Geoscience Reference
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of new oceanic crust along the axis of the ridge and slip on the normal faults.
The heterogeneous structure of slow-spreading ridges is a result of the spatial
and temporal variability of tectonic and volcanic processes (Fig. 9.17). Within
individual ridge segments (Section 9.5.3) the fault distribution is such that there
are many small faults towards the segment centre whereas the segment ends are
characterized by just a few large faults. Fault size and spacing both increase from
the segment centre to its end, although the tectonic strain is constant. Under-
standing of the distribution and size of faults along mid-ocean-ridge segments is
greatly improving with the increased availability of high-resolution technology
in the form of multibeam and side-scan sonar, coupled with ground-truthing from
submersibles.
Multibeam and side-scan sonar imaging have led to spectacular discoveries
of what seem to be major detachment surfaces on the Mid-Atlantic Ridge (near
the Kane and Atlantis transform faults) and on the Central Indian Ridge (50 km
north of the Rodriguez triple junction) (Fig. 9.19 and Plate 20). The surfaces
are at inner corners (RT) of ridge-transform intersections (see Section 9.5.2).
The surfaces are corrugated and striated platforms that dip gently towards the
ridge axis for tens of kilometres parallel to the ridge axis. The geometry of these
surfaces is such that, with over 10-15 km of displacement, the lower crust and
upper mantle should be exposed. Initial sampling by dredging has yielded gabbros
and serpentinite from the corrugated surface, while it is overlain by upper-crustal
basaltic rocks on its median valley side. The corrugations which are parallel to
the spreading direction have a wavelength of
1.0 km and an amplitide of tens of
metres. The fine-scale (wavelength 50-100 m) striations which also parallel the
spreading direction seem to be alternate exposures of hard rock and sediment-
filled depressions. These corrugations and striations seem to be directly related to
the formation of the detachment fault. Although erosion has removed the surface
corrugations, Atlantis Bank, a platform adjacent to the Atlantis II fracture zone on
the Southwest Indian Ridge, seems to be another example. The 1500-m deep ODP
hold 735B has drilled through 1300 m of lower-crustal gabbro and lherzolite. The
mechanisms for formation of these surfaces are similar to those suggested for the
formation of continental-core complexes: slip on low-angle detachment faults and
subsequent tilting of upper-crustal fault blocks. Submersible diving and detailed
shallow drilling, linked with magnetic and geochemical studies, should provide
information on the detailed operation of processes at slow-spreading ridges and
may also have implications for our understanding of continental geology.
∼
9.4.2 Crustal magma chambers
As has been shown, the region of partial melting in the upper mantle is probably
relatively broad. This is in sharp contrast to the very narrow zone of intrusion at
the ridge axis: magnetic data (see Section 3.2) suggest that the standard deviation
of the distribution of dyke injections must be only a few kilometres or less, though
