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CONTINENTAL RIFTS AND RIFTED MARGINS 209
(Section 10.6.1). This periodicity of ocean formation and margins (Fig. 7.41e). This phase will continue until the
closure is known as the Wilson cycle, named after J. Tuzo two continents collide and the ocean basin closes com-
Wilson in recognition of his contributions to the theory pletely (Fig. 7.41f). Continent–continent collision leads
of plate tectonics (Dewey & Burke, 1974). to the formation of a Himalaya-type orogen (Section
Figure 7.41 shows a schematic illustration of the 10.1) and the exhumation of deep crustal rocks. At this
various stages in the Wilson cycle beginning with the time subduction zones must initiate at other continen-
initial break-up of a stable continental craton (Fig. tal margins in order to maintain constant global surface
7.41a) and the thinning of continental lithosphere. area. The forces associated with these new subduction
Rifting (Fig. 7.41b) is followed by the development of zones place the continent under tension and, if other
a thinned, rifted continental margin and eventually conditions are extant (Section 7.5), the rifting process
gives way to sea floor spreading as the two continents begins again. Present day analogues of the oceans
separate across an expanding ocean (Fig. 7.41c). The shown in Fig. 7.41 are: Fig. 7.41c (expanding oceans) =
termination of basin opening may occur in response the Gulf of Aden, Woodlark Rift, and the Atlantic
to plate collisions, which could trigger subduction at Ocean; Fig. 7.41d,e (contracting oceans) = the Pacifi c
one or more rifted margins (Fig. 7.41d). Basin closure Ocean. Chapters 9 and 10 provide discussions of the
also may compensate for oceanic lithosphere that is processes that operate during the destructive part of
newly formed elsewhere. The contracting ocean is a the Wilson cycle as ocean basins close and continents
consequence of subduction at one or both continental collide.