Geoscience Reference
In-Depth Information
The preceding discussion of some of the seismic experiments carried out over
the East Pacific Rise shows clearly that, just as is indicated for the Mid-Atlantic
Ridge, the East Pacific Rise is not such a simple one-dimensional feature as was
first thought. Although a magma chamber seems to be present in some locations,
it apparently varies in size and extent along the axis and is not a continuous
feature. In contrast, the Moho discontinuity appears to be continuous under all
oceanic crust except for a very narrow zone some few kilometres in width along
the ridge axis.
9.5 Transform faults
9.5.1 Geometry
Transform faults
are plate boundaries at which material is neither created nor
destroyed. The relative motion between the two plates is parallel to the strike of
the fault. By referring to Sections 2.3 and 2.4,you will appreciate that, since
lines of constant velocity are small circles about the pole of rotation between
two plates, transform faults are arcs of small circles. From this fact, the relative
motions of the plates can be determined, as discussed in Chapter 2.Ifthe relative
motion between the two plates is not exactly parallel to the fault, there is a small
component of convergence or divergence. A transform with a small component
of divergence is called a
leaky transform fault
.Anexample of a leaky fault is
the plate boundary between Eurasia and Africa, from the Azores Junction on the
Mid-Atlantic Ridge eastwards towards Gibraltar (Fig. 9.30). Earthquake fault-
plane solutions for this boundary near the Azores illustrate the small amount
of extension occurring there. Transform faults that have a significant amount
of compression in addition to strike-slip are referred to as transpressional. An
example of such a fault is the plate boundary through the South Island of New
Zealand.
Figure 9.30.
Slip vectors
for the motion of Africa
relative to Eurasia along
their plate boundary. The
length of each arrow
indicates the magnitude
of the velocity. The solid
circle denotes the pole
position. Shading
indicates the regions of
most intense seismicity.
(From Anderson and
Jackson (1987).)
