Propagating Rifts And Microplates

The direction of spreading at an ocean ridge does not always remain constant over long periods of time, but may undergo several small changes. Menard & Atwater (1968) proposed that spreading in the northeastern Pacific had changed direction five times on the basis of changes in the orientation of major transform faults (Section 5.9) and magnetic anomaly patterns. Small changes in spreading direction have also been proposed as an explanation of the anomalous topography associated with oceanic fracture zones (Section 6.12).

Menard & Atwater (1968) made the assumption that the reorientation of a ridge would take place by smooth, continuous rotations of individual ridge segments until they became orthogonal to the new spreading direction (Fig. 6.19a). The ridge would then lie at an angle to the original magnetic anomaly pattern. Long portions of ridges affected in this way might be expected to devolve into shorter lengths, facilitating ridge rotation and creating new transform faults (Fig. 6.19b). The change in spreading direction is thus envisaged as a gradual, continuous rotation that produces a fan-like pattern of magnetic anomalies that vary in width according to position.

An alternative model of changes in spreading direction envisions the creation of a new spreading center

Fig. 6.19 (a) Ridge rotation model of spreading center adjustment; (b) evolution of a stepped ridge following rotation (modified from Hey et al., 1988, by permission of the American Geophysical Union. Copyright © 1988 American Geophysical Union).
Fig. 6.20 (a) Ridge adjustment by rift propagation; (b) evolution of a stepped ridge following propagation (modified from Hey et al., 1988, by permission of the American Geophysical Union. Copyright © 1988 American Geophysical Union).

and its subsequent growth at the expense of the old ridge. This mechanism has been termed the propagating rift model (Hey, 1977; Hey et al., 1980). Thus the old, "doomed," rift is progressively replaced by a propagating spreading center orthogonal to the new spreading direction (Fig. 6.20a). Kleinrock & Hey (1989) have described the complex processes that take place at the tip of the propagating rift. The boundaries between lithosphere formed at old and new ridges are termed pseudofaults. Pseudofaults define a characteristic V-shaped wake pointing in the direction of propagation. Between the propagating and failing rifts, lithosphere is progressively transferred from one plate to the other, giving rise to a sheared zone with a quite distinctive fabric. Therefore, abrupt changes in both the topographic and magnetic fabric of the sea floor occur at the pseudofaults and failed rift, and the new ridge propagates by the disruption of lithosphere formed by symmetric accretion at the old ridge. Figure 6.20b shows a possible way in which the propagating model could give rise to evenly spaced fracture zones. These new fracture zones are bounded by pseudofaults and/or failed rifts, because the fracture zones do not form until propagation is completed. They thus contrast with the ridge rotation model (Fig. 6.19b) which does not produce failed rifts and in which the fracture zones are areas of highly asymmetric sea floor spreading. The propagation model predicts abrupt boundaries between areas of uniform magnetic anomaly and bathymetric trends of different orientation. The rotation model predicts a continuous fanlike configuration of magnetic anomalies whose direction changes from the old to new spreading direction. Consequently, detailed bathymetric and magnetic surveys should be able to distinguish between the two models.

Hey et al. (1988) reported the results of a detailed investigation of the region where the direction of spreading of the Pacific-Farallon boundary changed direction at about 54 Ma, just north of the major bend of the Surveyor Fracture Zone, using side-scan sonar, magnetometry, and seismic reflection. They found that the change in direction of sea floor fabric revealed by sonar is abrupt, in accord with the propagating rift model. Similar conclusions were reached by Caress et al. (1988). Hey et al. (1980) described the results of a survey of an area west of the Galapagos Islands at 96°W. They concluded that here a new ridge is progressively breaking through the Cocos plate, and the magnetic data in particular (Fig. 6.21) provide convincing evidence that the ridge propagation mechanism is operative. This interpretation was confirmed by detailed mapping of the bathymetry in this area (Hey et al., 1986). This clearly revealed the V-shaped pattern of the pseudofaults, the active and failed rifts, and the oblique tectonic fabric in the sheared zone of transferred lithosphere. The propagating rift model also elegantly explains the way in which the change in orientation of the Juan de Fuca Ridge (Fig. 4.1) has been achieved within the past 10 Ma (Wilson et al., 1984).

Engeln et al. (1988) pointed out that the propagating rift model described above assumes that the newly formed rift immediately attains the full accretion rate between the two plates, thereby rendering the preexisting rift redundant. However, if spreading on the new rift is initiated at a slow rate, and only gradually builds up to the full rate over a period of millions of years, the failing rift continues to spread, albeit at a slower and decreasing rate, in order to maintain the net accretion rate. In contrast to the original propagating rift model, in this model the two rifts overlap and the area of oceanic lithosphere between them increases with time. In addition, as a result of the gradients in spreading rate along each rift, the block of intervening lithosphere rotates. This rotation in turn produces compression in the oceanic lithosphere adjacent to the tip of the propagating rift and transtension (Section 8.2) in

Fig. 6.21 (a) Predicted magnetic lineation pattern resulting from ridge propagation; (b) observed magnetic anomalies near 96°W west of the Galapagos Islands (redrawn from Hey et al., 1980, by permission of the American Geophysical Union. Copyright © 1980 American Geophysical Union).

the region between the points where the propagating rift was initiated and the original rift started to fail. After a few million years this transpression gives rise to an additional propagating rift.

This second propagating rift model was put forward to explain the remarkable phenomenon of microplates

Fig. 6.22 Map showing the location and extent of the Galapagos, Easter and Juan Fernandez microplates in the southeast Pacific Ocean. Arrows on ridge segments indicate active or previously active propagating rifts (modified from Bird et al., 1998, by permission of the American Geophysical Union. Copyright © 1998 American Geophysical Union).

Fig. 6.22 Map showing the location and extent of the Galapagos, Easter and Juan Fernandez microplates in the southeast Pacific Ocean. Arrows on ridge segments indicate active or previously active propagating rifts (modified from Bird et al., 1998, by permission of the American Geophysical Union. Copyright © 1998 American Geophysical Union).

in the southeast Pacific (Fig. 6.22). Detailed studies of the Easter and Juan Fernandez microplates show that their bathymetric fabrics and structural evolution are very similar, and fit well with the predictions of the model of Engeln et al. (1988) (Searle et al., 1989; Rusby & Searle, 1995; Larson et al., 1992; Bird et al., 1998). The tectonic elements of the Juan Fernandez microplate (Fig. 6.23) clearly show the characteristic pseudofaults of the original propagating rift to the east, and the subsequent propagating rift to the southwest of the microplate. Microplates are thought to exist for no more than 5-10 million years, by which time the initial rift succeeds in transferring the oceanic lithosphere of the microplate from one plate to another, in the case of the Juan Fernandez microplate, probably from the Nazca to the Antarctic plate (Bird et al.,1998). Tebbens et al. (1997) have documented an analogous example in the late Miocene when a newly formed rift, propagating northwards from the Valdivia Fracture Zone on the Chile Ridge, ultimately transferred lithosphere from the Nazca to the Antarctic plate. Brozena & White (1990) have reported ridge propagation from the South Atlantic, so this phenomenon appears to be independent of spreading rate.

The cause of the initiation of ridge propagation is unknown but several researchers have noted that propagating rifts tend to form in the vicinity of hot spots and on the hot spot side of the pre-existing ridge crest (e.g. Bird et al., 1998; Brozena & White, 1990). An important corollary of the mere existence of propagating rifts is that the ridge-push force at spreading centers (Section 12.6) is not a primary driving mechanism as it appears to be quite easily overridden during ridge propagation.

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