Geology Reference
In-Depth Information
Apparent
Plate B
Inactive
trace
Plate A
offset
Figure 3.2. Sketch of one kind of transform fault, the kind that joins two spreading
ridge segments.
to define
transform faults
, for which he is perhaps best known. Figure 3.2 illustrates
one kind of transform fault. Wilson recognised that, if the plates on either side of
the ridge segments are separating, then the sense of motion on the fault joining
the ridge segments will be right lateral, whereas the apparent offset of the ridge
crest is left lateral. At the point where the fault meets a ridge segment, the right
lateral motion is
transformed
into horizontal extension along the ridge segment. A
transform fault can also connect with a subduction zone, so a transform fault is a
transcurrent (strike-slip) fault at whose ends the strike-slip motion is transformed
into horizontal extension or shortening.
Wilson's famous paper of 1965, 'A new class of faults and their bearing on
continental drift' [23] is perhaps still not as well appreciated as it should be. In this
paper Wilson not only defined transform faults, but also went on for the first time
to describe a globally connected set of mobile belts that divide the Earth's surface
into a giant jigsaw of moving pieces, which he called
plates
. In other words, he
conceived for the first time of plate tectonics. It is clear from the second part of
his title, and from his comprehensive discussion of continental drift of two years
earlier [28], that he knew he had solved the puzzle of continental drift in the course
of conceiving a grander and more comprehensive theory. It is worth quoting the
opening of the paper [23].
Many geologists [29] have maintained that movements of the Earth's crust are concentrated
in mobile belts, which may take the form of mountains, mid-ocean ridges or major faults
with large horizontal movements. These features and the seismic activity along them often
appear to end abruptly, which is puzzling. The problem has been difficult to investigate
because most terminations lie in ocean basins.
This article suggests that these features are not isolated, that few come to dead ends, but
that they are connected into a continuous network of mobile belts about the Earth which
divide the surface into several large rigid plates.
