Dynamics of delamination as a driver of Tonga-Kermedec subduction initiation

Abstract:

The interpretation of seismic profiles over northern Zealandia and borehole data recovered from IODP Expedition 371 reveals that the Lord Howe Rise uplifted and then subsided during the early Cenozoic during Tonga Kermedec subduction initiation. Meanwhile the New Caledonia Trough (NCT) underwent a large scale subsidence. The initial uplift, as well as the pervasive compression over the region, are interpreted to be associated with subduction initiation. The subsequent NCT subsidence is interpreted as a consequence of the crustal delamination.

We show a sequence of computational models with a visco-elasto-plastic rheological models that shed light on the dynamics of the proposed delamination model of Tonga-Kermedec subduction. In the model, we build up a trench perpendicular model with a pre-existing weak zone, as the Tonga-Kermedec subduction zone is believed as a case of subduction resurrection. A compressional force is applied on the far side of the plate with either a Neuuman or Dirichlet boundary condition to re-activate the subduction zone and induce the subduction. As the rupture zone nucleated and the tip of oceanic crust metamorphosed into dense eclogite root, the subducting plate starts to peel off from the overriding plate and roll back, which lead to more metamorphism. Eventually with the plate convergence sufficient crust turn into dense eclogite, the negative buoyance replace the tectonic compression as the dominant force, and delamination is triggered. The prediction of bathymetric vertical motion from our model is consistent with the observation. The initial Lord Howe Rise uplifting is a consequence of tectonic inter-plate compression, and the subsidence of the Lord Howe Rise and New Caledonia Trough corresponds to the increased negative buoyance from metamorphism and delamination. Our models also show the eclogite plays a key role in this process.