Geodynamic modelling of continental subduction beneath oceanic lithosphere

Subduction of an oceanic plate beneath either an oceanic, or a continental, overriding plate requires two main conditions to occur in a steady state: i) a high enough subduction rate (~5 cm/yr, Schellart in print); and ii) a weak (efficiently softened/lubricated) subduction channel (Gerya and Meilick, 2011). The first requirement prevents thermal diffusive re-equilibrium of the subducting slab within the asthenospheric ambient mantle, maintaining the slab cold and dense enough to provide the slab-pull subduction driving force. The second condition, is achieved with the contribution of a strong dehydration of the serpentinized oceanic plate, with resulting pervasive fluid circulation in the subduction channel significantly promoting its weakening, thus preventing strong coupling between the subducting and the overriding plate. Avoiding such a coupling has been shown to be key to maintain stable subduction, since it generally leads to a halt in the subduction process and to slab break-off (Duarte et al., 2015). Both these conditions are seemingly not favoured in a continental subduction scenario, since continental lithosphere is positively buoyant and much less, or not al all, serpentinized. Hence, the (geo)dynamics governing continental subduction is still not fully understood.We thus carried out a set of geodynamic numerical modelling experiments to further understand the first order geodynamic constraints governing continental subduction in the specific scenario that considers the subduction of a continental plate beneath an oceanic one, i.e., upon the arrival of a continental plate at an intra-oceanic subduction zone. The 2D numerical experiments were conceived and constructed using the Underworld code (Moresi et al., 2007), to better understand the influence on continental subduction efficiency, as well as on related synthetic ophiolite obduction, of considering either a scenario of dominant trench retreat (roll-back) or trench advance (roll forward) subduction regime. Roll-back subduction was prescribed in our models by fixing the trailing edge of the overriding plate, whereas roll-forward subduction was favoured (allowed) by leaving it free to move. Our experiments ensure dynamic self consistency in all cases.  Our preliminary results show that, although synthetic obduction is possible to achieve in both situations, the overall first order (geo)dynamic differences implied by the two different simulated regimes, bear important consequences on the timing, overall kinematic configuration and local stress/strain distribution of the considered continental subduction-exhumation cycle in each case.AcknowledgmentsThis work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020- IDLReferencesDuarte, J.C., Schellart, W.P., Cruden, A.R., 2015. How weak is the subduction zone interface? Geophysical Research Letters 42, 2664–2673. doi:10.1002/2014GL062876.Gerya, T.V., Meilick, F., 2011. Geodynamic regimes of subduction under an active margin: effects of rheological weakening by fluids and melts. Journal of Metamorphic Geology 29, 7–31. doi:0.1111/j.1525-1314.2010.00904.x.Moresi, L., Quenette, S., Lemiale, V., Mériaux, C., Appelbe, B., Muhlhaus, H.B., 2007. Computational approaches to studying non-linear dynamics of the crust and mantle. Physics of the Earth and Planetary Interiors 163, 69–82. doi:10.1016/j.pepi.2007.06.009.Schellart, W.P., in print. Subduction zones: A short review, in Dynamics of Plate Tectonics and Mantle Convection, Editor: João Duarte, ISBN: 9780323857338.