Shallow Structure Of The Axial Region

As noted above (Section 6.1), normal oceanic crust, that is, not formed in the vicinity of hot spots or transform faults, has a remarkably uniform seismic thickness of 7 ± 1 km if generated at a full spreading rate in excess of 20 mm a-1. For a homogeneous mantle this implies a comparable thermal gradient beneath all such ridge crests, and a similar degree of partial melting of the mantle, which produces the uniform thickness of mafic crust. The essential uniformity of the thermal regime beneath ridges is also implied by the lithospheric age versus depth relationship (Section 6.4). However, the rate at which magma is supplied to the crust will depend on the spreading rate. On fast-spreading ridges the magma supply rate is such that the whole crestal region at relatively shallow depth is kept hot and a steady state magma chamber exists. Indeed the crust above the magma chamber would be even hotter and weaker but for the cooling effect of hydrothermal circulation (Section 6.5). On slow-spreading ridges the lower rate of magma supply enables the crust to cool by conduction, as well as hydrothermal circulation, between injections of magma from the mantle. As a result the crust is cooler, and a steady state magma chamber cannot be maintained. At spreading rates of less than 20 mm a-1 this conductive cooling between injections of magma extends into the mantle and inhibits melt generation. This reduces the magma supply, as well as the magma supply rate, and hence the thickness of mafic crust produced, as observed on the Southwest Indian Ocean and Gakkel ridges (Section 6.1). It also makes the existence of even transient magma chambers beneath such ridges rather unlikely except beneath the volcanic centers (Section 6.6).

The relatively smooth axial topography of fast-spreading ridges is characterized by an axial high, up to 400 m in height and 1-2 km wide, and fault scarps with a relief of tens of meters, the fault planes dipping either towards or away from the ridge axis. Active volcanism is largely confined to the axial high, and the smooth topography is thought to result from the high eruption rate and the low viscosity of the magma. The axial high appears to correspond to and to be supported by the buoyancy of the axial magma chamber beneath. Studies of major fault scarps and drill core from DSDP/ODP drill hole 504B, all in Pacific crust, reveal that at depth the lava flows dip towards the ridge axis at which they were erupted and that the dikes beneath them dip away from the ridge axis (Karson, 2002). This geometry indicates a very narrow and persistent zone of dike intrusion, and isostatic subsidence as the thickness of the lava flow unit increases away from the point of extrusion (Section 6.10). This relatively simple structure of the upper crust at the crests of fast-spreading ridges is illustrated in Fig. 6.16.

The shallow structure at the crests of slow-spreading ridges is fundamentally different to that on fast-spreading ridges (Smith & Cann, 1993). As a result of less frequent eruptions of magma and a cooler, more brittle upper crust, extension by normal faulting is more pronounced. The fault scarps have approximately 100 m of relief and the fault planes dip towards the ridge axis. Volcanism is essentially confined to the inner valley floor, and at any one time appears to be focused along specific axis-parallel fissures, forming axial volcanic ridges 1-5 km wide and tens of kilometers in length. As these ridges move off axis, as a result of further accretion, they may be cut by the faults that ultimately form the bounding scarps of the median valley. The spacing of these bounding faults appears to be about one-third to one-half of the width of the inner valley, that is, several kilometers. Within the inner valley floor the topography is fissured and cut by small throw normal faults, the density of these features giving an indication of its age. There is clear evidence of alternate phases of volcanic and tectonic (magmatic and amagmatic) extension of the crust, as one would expect if there are transient magma chambers beneath, which supply discrete packets of magma to the inner valley floor.

Very slow-spreading ridges are characterized by thin mafic crust and large regions of peridotite exposures where the mantle appears to have been emplaced directly to the sea floor. However there are also mag-matic segments analogous to the second order

Melt lens 10's of m thick

Axial Summit Graben-10's of m deep

Basalts-200-800 m thick

Melt lens 10's of m thick

Axial Summit Graben-10's of m deep

Basalts-200-800 m thick

h

Fig. 6.16 Schematic diagram of the upper crustal structure for a fast-spreading ridge (redrawn with permission from Karson, et al., 2002 by permission of the American Geophysical Union. Copyright © 2002, American Geophysical Union).

segments on slow-spreading ridges. These have abundant volcanoes, typically in the form of axial volcanic ridges. These are 15-25 km long, and rise 400-1500 m from the axial valley floor. In the amagmatic sections the rift valley is often deeper than on slow-spreading ridges, up to 5000 m deep in places, and the rift valley walls have up to 2000 m of relief. On the Gakkel Ridge the western section is magmatic, the central section essentially amagmatic, less than 20% of the rift valley having a basaltic cover, and the ultraslow-spreading eastern section is very different again. It has six large volcanic centers on it that extend for 15-50 km along axis and are 50-160 km apart. These volcanic edifices are larger and more circular than those on other ridges. The amplitudes of the magnetic anomalies recorded between the volcanic centers suggest that the basaltic cover is thin in these tectonized zones. These marked along-axis contrasts in the extent of magma supply, which do not correlate with spreading rate, pose interesting questions regarding the generation and/or migration of melts beneath the ridge (Section 6.8) (Michael et al., 2003).

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