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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.
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