Testing the influence of far-field topographic forcing on subduction initiation at a passive margin

- We test far-field topographic forces on subduction initiation.
- Topographic forces sum up to a magnitude enough to initiate subduction.
- The SE Brazilian margin shows signs of inversion.
- Gravitational models show that the Andes are necessary to initiate subduction.
- We now call the early subduction stage the “Brazilian Stage”.

Despite favourable gravitational instability and ridge-push, elastic and frictional forces prevent subduction initiation from arising spontaneously at passive margins. Here, we argue that forces arising from large continental topographic gradients are required to initiate subduction at passive margins. In order to test this hypothesis, we use 2D numerical models to assess the influence of the Andean Plateau on stress magnitudes and deformation patterns at the Brazilian passive margin. The numerical results indicate that “plateau-push” in this region is a necessary additional force to initiate subduction. As the SE Brazilian margin currently shows no signs of self-sustained subduction, we examined geological and geophysical data to determine if the margin is in the preliminary stages of subduction initiation. The compiled data indicate that the margin is presently undergoing tectonic inversion, which we infer as part of the continental-oceanic overthrusting stage of subduction initiation. We refer to this early subduction stage as the “Brazilian Stage”, which is characterized by > 10 km deep reverse fault seismicity at the margin, recent topographic uplift on the continental side, thick continental crust at the margin, and bulging on the oceanic side due to loading by the overthrusting continent. The combined results of the numerical simulations and passive margin analysis indicate that the SE Brazilian margin is a prototype candidate for subduction initiation.