Page 155 - Global Tectonics

OCEAN RIDGES 141

extent of melting beneath such ridges (Section 6.9), The relatively smooth axial topography of fast-
small variations in mantle temperature and/or compo- spreading ridges is characterized by an axial high, up to
sition would lead to greater proportional changes 400 m in height and 1–2 km wide, and fault scarps with
in the volume of magma produced (Michael et al., a relief of tens of meters, the fault planes dipping either
2003). 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
6.9 SHALLOW appears to correspond to and to be supported by the
buoyancy of the axial magma chamber beneath. Studies
STRUCTURE OF THE of major fault scarps and drill core from DSDP/ODP
drill hole 504B, all in Pacific crust, reveal that at depth

AXIAL REGION the lava fl ows 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 indi-
cates a very narrow and persistent zone of dike intru-
As noted above (Section 6.1), normal oceanic crust, sion, and isostatic subsidence as the thickness of the lava

that is, not formed in the vicinity of hot spots or flow unit increases away from the point of extrusion
transform faults, has a remarkably uniform seismic (Section 6.10). This relatively simple structure of
thickness of 7 ± 1 km if generated at a full spreading the upper crust at the crests of fast-spreading ridges is
−1
rate in excess of 20 mm a . For a homogeneous mantle illustrated in Fig. 6.16.
this implies a comparable thermal gradient beneath all The shallow structure at the crests of slow-spreading
such ridge crests, and a similar degree of partial ridges is fundamentally different to that on fast-spread-
melting of the mantle, which produces the uniform ing ridges (Smith & Cann, 1993). As a result of less

thickness of mafic crust. The essential uniformity of frequent eruptions of magma and a cooler, more brittle
the thermal regime beneath ridges is also implied by upper crust, extension by normal faulting is more pro-
the lithospheric age versus depth relationship (Section nounced. The fault scarps have approximately 100 m of
6.4). However, the rate at which magma is supplied to relief and the fault planes dip towards the ridge axis.

the crust will depend on the spreading rate. On fast- Volcanism is essentially confined to the inner valley
spreading ridges the magma supply rate is such that fl oor, and at any one time appears to be focused along
the whole crestal region at relatively shallow depth is specific axis-parallel fissures, forming axial volcanic

kept hot and a steady state magma chamber exists. ridges 1–5 km wide and tens of kilometers in length. As
Indeed the crust above the magma chamber would these ridges move off axis, as a result of further accre-
be even hotter and weaker but for the cooling effect tion, they may be cut by the faults that ultimately form
of hydrothermal circulation (Section 6.5). On slow- the bounding scarps of the median valley. The spacing
spreading ridges the lower rate of magma supply of these bounding faults appears to be about one-third
enables the crust to cool by conduction, as well as to one-half of the width of the inner valley, that is,
hydrothermal circulation, between injections of several kilometers. Within the inner valley fl oor the

magma from the mantle. As a result the crust is topography is fissured and cut by small throw normal
cooler, and a steady state magma chamber cannot be faults, the density of these features giving an indication
−1
maintained. At spreading rates of less than 20 mm a of its age. There is clear evidence of alternate phases of
this conductive cooling between injections of magma volcanic and tectonic (magmatic and amagmatic) exten-
extends into the mantle and inhibits melt generation. sion of the crust, as one would expect if there are tran-
This reduces the magma supply, as well as the magma sient magma chambers beneath, which supply discrete
supply rate, and hence the thickness of mafi c crust packets of magma to the inner valley fl oor.
produced, as observed on the Southwest Indian Ocean Very slow-spreading ridges are characterized by thin
and Gakkel ridges (Section 6.1). It also makes the mafic crust and large regions of peridotite exposures

existence of even transient magma chambers beneath where the mantle appears to have been emplaced
such ridges rather unlikely except beneath the volcanic directly to the sea floor. However there are also mag-

centers (Section 6.6). matic segments analogous to the second order