Abstract
We perform numerical modelling to simulate the shortening of an oceanic basin and the adjacent continental margins in order to discuss the relationship between compressional stresses acting on the lithosphere and the time dependent strength of the mid-oceanic ridges within the frame of subduction initiation. We focus on the role of stress regulating mechanisms by testing the stress–strain-rate response to convergence rate, and the thermo-tectonic age of oceanic and continental lithospheres. We find that, upon compression, subduction initiation at passive margin is favoured for thermally thin (Palaeozoic or younger) continental lithospheres (<160 km) over cratons (>180 km), and for oceanic basins younger than 60 Myr (after rifting). The results also highlight the importance of convergence rate that controls stress distribution and magnitudes in the oceanic lithosphere. Slow convergence (<0.9 cm/yr) favours strengthening of the ridge and build-up of stress at the ocean-continent transition allowing for subduction initiation at passive margins over subduction at mid-oceanic ridges. The results allow for identifying geodynamic processes that fit conditions for subduction nucleation at passive margins, which is relevant for the unique case of the Alps. We speculate that the slow Africa–Europe convergence between 130 and 85 Ma contributes to the strengthening of the mid-oceanic ridge, leading to subduction initiation at passive margin 60–70 Myr after rifting and passive margin formation.
Highlights
Studies of subduction systems show that oceanic subduction either nucleates at passive margins or within oceanic plates (Gurnis et al, 2004; Stern, 2004; Stern and Gerya, 2018; Crameri et al, 2020)
The results show that, overall, subduction at passive margins requires less stress than intraoceanic subduction suggesting that subduction at passive margins is possible for stress levels lower than that within the oceanic plate (Figure 4B, blue dash-dot line), which is predicted for oceanic lithospheres older than ∼40 Myr (Figure 4B)
Models with low convergence rate (∼0.8 cm yr−1) predict strain localization and subsequent subduction initiation at the passive margin (Figure 5A, 6.6% BS), whereas models with high convergence rate predict intraoceanic subduction (Figure 5B, 7%BS). These results show that stress loading preferentially occurs at passive margins for cases of low convergence rate (Figure 5C), because deformation is distributed over many small structures within the brittle layer that accommodate low amounts of strain and is even more distributed within the ductile layers of the models
Summary
Studies of subduction systems show that oceanic subduction either nucleates at passive margins or within oceanic plates (Gurnis et al, 2004; Stern, 2004; Stern and Gerya, 2018; Crameri et al, 2020). Active compressional tectonics is well-documented in several oceanic basins (e.g., Forsyth, 1973; Wysession et al, 1991; Stein and Stein, 1993), recent and past examples of subduction initiation at or close to mid-ocean ridges as suggested for the Tethys or Pacific realms, are less well documented and subject to debate (Agard et al, 2016; Crameri et al, 2020), suggesting that mechanisms such as the dissipation of mechanical energy into heat regulates the stress level in the oceanic lithosphere (Brun and Cobbold, 1980; Schmalholz et al, 2009). How such mechanisms contribute to favour subduction initiation at passive margins over subduction at mid-oceanic ridges remain unclear
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