Page 161 - Fundamentals of Geomorphology

144 STRUCTURE

( ) Horst with step faults () Tilted block
a
b

Figure 5.25 Up-faulted structures. (a) Horst. (b) Tilted block.

listric fault. Axial drainage runs along the fault axis, where Where compressional stresses dominate a strike-slip
lakes often form. Roll-over drainage develops on the fault, ridges and linear and en échelon scarplets may
roll-over section of the rift (Figure 5.24b). develop.

Offset drainage
Strike-slip faults
Shutter ridges and sag ponds Offset drainage is the chief result of strike-slip faulting.
The classic example is the many streams that are offset
If movement occurs along a strike-slip fault in rugged across the line of the San Andreas Fault, California, USA
country, the ridge crests are displaced in different direc- (Figure 5.27).
tions on either side of the fault line. When movement
brings ridge crests on one side of the fault opposite valleys
on the other side, the valleys are ‘shut off’. The ridges are Lineaments
therefore called shutter ridges (Figure 5.26). Any linear feature on the Earth’s surface that is too precise
Where tensional stresses dominate strike-slip faults, to have arisen by chance is a lineament. Many lineaments
subsidence occurs and long, shallow depressions or sags are straight lines but some are curves. Faults are more
may form.These are usually a few tens of metres wide and or less straight lineaments, while island arcs are curved
afewhundredmetreslong,andtheymayholdsag ponds. lineaments. Most lineaments are tectonic in origin. Air
photography and remotely sensed images have greatly
facilitated the mapping of lineaments. At times, ‘the
search for lineaments verges on numerology, and their
alleged significance can take on almost magical prop-
erties’ (Ollier 1981, 90). Several geologists believe that
two sets of lineaments are basic to structural and phys-
iographic patterns the world over – a meridional and
orthogonal set, and a diagonal set. In Europe, north–
south lineaments include the Pennines in England,
east–west lineaments include the Hercynian axes, and
diagonal lineaments include the Caledonian axes (e.g.
Affleck 1970). Lineaments undoubtedly exist, but estab-
lishing worldwide sets is difficult owing to continental
Figure 5.26 Shutter ridges along a strike-slip fault. drift. Unless continents keep the same orientation while
Source: Adapted from Ollier (1981, 68) they are drifting, which is not the case, the lineaments