Passive margins are segmented into discrete elements by segment boundaries exhibiting a variety of structural styles. The Namibian margin is bounded by a Hinge Line that is offset by segment boundaries. The most important is the Cape Cross Segment Boundary. Here, pre-existing basement lineaments and ‘Karoo’ rift elements were reactivated during late Mesozoic rifting. It marks the northern limit of the earliest of three rift sequences and relates to a temporal change in break-up and formation of the South Atlantic. A similar change occurs across other important segment boundaries but smaller examples lack this. Major segment boundaries with a temporal change in rifting and break-up exhibit a strong connection to the basement province.