CONGESTED SUBDUCTION ZONES: THE RECORD OF COLLISIONS IN A CONVERGENT MARGIN'S HISTORY

When buoyant material is entrained into a subduction zone, the dynamics of the subsequent evolution can become extremely complicated. Whether that buoyant material chokes the subduction zone or is quickly accreted onto the over-riding plate, whether there is a long-lived collision, whether slab break-off occurs - all of these outcomes are dependent on the local buoyancy balance but also on the integrated force balance along the entire margin.

We find that buoyant material ingested by the subduction zone produces a relative advance of the local region of the trench (either reduced rollback or absolute advance) naturally leading to the characteristic indentation of the plate boundary by the buoyant intruder (e.g. Mason et al, 2010). Depending on the strength and buoyancy of the plateau relative to the oceanic lithosphere, it may be subducted or it may be accreted with the associated formation of a slab window. Extending this model to ocean-continent convergent zones (Moresi et al, 2014), we show how the indentation of

buoyant exotic material also dominates terrane accretion.

When large blocks of material congest a subduction zone, the subduction zone needs to undergo signficiant re-arrangement for convergence to continue. We have modelled this process and observe characteristic patterns in the deformation of the over-riding plate, in the timing of the escape of material from behind the indenter, and in the oroclinal geometry that remains once the collision has completed.

We focus our study on numerical models of the SE Australian accreted terrains, the collision of the Hikurangi plateau at the Gondwana margin and, presently, the South Island of New Zealand, and the ongoing collision of the Yakutat terrain in Alaska.

References

Mason, W. G., Moresi, L., Betts, P. G., & Miller, M. S. (2010). Three-dimensional numerical models of the influence of a buoyant oceanic plateau on subduction zones. Tectonophysics, 483(1-2), 71–79. doi:10.1016/j.tecto.2009.08.021

Moresi, L., Betts, P. G., Miller, M. S., & Cayley, R. A. (2014). Dynamics of continental accretion. Nature. doi:10.1038/nature13033