Australia: The Land Where Time Began

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Oceanic Fracture Zones

Fracture zones define oceanic transform faults in the absence of sedimentary cover. Oceanic fracture zones are bathymetric depressions, that are long and linear, normally following arcs on the surface of the Earth perpendicular to the offset ridge (Bonatti & Crane, 1984). Kearey et al. suggest the apparent simplicity of fracture zones of the seafloor is probably the result of the usual method of study, from the ocean surface several kilometres above the seafloor. It has been shown by direct observation of a fracture zone on the Mid-Atlantic Ridge (Choukroune et al., 1978) that it was actually comprised a 300-1000 m wide zone in which there was a complex swarm of faults. It has been suggested that on fast-spreading ridges, such as the East Pacific Rise, these zones of multiple faults are more common than elsewhere.

The active transform segment, as well as its fossilised trace, are both marked by fracture zones. Collette (1979) has suggested that the fractures are a result of the thermal contraction in the direction of the ridge axis, that causes internal stresses that are much larger than the breaking strength of the rocks, indicating the possibility that fracture zones develop along the lines of weakness that result.

Rocks have been recovered by dredging that are composed of normal oceanic crust and rocks that have undergone a much higher degree of shearing and metamorphism. Large blocks of serpentine are very commonly present at the bases of fracture zones. Recovered specimens from the thick crustal sections in the large equatorial fracture zones in the Atlantic, have been examined by Bonatti & Honnorez (1976) and Fox et al. (1976). They were found to consist of ultramafic, gabbroic and basalt rock types, as well as their equivalents that had been metamorphosed and tectonised. Within fracture zones it is common to find serpentine, as well as alkali basalt volcanism, hydrothermal activity, and metallogenesis. In the Verna Fracture Zone, investigations (Auzende et al., 1989) indicated a sequence that was similar to normal oceanic layering. In the equatorial Atlantic, St Peter Rock and St Paul Rock, on a ridge associated with the St Paul Fracture Zone, are composed of mantle peridotite.

Fracture zone crust in the North Atlantic has a thickness and structure that are very heterogeneous (Detrick et al., 1993b). It is often thin (<3 km), and of low seismic velocity, and lacking layer 3. There may be crustal thinning up to several tens of kilometres from the fracture zone. It has been suggested that this structure possibly represents a basaltic layer that is thin and has been intensely fractured and altered hydrothermally, and is underlain by serpentinised ultramafic rocks. It has been suggested the apparent variations in thickness may reflect different extents of serpentinisation. It is thought a reduced supply of magma at the ridge offset may have resulted in the thin mafic crust.

According to Kearey et al., oceanic crust of different ages must be brought into juxtaposition by fracture zones. A scarp would be expected to develop across the fracture zone from the younger, higher crust to the lower, older crust, because the seafloor depth depends upon its age (Menard & Atwater, 1969; DeLong et al., 1977). The rate of oceanic lithosphere subduction is inversely dependent on the square root of its age (DeLong et al., 1977), the higher, younger crust subsiding more rapidly than the older, lower side. A small component of dip-slip motion away from the active transform fault, along the fracture zone, results in the combination of contraction in the vertical plane, and horizontally, perpendicular to the ridge axis direction. It has been suggested (DeLong et al., 1977) that fracture zone seismicity and deformation of rocks within the floor and walls could result from this small amount of dip-slip motion.

Major fracture zones, that can provide more tha 6 km of vertical relief, are often associated with transverse ridges, running parallel to the fractures (Bonatti , 1978) on one margin or both. As their elevation may be greater than that of the crust at the spreading ridge, they are often anomalous. As a result, depths differ from that of "normal" crust of the same age, as the age-depth relationship of normal oceanic lithosphere doesn't apply. The ridges are believed to be the result of tectonic uplift of crustal blocks and upper mantle, and not from volcanic activity occurring in the fracture zone nor by hot spot activity. Because of this normal processes of lithosphere accretion cannot be the explanation of transverse ridges. It has been suggested that the most likely mechanism producing this uplift is horizontal stresses, compressional and tensional, across the fracture zone, that result from small changes in the spreading direction, the transform movement no longer being exactly orthogonal to the ridge (Bonatti, 1978). Episodic compression and extension, affecting different parts of the fracture, can result from small changes. This has caused the emergence of parts of the transverse ridges as islands, e.g. the St Peter Rock and the St Paul Rock, and their subsequent subsidence (Bonatti & Crane, 1984).

It has been noted (Lowrie et al., 1986) that the scarp height may be preserved for more than 100 My in some fracture zones. They believe that sime parts of fracture zones, that are characterised by volcanism, are weak, maintaining the theoretical depths predicted for lithosphere that is cooling. other parts are apparently welded together, locking in their initial differential bathymetry. Lithosphere flexure on both side of the fracture zone would then result from differential cooling stresses  according to the authors (Source 1), the possible existence of a systematic patterns of distribution of strong and weak portions of the fracture zone, will be revealed by future research.

In some oceanic transform faults that have an extension component across the fault, the direction of the fault not corresponding exactly to the spreading direction on either side. In these faults, the trajectory of the fault may be adjusted by developing into a series of fault segments that are connected by small lengths of spreading centre, bring the direction of the fault to be approximately parallel to the spreading direction. A leaky transform transform fault in one in which new crust is originalted (Thompson & Melson, 1972; Taylor et al., 1994). When a small shift occurs in the position of the pole of rotation around which a small circle is described by the fault is an alternative mechanism by which a leaky transform fault can develop. Such a fault would become leaky to adjust to the new small circle.

See Source 1 for more detailed information on all aspects of plate tectonics

Sources & Further reading

  1. Kearey, Philip, Klepeis, Keith A. & Vine, Frederick J., 2009, Global Tectonics, 3rd Edition, Wiley-Blackwell.
Author: M. H. Monroe
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Last Updated 15/04/2012

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading