Numerical modelling of intra-oceanic rifting: the rift-to-drift transition time frame

Numerical modelling of rifting has been focused on cases involving extension and breakup of the continental lithosphere. However, the oceanic lithosphere has also been known to undergo rifting in specific geo-tectonic settings, as in the case of the Terceira ridge in the Azores triple junction (N-Atlantic). The rift-to-drift evolution of a segment of oceanic lithosphere potentially bears major implications for the Wilson cycle evolution of an oceanic basin, justifying the importance of carrying out the present numerical modelling study.

We used the Underworld geodynamic code to carry out 2D numerical models of oceanic rifting. To this extent, we systematically tested two main parameters which control the timing of the evolution from initial oceanic extension to breakup and drifting, namely: a) different total extension rates between 4 mm/yr and 160 mm/yr, and b) different oceanic plate ages ranging between 10 Myr and 90 Myr, which act as proxies for the lithospheric thickness.

Our results show that during oceanic rifting, the time required to achieve breakup of the extending oceanic lithosphere decreases logarithmically with an increasing extensional rate (i.e., the time needed to achieve breakup reaches a plateau). Our modelling also shows that lithospheric thickness plays a secondary, yet significant role in the type of oceanic rift that is formed (i.e., its structural configuration). This oceanic rift structure can comprise either a unique major graben or two main grabens, as preferable sites of extensional strain localization. Furthermore, when two main grabens develop, one of them often accommodates the bulk of the deformation, while the other wanes and eventually aborts. In this case, a more distributed pattern of extensional strain (comprising two main grabens) seemingly implies some delay in achieving full oceanic break-up, when compared with the single major graben scenario.

Acknowledgements: numerical modelling was financed by Projeto GEMMA - PTDC/CTA-GEO/2083/2021, Fundação para a Ciência e Tecnologia. This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020- IDL.