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.