Geology Reference
In-Depth Information
( ) Horst with step faults
a
()Tilted block
b
Figure 5.25 Up-faulted structures. (a) Horst. (b) Tilted block.
listric fault. Axial drainage runs along the fault axis, where
lakes often form. Roll-over drainage develops on the
roll-over section of the rift (Figure 5.24b).
Where compressional stresses dominate a strike-slip
fault, ridges and linear and en échelon scarplets may
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
across the line of the San Andreas Fault, California, USA
(Figure 5.27).
If movement occurs along a strike-slip fault in rugged
country, the ridge crests are displaced in different direc-
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
therefore called shutter ridges (Figure 5.26).
Where tensional stresses dominate strike-slip faults,
subsidence occurs and long, shallow depressions or sags
may form. These are usually a few tens of metres wide and
a few hundred metres long, and they may hold sag ponds .
Lineaments
Any linear feature on the Earth's surface that is too precise
to have arisen by chance is a lineament . Many lineaments
are straight lines but some are curves. Faults are more
or less straight lineaments, while island arcs are curved
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
drift. Unless continents keep the same orientation while
they are drifting, which is not the case, the lineaments
Figure 5.26 Shutter ridges along a strike-slip fault.
Source: Adapted from Ollier (1981, 68)
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