The surface of Mars is cut by long linear faults with displacements of metres to kilometres, most of which are thought to have been formed by extension(1). The surface has also been modified by enormous floods, probably of water, which often flowed out of valleys formed by the largest of these faults(2). By analogy with structures on Earth(3,4), we propose here that the faults are in fact the surface expression of dykes(5), and not of large-scale tectonic movements. We use a numerical model to show that the intrusion of large dykes can generate structures like Valles Marineris. Such dykes can provide a heat source to melt ground ice, and so provide a source of water for the floods that have been inferred to originate in some of the large valleys(2).