Tectonic Inheritance, Continental Rifts and the Transition to Segmented Seafloor Spreading: Results From Analogue and Numerical Models

We use analogue and numerical models to investigate the role of an inherited structural grain on continental rifting and the implications for oceanic ridge segmentation after continental breakup. Our models are based on geological observations from the western branch of the East African Rift System. The basins of this rift system are located within Proterozoic mobile belts at the edge of the Tanzanian craton; the Archean cratonic area has undergone very limited extension. Surface geology suggests that the geometry of the weak zone surrounding the strong Tanzanian craton is an important parameter in controlling rift development and architecture. Our models confirm this finding and show that the varying extension direction along the rift, caused by the curved mobile belts surrounding the craton, may explain its geometry and segmentation. It accounts for rift asymmetry, segmentation characteristics and border fault architecture. Areas of oblique extension give rise to a segmented rift structure, whereby the individual segment orientation is influenced by the extension direction. The segments are loci of strain localization and mantle upwelling, and are expected to develop into spreading ridge segments after breakup. Thus, distinct basins where deformation has localized may provide a mould for initial mid-oceanic ridge segments. Continental breakup along the curved rift system is then expected to result in a curved, segmented mid-oceanic ridge, such as observed in the southern Atlantic Ocean.