2D Numerical modelling of continental subduction and synthetic obduction

Continental subduction beneath an overriding oceanic plate is known to occur in nature, following the arrival of a continental margin at an intra-oceanic subduction zone, and often implying synthetic (i.e., Tethyan type) obduction. However, the main geodynamic constraints and geological/geophysical parameters governing this process, its viability and likelihood, are still not fully understood.

In the present work, we use 2D geodynamic numerical modelling to specifically investigate the geodynamic causes that might determine the amplitude of the subduction-exhumation (time-depth) cycle, as well as the viability of ophiolite emplacement and associated inward continental reach of ophiolitic nappes.

Using the finite-element code Underworld (Moresi et al., 2007) we constructed a 2D model with top free surface boundary conditions (to account for obduction-related topography build-up), lateral periodic boundary conditions (to compensate for the absence of asthenospheric mantle toroidal flow), and no-slip basal boundary conditions (to simulate subducting slab anchoring at the upper-lower mantle discontinuity when the slab reaches this interface at 660 km depth).

All simulations considered an initial intra-oceanic subduction zone, in which the subducting plate is linked to a trailing continental segment that eventually arrives at the subduction trench. To evaluate the geodynamic viability and efficiency of subsequent continental subduction and ophiolite emplacement, we used buoyancy driven models (i.e., without any externally imposed velocity boundary conditions), and investigated the following variable parameters: existence vs. absence of a weak (serpentinized) crustal layer in the overriding plate; variable age of the oceanic overriding plate (10, 20 and 60 Myrs) vs. a constant 70 Myrs subducting plate; different length of the oceanic segment of the subducting plate; and fixed vs. free subducting plate trailing edge boundary conditions.

Our preliminary results reveal a clear facilitation of ophiolite emplacement by the considered weak (serpentinite) crustal layer (in the overriding plate). Also, younger, less dense, and relatively weaker, overriding plates are shown to likewise favour more efficient obduction, including ophiolitic nappe allochthonous transport and formation of ophiolitic thrust windows and klippen. Finally, a higher length of the oceanic segment of the subducting plate and fixed trailing edge boundary conditions are shown to better comply with the geodynamic requirements assisting efficient, more realistic, amplitude subduction-exhumation cycles during continental subduction, as well as associated ophiolite obduction processes.        

Acknowledgements:

This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UID/50019/2025 and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020).

References:

Moresi, L., Quenette, S., Lemiale, V., Mériaux, C., Appelbe, B., & Hans-Bernd Mühlhaus (2007). Computational approaches to studying non-linear dynamics of the crust and mantle. Physics of the Earth and Planetary Interiors, 163 (1), 69-82. (Computational Challenges in the Earth Sciences) doi: 488 10.1016/j.pepi.2007.06.009