Figure 6 Schematic cross-section of the sequence of events leading to the formation, development and closure of an ocean basin. (a) Crustal thinning. At first, the upper parts of the crust extend by developing a series of brittle normal faults. This can be initiated either by passive rifting (a)(ii), or by active rifting (a)(iii). After the crust extends, it will eventually undergo ‘thermal sag’, creating a basin in which sedimentary and/or volcanic rocks can accumulate. (b) Embryonic ocean basin formation. As extension continues, the lower lithosphere will rise and melt. (c) Active rifting and formation of a passive continental margin. (d) Onset of subduction