Page 101 - Global Tectonics

88 CHAPTER 4

Wilson (1965) defined six classes of transform fault zones that offset the Mid-Atlantic Ridge to the left at
that depend upon the types of nonconservative features equatorial latitudes (Fig. 4.17). Events along the ridge
they join (Fig. 4.16). These may be an ocean ridge, the axis are consistent with normal faulting along north–
overriding plate at a trench or the underthrusting plate south planes. Events along the fracture zones are much
at a trench. Figure 4.16a shows the six possible kinds of more common and the energy release is about a
dextral transform fault; a further six based on sinistral hundred times greater than along the ridge crest.
movement are also possible. Figure 4.16b shows how Between the offset ridge segments events are of strike-
the transform faults would develop with time. Cases (i) slip type with one nodal plane consistent with dextral
and (v) will remain unchanged, cases (ii) and (iv) will transform motion. Events along the fracture zone
grow, and cases (iii) and (vi) will diminish in length with beyond the ridge extremities are rare. These results

the passage of time. provided striking confirmation of the transform fault

concept and further, independent, confirmation of the
hypothesis of sea fl oor spreading.
4.2.2 Ridge–ridge Before the recognition of transform faulting,
the parallel fracture zones which appear to displace the
transform faults crest of the Mid-Atlantic Ridge in equatorial latitudes
between Africa and South America were believed to
Sykes (1967) determined focal mechanism solutions for represent sinistral transcurrent faults that displaced an
earthquakes occurring in the vicinity of the fracture originally straight crest (Fig. 4.15b). However their

20°N
Mid-Atlantic Ridge
Earthquake epicenters
Earthquake mechanisms
Africa
15°
Ridge crest
Fracture zones

1000 fm
10° 4

3 2
5°

0° St. Paul’s Rocks
1000 fm
South America
5°S
50° 40° 30° 20° 10°
Figure 4.17 Epicenters of earthquakes that occurred on the Mid-Atlantic ridge in the equatorial Atlantic between
1955 and 1965. The arrows beside four of the earthquakes indicate the sense of shear and the strike of the fault plane

inferred from focal mechanism solutions (modified from Sykes, 1967, by permission of the American Geophysical Union.
Copyright © 1967 American Geophysical Union).