Continental breakup and slab pull driving force

Although slab pull is recognized as the main driving force of tectonic plates, marginal basins formation is generally explained by slab roll back or mantle plume impingement. The link between the slab pull force and the continental breakup of the lower plate is still poorly investigated, maybe due to the scarcity of proven examples? The goal of this study is to identify the mechanical conditions for which the slab pull force can be transmitted to the continental lithosphere of the lower plate and generates a continental rifting and breakup. The first condition requires to transfer the slab pull force across the oceanic domain and generate tensional setting into the attached continental margin. Then the ocean needs to be free of any Mid-oceanic ridge, which means that the continental breakup of the lower plate can only happen after the subduction or the inactivation of the ridge. The other conditions cannot be assessed as easily, and therefore motivates our modelling.

We perform a set of 2D thermo-mechanical regional-scale simulations of ridge-free subduction with slab pull evolving self-consistently during the sinking of the slab. The aim is to understand how, when and where slab pull can lead to continental breakup. Two parametric studies are presented. One investigates the tectonic plates kinematic relatively to the upper mantle and another one focused on the strength of both the oceanic and the continental part of the lower plate. In the simulations, the continental rifting is driven by tensional forces internally generated by the subduction zone. Kinematic conditions are only prescribed to the boundaries of the simulation domains to simulate convergent setting and promote subduction. Our numerical simulations reveal that a significant increase of the slab pull induced by the crossing of the 410 km phase transition is responsible for the lower plate breakup. If the oceanic domain is weaker than the continental margin, the slab pull leads to the slab break-off. On the contrary, if the continental domain is weaker, we observe a continental breakup at around 500 km apart from the passive margin. If the lower plate moves compared to the asthenosphic mantle below it, the horizontal basal shear at the LAB prevents the localization of the deformation and leads to an aborted rift.

To synthetize in natural examples, we show that the slab pull can lead to continental breakup when the Mid-oceanic ridge is already subducted, the continental domain is weaker than the oceanic domain, and the horizontal displacement of the lower plate is the same as that of the astenospheric mantle underneath. In light of this new constrains, we discuss the plate reconstruction models proposed for (1) the Cimmerian blocks detachment from the Gondwana during the Permian and (2) the Oligocene South China Sea opening.