Geoscience Reference
In-Depth Information
a 2-4-km-thick zone with high velocity gradients and velocities appropriate for
layer 2 and the upper part of layer 3. This is underlain by a 2-3-km-thick layer
with velocity of 7.2-7.6 km s
−
1
and then by the Moho. In trying to decide the
origin of this 7.2-7.6 km s
−
1
layer it is important here to keep in mind that the
Moho here is strictly a seismic boundary. This seismic Moho and the petrological
Moho need not be locally coincident beneath fracture zones. Water penetrating
through the fracture zone could have serpentinized the uppermost mantle, result-
ing in a reduction of the seismic velocity of peridotites. This would mean that
the seismic Moho would occur beneath any serpentinized peridotite, whereas
the petrological crust-mantle boundary would be drawn above it. If this is the
case, hydrothermal circulation must have penetrated the cracked and therefore
more permeable lithosphere along the fracture zone down to depths of at least
6 km. Alternatively, the 7.2-7.6 km s
−
1
material may perhaps represent interlay-
ered gabbro and dunite, a much thickened transition zone between crust and
mantle. Whichever explanation is correct, the fracture-zone crust is thinner than
the normal adjacent crust, with most of the thinning taking place in layer 3, and
the sudden change in crustal structure beneath the fracture zone strongly implies
that there is a major reduction in magma supply at the ends of ridge segments.
A further example of the importance of realizing that the seismic Moho cannot
uniquely be assigned to a specific geological contact comes from the Mid-Atlantic
Ridge at 35
◦
N. In a 90-km-long segment south of the Oceanographer transform
fault, the seismic Moho corresponds to a gabbro-plus-dunite/ultramafic contact;
to a gabbro/ultramafic contact; and to a serpentinized alteration front beneath the
thin crust of a non-transform offset within the segment and beneath the inside
corner at the southern end of the segment.
Results of a three-dimensional tomographic study across the Clipperton trans-
form fault (Table 9.4)onthe East Pacific Rise do not show any substantial reduc-
tion in crustal thickness. The P-wave velocities are significantly reduced, by
∼
1kms
−
1
,overa10-km-wide zone centred on the fault and extending through-
out the crust. The low velocities are consistent with an increase in porosity caused
by brittle deformation of the crust. Sea water can therefore penetrate throughout
the crust along the fault. The constant crustal thickness is consistent with a two-
dimensional plate-driven mantle flow regime, rather than a three-dimensional
segment-centred flow, beneath the ridge - some models suggest that the flow
should be two-dimensional for spreading rates in excess of 4 cm yr
−
1
(Fig. 9.38).
As yet no seismic models of the fastest-slipping transform faults have been made.
However, beneath the Tamayo and Orozco transform faults on the northern East
Pacific Rise, which both have slip rates of approximately 6 cm yr
−
1
, the crust is
1-3 km thinner than normal oceanic crust.
Gravity anomalies are usually associated with transform faults; the local topog-
raphy is uncompensated. The interpretation of the excess mass is a matter of
current debate (gravity models are not unique), but it could be partially due
to anomalous material at a shallow depth. Until the details of the tectonics
