Geodynamics of congested subduction zones - implications for evolution of the Tasmanides

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 11 th 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

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). North Queensland (Mossman) Thomson Orogen (Neoproterozoic Triassic) New England Orogen (Cambrian-Triassic). North Queensland (Neoproterozoic-Triassic). Thomson Orogen New England Orogen Lachlan Orogen Turbidites 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 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 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 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 Stage 1 in initial accretion of the microcontinent 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 Moresi et al., 2014

VanDieland accretion 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 A Oceanic Plate Over-riding Plate Margin shortens and bends 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

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. Initial collision 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 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 material around the accreted micro-continent. Oroclines associated with edges of accreted microcontinents (the pinning point. 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.