Australia: The Land Where Time Began
The Propagating Rifts and Microplates
Over long time periods the spreading direction is not always constant, possibly undergoing several changes. On the basis of orientation changes of major transform faults and patterns of magnetic anomalies, it has been proposed that in the northeast Pacific spreading has changed direction 5 times (Menard & Atwater, 1968). Anomalous topography associated with oceanic fracture zones have been proposed to result from small changes of direction of spreading.
Reorientation of a ridge was assumed to occur by continuous smooth motion of individual segments of the ridge until they became orthogonal to the new direction of spreading, the ridge then being at an angle to the original pattern of magnetic anomaly. The authors suggest that long sections of ridges that are affected in this way might be expected to devolve into shorter sections, that facilitates rotation of the ridge and formation of new transform faults. A fan-like pattern of magnetic anomalies, of varying width depending on their position, is considered to result from a change in spreading direction.
The formation of a new spreading centre has been suggested as an alternative model of spreading direction changes, the propagating Rift Model (Hey, 1977; Hey et al., 1980) that includes the subsequent growth at the expense of the old ridge. In this model there is progressive replacement of the old ridge by a new propagating spreading centre that is orthogonal to the new direction of spread. The complex processes occurring at the tip of a propagating rift have been described (Kleinrock & Hey, 1989). Pseudofaults are the boundaries between lithosphere formed at an old ridge and a new ridge, they define a characteristic wake that is V-shaped, and points in the direction of propagation. Lithosphere is progressively transferred from one plate to the other, this gives rise to a sheared zone that has a fabric that is quite distinctive. Lithosphere formed by symmetric accretion at the old ridge is disrupted by the propagation of the new ridge, and abrupt changes in the sea floor topographic and magnetic fabric occur at the pseudofaults and failed rift. A possible way in which the propagating model could result in fracture zones that are evenly spaced is given in Fig. 6.20b, Source 1. Pseudofaults and/or failed rifts bound the new fracture zones, as propagation is complete before these fracture zones form. This contrasts with the situation in the ridge rotation model, in which failed rifts are not produced, and fracture zones are areas of sea floor spreading that are highly symmetric. Abrupt boundaries between areas with uniform magnetic anomaly and bathymetric trends of different orientation are predicted by the propagation rift model. Magnetic anomalies with direction changes from the old spreading direction to the new spreading direction are predicted by the rotation model to have a continuous fan-like configuration. The result is that it should be possible to determine which of the 2 models is valid by detailed bathymetric and magnetic surveys.
Results have been reported (Hey et al., 1988) of a detailed investigation, involving side-scan sonar, magnetometry and seismic reflection, in a region of spreading direction change of the Pacific-Farallon boundary that occurred about 54 Ma, just to the north of the major bend of the Surveyor Fracture Zone. It was shown that there was an abrupt change in the seafloor fabric, in agreement with predictions of the propagating rift model. Other survey results led to a similar conclusion (Caress et al., 1988). A survey was carried out in an area to the west of the Galapagos Islands, at 96o W (Hey et al., 1980). Their conclusion was that a new ridge was progressively breaking through the Cocos Plate, the evidence is convincing, especially from the magnetic data, indicating that the ridge propagating mechanism is operating in this area. Detailed bathymetry mapping of the area confirmed this interpretation (Hey et al., 1986). The pseudofaults were clearly shown to have a V-shaped pattern, rifts that were active and those that had failed, and in the sheared zone, the oblique tectonic fabric of the transferred lithosphere. The mechanism of the orientation change in the Juan de Fuca Ridge, that has been achieved in the last 10 My, is also explained by the propagating rift model (Wilson et al., 1984).
It has been pointed out (Engeln et al., 1988) that an assumption of the propagating rift model is that the full accretion rate between the 2 plates is attained immediately by the new rift, which would make the pre-existing rift redundant. If the new rift initially spreads at a lower rate, building up to the full rate over millions of years, the failing rift would continue spreading, though at a lower, decreasing rate, so that the overall accretion rate was maintained. According to this new propagating rift model the 2 rifts overlap, the oceanic lithosphere between them increasing over time. There is also rotation of the intervening block of lithosphere as a result of the gradients of spreading rate along each rift. Compression of the oceanic lithosphere adjacent to the tip of the propagating rift, and transtension in the region between the points where the propagating rift was initiated and the start of failure of the original rift, are results of the rotation. An additional propagating rift is initiated by the transpression after a few million years.
An explanation for the existence of microplates in the southeast Pacific was the reason for the development of the second propagating rift model. It has been shown by detailed studies of the Easter Microplate and the Juan Fernandez Microplate that there is a high degree of similarity between the bathymetric fabric and structural evolution of the 2 microplates. These findings conform to the predictions of the second propagating rift model (Engeln et al., 1988) (Searle et al., 1989; Rusby & Searle, 1995; Larson et al., 1992; Bird et al., 1998). The characteristic pseudofaults of the original propagating rift to the east are clearly shown by the tectonic elements of the Juan Fernandez Microplate, as well as the subsequent propagating rift to the southwest of the microplate. It is believed that microplates have a lifespan of about 5-10 My, the lithosphere of the microplate having been transferred by the initial rift from one plate to another. It has been suggested that in the case of the Juan Fernandez Microplate, it was probably from the Nazca Plate to the Antarctic Plate (Bird et al., 1998). An analogous example from the Late Miocene has been documented (Tebbens et al., 1997), a rift that was newly formed, that was propagating northwards from the Valdivia Fracture Zone of the Chile Ridge in which lithosphere from the Nazca Plate was ultimately transferred to the Antarctic Plate. Ridge propagation in the South Atlantic has been reported (Brozena & White, 1990), making it apparent that this phenomena is independent of rate of spreading.
It has been noted by a number of researchers that there is a tendency for propagating rifts to form in the vicinity of hot spots, and on the same side of the pre-existing ridge crest as a hot spot (e.g. Bird et al., 1998; Brozena & White, 1990), though the actual cause of the initiation of ridge propagation is unknown. The ridge-push force at spreading centres is not a primary driving mechanism, as during ridge propagation it appears quite easy to be overridden, is an important corollary of the existence of propagating rifts.
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