accreted microcontinents (the pinning point.
• Arc does not automatically restart beneath the micro-continent.
2D v 3D interpretations !!
36
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.
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