Page 160 - Global Tectonics

146 CHAPTER 6

Fig. 6.19 (a) Ridge rotation model of spreading center Fig. 6.20 (a) Ridge adjustment by rift propagation;
adjustment; (b) evolution of a stepped ridge following (b) evolution of a stepped ridge following propagation

rotation (modified from Hey et al., 1988, by permission (modified from Hey et al., 1988, by permission of the
of the American Geophysical Union. Copyright © 1988 American Geophysical Union. Copyright © 1988
American Geophysical Union). American Geophysical Union).

and its subsequent growth at the expense of the old giving rise to a sheared zone with a quite distinctive
ridge. This mechanism has been termed the propagating fabric. Therefore, abrupt changes in both the topo-

rift model (Hey, 1977; Hey et al., 1980). Thus the old, graphic and magnetic fabric of the sea floor occur at
“doomed,” rift is progressively replaced by a propagat- the pseudofaults and failed rift, and the new ridge prop-
ing spreading center orthogonal to the new spreading agates by the disruption of lithosphere formed by sym-
direction (Fig. 6.20a). Kleinrock & Hey (1989) have metric accretion at the old ridge. Figure 6.20b shows a
described the complex processes that take place at the possible way in which the propagating model could give
tip of the propagating rift. The boundaries between rise to evenly spaced fracture zones. These new fracture
lithosphere formed at old and new ridges are termed zones are bounded by pseudofaults and/or failed rifts,

pseudofaults. Pseudofaults define a characteristic V- because the fracture zones do not form until propaga-
shaped wake pointing in the direction of propagation. tion is completed. They thus contrast with the ridge
Between the propagating and failing rifts, lithosphere is rotation model (Fig. 6.19b) which does not produce
progressively transferred from one plate to the other, failed rifts and in which the fracture zones are areas of