Geoscience Reference
In-Depth Information
is generally long-lived and may be inherited from the earliest stages of continental
rifting is supported by three-dimensional modelling of melting and upwelling in
a mantle with non-uniform rheology: model segments persist for tens of millions
of years. Second, smaller offset discontinuities probably form to accommodate
the change in relative plate motion with distance from the rotation pole and the
magmatic supply regime. In this way, ridge segments with constant spreading
rates are created.
The orthogonal transform-fault/ridge-segment geometry, while being
normal
,
is not universal. The orthogonal geometry develops provided that the resistance to
slip at a transform (shearing yield stress) is less than the resistance to spreading
(tensional yield stress) at the ridge axis. At slow-spreading ridges where the
lithosphere is strong and thick close to the ridge axis (Eq. (7.65)), it may be
difficult for transform faults to develop, or for the ridge to adjust to any change
in the pole position. Oblique spreading is more likely to occur on slow-spreading
ridges than on fast-spreading ridges. Both the Reykjanes Ridge and the Mohns
Ridge (Fig. 2.2) are spreading obliquely, in the case of the Mohns Ridge at an
angle of 30-40
◦
to the ridge. The spreading direction along the Mohns Ridge
changed 27 Ma ago with a major reorganization in the North Atlantic. Since then
the Ridge has kept its old trend in the form of an axial valley. Within the valley,
however, there is a series of
en echelon
horsts and grabens (perpendicular to
the current spreading direction) linked by orthogonal transfer zones. The thicker
crust on the Reykjanes Ridge (Section 9.3.2) apparently affects the relative yield
stresses, giving rise to oblique spreading without transform faults. There are no
transform faults between 57
◦
N and Iceland (a distance of over 900 km) while to
the south, where the crust is thinner, the normal orthogonal fault/ridge geometry
prevails.
9.5.2 Topography and crustal structure
Transform faults are major bathymetric features, visible on magnetic-anomaly
maps and marked by earthquake epicentres. Figure 9.32 and Problem 9 suggest
that transform faults might ideally be marked by a single vertical fault; but,
instead, active transforms are anomalous linear valleys, usually less than 15 km
wide, bounded by inward-facing scarps and depressed by from 1 to 5 km. In the
idealized single-fault model, the oceanic crust is exposed along the fault wall and
easily accessible to geologists for sampling by dredging and drilling. However,
in practice, instead of a single fault scarp, there are usually many, each typically
with a throw of only a few hundreds of metres or less.
Some of the jargon used in the literature to describe the various provinces along
transform faults is illustrated in Fig. 9.34,aschematic diagram of the bathymetry
and faulting associated with a slow-slipping transform. The valley along the line
of a transform fault is the transform valley. The transform domain or transform
tectonized zone (TTZ) is the region which has been affected by deformation
