A model in which the ancient, 100–200-km-wide magnetic lineations in the southern hemisphere of Mars are formed by repeated dike intrusion is proposed. On the basis of magnetizations observed at terrestrial mid-ocean ridges, a likely upper bound on the Martian crustal magnetization is about 15 A m−1. This upper bound gives a minimum magnetized layer thickness of 35–60 km. Because the magnetic material is likely to be magnetite, the long-term stability of the anomalies probably requires ilmenite lamellae to have exsolved from the magnetite. This exsolution process provides an upper bound on the cooling time available of 102–105 yr. These bounds can be satisfied if the magnetic anomalies are created by successive intrusion of dikes from 20 to 2000 m wide. Production of a Martian crust 35–60 km thick by rifting requires either mantle potential temperatures in excess of 1430 °C or stretching factors greater than 5.