During continental breakup, the width of a developing rift system is thought to be primarily controlled by crustal rheology, where weak and decoupled crust may develop into a wide (up to 300 km) rift system and strong crust can lead to localised thinning. Simultaneously, the development of magma-rich margins is increasingly being recognised to result from lithospheric mantle thinning prior to crustal thinning, allowing the development of both narrow and wide magma-rich continental margin systems. Seaward Dipping Reflectors (SDRs) and flat lying flows (FLFs) at magma-rich margins are generally considered to develop above rifting upper continental crust and flowing ductile lower continental crust, respectively, which in many instances contribute to isostatic buoyancy and therefore the subaerial eruption of lavas. Subsequent to continental breakup, therefore, ocean basin flooding readily occurs, leading to the production of classical oceanic crustal structure in a submerged basin (i.e. pillow basalts, sheeted dykes, and gabbro). What happens, however, if basin flooding is significantly delayed relative to breakup of the continental lithosphere? Here, we review evidence from the Mozambique Basin (and other magma-rich basins around the globe) to understand if basin flooding can postdate continental breakup and lead to the development of SDRs outboard of the continent ocean transition. In the Mozambique Basin, we find this unusual situation may have occurred locally despite the basin likely residing below sea level. This circumstance was facilitated by long-offset continent-continent transform faults isolating the basin within the continent interior during plate separation. Our findings have implications for the development of appropriate models of crustal structure at magma-rich continental margins and, therefore, our ability to appropriately interpret geophysical datasets, which often permit contrasting interpretations of crustal composition and distribution.
