Wilson cycle The hypothesis, named after Tuzo Wilson who proposed it, that an ocean basin has a lifespan with several stages: from opening, through development, to final closing and the destruction of the basin. Six stages, and the plate tectonic processes involved, have been identified in different parts of the Earth today, and have been postulated for orogenic belts back to the early Proterozoic. The earliest (embryonic) stage involves uplift and crustal extension with the formation of rift valleys (e.g. the E. African Rifts). The young stage involves further subsidence, plus sea-floor spreading; the result is a narrow, parallelsided sea, possibly with evaporites from intermittent desiccation (e.g. the Red Sea). At this stage, if the uplift was broadly domal with a pattern of three radial rifts forming a triple junction, two of the rifts may widen leaving the third to form an aulacogen (e.g. the Ethiopian Rift). The next (mature) stage is exemplified by the Atlantic Ocean, a wide ocean basin flanked by continental shelves and with the production of new, hot, oceanic crust along an oceanic ridge. Eventually this expanding system becomes unstable, and part of the cooled lithosphere, away from the ridge, sinks into the asthenosphere, forming an oceanic trench with an associated island arc. The shrinking Pacific Ocean is thought to be at this stage. Further shrinking, with the compression, metamorphism, and uplift of accretionary wedges to form young mountain ranges, marks the terminal stage (e.g. the Mediterranean). Finally, all the oceanic crust between the continental masses has subducted and the continents converge in a collision zone, being joined along a suture. The suture (e.g. the Indus-Yarlung Zangbo suture in the Himalayas) marks the relic scar between the plates, and the plate margin finally becomes inactive.

Wilson cycle The hypothesis proposed by the geophysicist John Tuzo Wilson (1908–93) that an ocean develops through six distinct stages driven by the movement of crustal plates. The cycle begins with the lifting and extension of the crust to form a rift valley. Further crustal subsidence and sea-floor spreading produce a narrow sea with approximately parallel sides (e.g. the Red Sea). Further plate movement causes the sea to widen into an ocean flanked by continents (e.g. the Atlantic Ocean). The system becomes unstable and part of the cooled crust, away from the mid-oceanic ridge (see ridge (3)) sinks, forming a trench with an associated island arc (e.g. the Pacific Ocean). The ocean is then shrinking and continues to do so, with wedges of rock being lifted to form young mountain ranges (e.g. the Mediterranean). Finally, when all the crust between the continents has been subducted, the continents converge and join along a suture (e.g. in the Himalayas).