Page 99 - Global Tectonics

86 CHAPTER 4

Figure 4.14 Relative positions of Europe and Africa with respect to North America illustrating their separation during
the Mesozoic and Cenozoic. Ages of reconstruction shown in millions of years (redrawn from Pitman & Talwani, 1972,
with permission from the Geological Society of America).

but rather the motion is strike-slip, with adjacent litho- Wilson termed this new class of faults transform faults,
sphere in tangential motion. because the lateral displacement across the fault is taken
The existence of large lateral relative movements of up by transforming it into either the formation of new

the lithosphere was first suggested from marine mag- lithosphere at a terminated ocean ridge segment or

netic anomalies in the northeastern Pacific (Fig. 4.1), lithosphere subduction at a trench. Figure 4.15 shows
which were found to be offset along fracture zones. the plan view of an ocean ridge crest that has been
Combined left lateral offsets along the Mendocino and displaced by transcurrent and transform faulting. The
Pioneer faults amount to 1450 km, while the right transcurrent, or strike-slip, fault (Fig. 4.16b) causes a
lateral offset across the Murray Fault is 600 km in the sinistral offset along a vertical plane which must stretch

west and only 150 km in the east (Vacquier, 1965). to infinity beyond the ridge crests. The transform fault
However, in interpreting these fracture zones as (Fig. 4.15a), however, is only active between the offset
large scale strike-slip faults, a major problem arises in ridge crests, and the relative movement of the litho-
that there is no obvious way in which the faults termi- sphere on either side of it is dextral. Transform faults
nate, as it is certain that they do not circumnavigate the differ from other types of fault in that they imply, indeed
Earth to join up with themselves. Wilson (1965) pro- derive from, the fact that the area of the faulted medium,
posed that the faults terminate at the ends of ridges or in this case lithosphere, is not conserved at ridges and
trenches, which they commonly meet at right angles. trenches.