Page 160 - Fundamentals of Geomorphology

SMALL-SCALE TECTONIC AND STRUCTURAL LANDFORMS 143

If the fault moves repeatedly, the streams are rejuvenated A half-graben is bounded by a major fault only on one
to form wineglass or funnel valleys. Some fault scarps side (Figure 5.24b).This is called a listric (spoon-shaped)
occur singly, butmanyoccur in clusters. Individualmem- fault. The secondary or antithetic fault on the other side
bers of fault-scarp clusters may run side by side for long is normally a product of local strain on the hanging
distances, or they may run en échelon (offset but in wall block. Examples are Death Valley in the Basin and
parallel), or they may run in an intricate manner with Range Province of the USA, and the Menderes Valley,
no obvious pattern. Turkey.
A horst is a long and fairly narrow upland raised by
Rift valleys, horsts, and tilt blocks upthrust between two faults (Figure 5.25a). Examples of
horsts are the Vosges Mountains, which lie west of the
Crustal blocks are sometimes raised or lowered between Rhine graben in Germany, and the Black Forest Plateau,
roughly parallel faults without being subjected to tilting. which lies to the east of it.
The resulting features are called rift valleys and horsts. Tilted or monoclinal blocks are formed where a
A rift valley or graben (after the German word for a section of crust between two faults is tilted (Figure 5.25).
ditch) is a long and narrow valley formed by subsidence The tilting may produce mountains and intervening
between two parallel faults (Figure 5.24a). Rift valleys are basins. In the Basin and Range Province of the western
not true valleys (p. 232) and they are not all associated USA, these are called tilt-block mountains and tilt-block
with linear depressions. Many rift valleys lie in zones of basins where they are the direct result of faulting.
tension in the Earth’s crust, as in the Great Rift Valley
of East Africa, the Red Sea, and the Levant, which is the Dip-faults and drainage disruption
largest graben in the world. Grabens may be very deep,
some in northern Arabia holding at least 10 km of alluvial Fault scarps may disrupt drainage patterns in several
fill. Rift valleys are commonly associated with volcanic ways. A fault-line lake forms where a fault scarp of
activity and earthquakes. They form where the Earth’s sufficient size is thrown up on the downstream side
crust is being extended or stretched horizontally, caus- of a stream. The stream is then said to be beheaded.
ing steep faults to develop. Some rift valleys, such as the Waterfalls form where the fault scarp is thrown up on
Rhine graben in Germany, are isolated, while others lie in the upstream side of a stream. Characteristic drainage
graben fields and form many, nearly parallel structures, patterns are associated with half-grabens. Back-tilted
as in the Aegean extensional province of Greece. drainage occurs behind the footwall scarp related to the

Figure 5.24 Down-faulted structures. (a) Graben. (b) Half-graben.
Source: After Summerfield (1991, 92)