Ocean ridges mark accretive, or constructive plate margins where new oceanic lithosphere is created. They represent the longest, linear uplifted features of the Earth's surface, and can be traced by a belt of shallow focus earthquakes that follows the crestal regions and transform faults between offset ridge crests (Fig. 5.2). The total length of the spreading margins on mid-ocean ridges is approximately 55,000 km. The total length of the active ridge-ridge transform faults is in excess of 30,000 km. The topographic expression of mid-ocean ridges is typically between 1000 and 4000 km in width. Their crests are commonly 2-3 km higher than neighboring ocean basins, and locally the topography can be quite rugged and runs parallel to the crests.
The gross morphology of ridges appears to be controlled by separation rate (Macdonald, 1982). Spreading rates at different points around the mid-ocean ridge system vary widely. In the Eurasian basin of the Arctic Ocean, and along the Southwest Indian Ocean Ridge, the full spreading rate (the accretion rate) is less than 20 mm a-1. On the East Pacific Rise, between the Nazca and Pacific plates, the accretion rate ranges up to 150 mm a-1. It is not surprising therefore that many of the essential characteristics of the ridges, such as topography, structure, and rock types, vary as a function of spreading rate. Very early on it was recognized that the gross topography of the East Pacific Rise, which is relatively smooth, even in the crestal region, contrasts with the rugged topography of the Mid-Atlantic Ridge, which typically has a median rift valley at its crest. This can now be seen to correlate with the systematically different spreading rates on the two ridges (Fig. 5.5), that is, fast and slow respectively. These two types of ridge crest are illustrated in Fig. 6.1, which is based on detailed bathymetric data obtained using deeply towed instrument packages. In each case, the axis of spreading is marked by a narrow zone of volcanic activity that is flanked by zones of fissuring. Away from this volcanic zone, the topography is controlled by vertical tectonics on normal faults. Beyond distances of 10-25 km from the axis, the lithosphere becomes stable and rigid. These stable regions bound the area where oceanic lithosphere is generated - an area known as the "crestal accretion zone" or "plate boundary zone".
The fault scarps on fast-spreading ridges are tens of meters in height, and an axial topographic high, up to 400 m in height and 1-2 km in width, commonly is present. Within this high a small linear depression, or graben, less than 100 m wide and up to 10 m deep is sometimes developed (Carbotte & Macdonald, 1994). The axial high may be continuous along the ridge crest for tens or even hundreds of kilometers. On slow-spreading ridges the median rift valley is typically 3050 km wide and 500-2500 m deep, with an inner valley floor, up to 12 km in width, bounded by normal fault scarps approximately 100 m in height. Again there is often an axial topographic high, 1-5 km in width, with hundred of meters of relief, but extending for only tens
Fig. 6.1 Bathymetric profiles of ocean ridges at fast and slow spreading rates. EPR, East Pacific Rise; MAR, Mid-Atlantic Ridge. Neovolcanic zone bracketed by Vs, zone of fissuring by Fs, extent of active faulting by Ps (redrawn with permission from MacDonald, 1982, Annual Review of Earth and Planetary Sciences 10. Copyright © 1982 by Annual Reviews).
of kilometers along the axis. At fast rates of spreading the high may arise from the buoyancy of hot rock at shallow depth, but on slowly spreading ridges it is clearly formed by the coalescence of small volcanoes 1-2 km in width, and hence is known as an axial volcanic ridge (Smith & Cann, 1993).
A detailed study of a median rift valley was made in the Atlantic Ocean between latitudes 36°30' and 37°N, a region known as the FAMOUS (Franco-American Mid-Ocean Undersea Study) area, using both surface craft and submersibles (Ballard & van Andel, 1977). The median rift in this area is some 30 km wide, bounded by flanks about 1300 m deep, and reaches depths between 2500 and 2800 m. In some areas the inner rift valley is 1-4 km wide and flanked by a series of fault-controlled terraces (Fig. 6.2). Elsewhere, however, the inner floor is wider with very narrow or no terraces developed. The normal faults that control the terracing and walls of the inner rift are probably the locations where crustal blocks are progressively raised, eventually to become the walls of the rift and thence ocean floor, as they are carried laterally away from the rift by sea floor spreading. Karson et al. (1987) described investigations of the Mid-Atlantic Ridge at 24°N using a submersible, deep-towed camera and side-scan sonar. Along a portion of the ridge some 80 km long they found considerable changes in the morphology, tectonic activity, and volca-nism of the median valley. By incorporating data supplied by investigations of the Mid-Atlantic Ridge elsewhere, they concluded that the development of the style of the median valley may be a cyclic process between phases of tectonic extension and volcanic construction.
Bicknell et al. (1988) reported on a detailed survey of the East Pacific Rise at 19°30'S. They found that faulting is more prevalent than on slow-spreading ridges, and conclude that faulting accounts for the vast majority of the relief. They observed both inward and outward facing fault scarps that give rise to a horst and graben topography. This differs from slower spreading ridges, where the topography is formed by back-tilted, inward-facing normal faults. Active faulting is confined to the region within 8 km of the ridge axis, and is asymmetric with the greater intensity on the eastern flank. The half extension rate due to the faulting is 4.1 mm a-1, compared to 1.6 mm a-1 observed on the Mid-Atlantic Ridge in the FAMOUS area.
Historically, for logistical reasons, the slowest spreading ridges, the Southwest Indian Ocean Ridge and the Gakkel Ridge of the Arctic Ocean, were the last to be studied in detail. In the Arctic the year-round ice cover necessitated the use of two research icebreakers (Michael et al., 2003). The results of these studies led Dick et al. (2003) to suggest that there are three types of ridge as a function of spreading rate: fast, slow, and ultraslow (Fig. 6.3). Although the topography of the ultraslow Gakkel Ridge is analogous to that of slow-spreading ridges, typically with a well-developed median rift, the distinctive crustal thickness (Fig. 6.3), the lack of transform faults, and the petrology of this ridge set it apart as a separate class. Note that there are two additional categories of ridge with spreading rates between those of fast and slow, and slow and ultraslow, termed intermediate and very slow respectively. Intermediate spreading rate ridges may exhibit the characteristics of slow or fast-spreading ridges, and tend to alternate between the two with time. Similarly, a very slow-spreading ridge may exhibit the characteristics of a slow or ultraslow ridge. It is interesting to note that at the present day the East Pacific Rise is the only example of a fast-spreading ridge and the Gakkel Ridge of the Arctic is the only ultraslow-spreading ridge. Differences between the crustal structure and petrology of fast, slow and ultraslow ridges are discussed in Sections 6.6-6.9.
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