2D numerical modelling of Tethyan-type ophiolite emplacement: The role of overriding plate age, serpentinization, and OCT width.

Ophiolites are exposed remnants of oceanic lithosphere that are emplaced onto a continental domain, and Tethyan-type ophiolites, specifically, are those that are emplaced within a continental passive margin. The emplacement process for this type of ophiolites occurs when a continental passive margin subducts, and subsequently exhumes, beneath an oceanic overriding plate (future ophiolite). It is the exhumation of the passive margin’s crust that triggers both the separation of the ophiolite from the remaining oceanic overriding plate (OP) and its ensuing emplacement within the continental domain.

Analogue and numerical models have demonstrated the feasibility of this process (Chemenda et al., 1996; Duretz et al., 2016; Porkoláb et al., 2021); however, its specific geodynamic constraints are still poorly understood. For example, the geological record appears to be heavily skewed towards the fast emplacement of very young lithosphere, but it is unclear whether it is possible to emplace older lithosphere via the same process. Here we use 2D numerical models to test the sensitivity of this process to three key parameters: a) overriding plate age (10-60Myr), b) width of ocean-continent transition (OCT, 0-500km), and c) existence/absence of a serpentinization layer in the OP. The models use temperature and strain-rate dependent visco-plastic rheologies, are driven by buoyancy forces (without imposed non-zero velocity conditions), and are run using the Underworld code (Moresi et al., 2003).

Preliminary results show that the continental subduction/exhumation cycle and the ophiolite emplacement process are highly sensitive to variations in initial model conditions. Nevertheless, the emplacement process is physically viable under a somewhat wide range of conditions, being optimized for a narrow OCT and adjacent continental margin subducting beneath a young and serpentinized OP. A 10 Myrs old OP leads to a fast continental subduction-exhumation cycle (15-20 Myrs), while a 60 Myrs old OP induces a slow (>30 Myrs) cycle, but still leads to ophiolite emplacement. A long and tapered margin (OCT, 500km) also promotes a slow (>30 Myrs) cycle, with only a thin melange of exhumed crust, which hinders the formation and emplacement of individual ophiolite klippen; the reverse is true for a very short OCT. The existence of a serpentinization layer greatly facilitates the emplacement of the ophiolite klippe.

Acknowledgments

This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia I.P./MCTES through national funds (PIDDAC)–UIDB/50019/2020-IDL and through scholarship SFRH/BD/146726/2019.

References

Chemenda, A., Mattauer, M., Bokun, A. (1996). Continental subduction and a mechanism for exhumation of high-pressure metamorphic rocks: New modelling and field data from Oman. EPSL, 143, 173–182.

Duretz, T., Agard, P., Yamato, P., Ducassou, C., Burov, E., Gerya, T. (2016). Thermo-mechanical modeling of the obduction process based on the Oman Ophiolite case. GR, 32, 1–10.

Moresi, L., Dufour, F., Mühlhaus, H. B. (2003). A Lagrangian integration point finite element method for large deformation modeling of viscoelastic geomaterials. Journal Comp. Physics, 184, 476–497.

Porkoláb, K., Duretz, T., Yamato, P., Auzemery, A., Willingshofer, E. (2021). Extrusion of subducted crust explains the emplacement of far-travelled ophiolites. Nature Commun., 12, 1499.