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Dike intrusion as a possible cause of linear Martian magnetic anomalies

¹ Bullard Laboratories, Department of Earth Sciences, Madingley Road, Cambridge CB3 0EZ, UK

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.

Key Words: crustal origin • cooling • magnetite • ilmenite

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