Publication of the results of continuous seismic reflection surveys around the continental margins (Beck & Lehner 1974, Seely et al. 1974, Lehner & Ruiter, in press) and discoveries made by drilling in the deep-sea by JOIDES and IPOD (Initial Reports of the Deep Sea Drilling Project) have shown that seismically active continental margins display structures very different from the structures associated with aseismic margins.

Lehner showed that after the initial rifting phase of evolving aseismic margins the main structural features are vertical movements manifested by the development of sedimentary basins at the margin of the new ocean and complementary basins on the craton. Some of these cratonic basins may be associated with aulacogens (Dewey & Burke 1974), others appear to be separated from the continental margin basins by a geanticlinal structure.

The structural style of the present aseismic margins of West Africa, northwest Australia and elsewhere can be summarized as asymmetric in cross section showing a ‘steep, mostly faulted landward flank on continental basement and a gentle seaward flank on oceanic basement' (Lehner).

Evidence is available from parts of the Mediterranean region (Smith) and from northern Australia (Garter et al. 1976) which shows how within few Ma an aseismic continental margin can become seismically active and be involved in orogenic processes such as major overthrusting.

Recent papers by Fischer (1975) and Bally (1975) have greatly influenced the way we think about the structural style of seismically active continental margins. Fischer’s suggestion that oceanic lithosphere has a maximum life of about

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