Passive margin asymmetry and its polarity in the presence of a craton

When continental lithosphere is extended to break-up it forms two conjugate passive margins. In many instances, these margins are asymmetric: while one is wide and extensively faulted, the conjugate thins more abruptly and exhibits little faulting. Recent studies have suggested that this asymmetry results from the formation of an oceanward-dipping sequential normal fault array and rift migration leading to the observed geometry of asymmetric margins. Numerical models have shown that fault sequentiality arises as a result of asymmetric uplift of the hot mantle towards the hanging wall of the active fault. The preferential localization of strain reinforced by strain weakening effects is random and can happen on either conjugate. However, along the long stretch of the South Atlantic margins, from the Camamu-Gabon to the North Santos-South Kwanza conjugates, the polarity can be very well correlated with the distance of the rift to nearby cratonic lithosphere. Here, we use numerical experiments to show that the presence of a thick cratonic root inhibits asthenospheric flow from underneath the craton towards the adjacent fold belt, while flow from underneath the fold belt towards the craton is favoured. This enhances and promotes sequential faulting and rift migration towards the craton and resulting in a wide faulted margin on the fold belt and a narrow conjugate margin on the craton side, thereby determining the polarity of asymmetry, as observed in nature.