Geoscience Reference
In-Depth Information
These old surfaces have erosion rates estimated to be of
the order of 0.25-0.3 m/Myr (0.00025-0.0003 mm/yr).
occupying the lowest point on the floodplain. The his-
tory of the Indus River system provides some evidence
of depositional sequences responding to crustal deforma-
tion. McDougall (1989) hypothesises that a major east-
erly switch of the Indus course occurred at some point
0.5-0.1 Ma owing to oblique compression in the active
right lateral Kalabagh fault zone. Jorgensen
et al.
(1993,
p. 310) discuss the interaction of climatic and tectonic
influences on the course of the Lower Indus River. They
note that:
2.5.4
Aeolian sequences
The antiquity of aeolian sequences is sometimes inferred
on the basis of the volume of sand incorporated in sand
seas or because of the presence of distinct bounding sur-
faces between different generations of dunes (Lancaster,
1992). Although it is recognised that some sand seas have
developed and been reworked, over timescales of the or-
der of 10
6
yr (Cooke, Warren and Goudie, 1993, p. 409),
direct chronological information is often lacking. A major
exception is in North America for sand ramps in Nevada,
where the Bishop ash (K/Ar age 0.74 Ma) is preserved at
the base of several of the ramps. The influence of tectonic
activity on aeolian activity is likely to be indirect and
a function of the timescale considered. Aeolian activity
and sedimentation tends to be episodic in nature. In the
short term, sediment availability may, for example, be a
function of fluvial processes responsible for transporting
material to a site for deflation. In the longer term, tectonics
may be a major control on erosion and therefore sediment
supply. If only short term records are available then it is
likely that they will only be amenable to interpretation
in terms of climate change rather than tectonism. Lumi-
nescence dating of aeolian sands has proved particularly
useful in identifying periods of aeolian activity during the
Late Quaternary and is discussed at length in Chapters 3
and 17.
...
all of the significant river pattern changes take
place at tectonic boundaries
anomalously steep
and gently sloping reaches of the river conform to
suspected reaches of uplift and subsidence, and avul-
sions may mark the points of relative upwarp in con-
trast to basin subsidence.
...
2.6.2
Tectonic controls on alluvial
sedimentation
Although the potential role of tectonics in alluvial fan
development clearly involves the topographic setting of
the fan, which in many cases may be at a fault-bounded
mountain front, the role of active tectonics in fan de-
velopment is still debated and climatic arguments tend
to be dominant (Blair and McPherson, 1994). In their
examination of alluvial fan sequences in the Dead Sea
rift in Israel, Frostick and Reid (1989) demonstrate that
the discrimination between climatically induced and tec-
tonically induced facies changes is extremely difficult.
On the basis of field evidence, they argue (p. 537)
that:
2.6
Selected examples of the
geomorphological impact of active
tectonics in arid environments
...
there is no reason to invoke tectonic destabiliza-
tion of the river system as a cause of influx of coarse
alluvium in the Dead Sea fan sequences
2.6.1
Tectonic disruption of fluvial systems
punctua-
tion of the sedimentary sequence by coarse sediment
was achieved by climatic [their emphasis] rather than
tectonic factors.
...
The 1980 EI Asnam earthquake in Algeria provides one
recent example of tectonic disruption of a fluvial system
as a result of surface deformation. Active fold develop-
ment resulted in the blocking of the Chelif River with
the production of a lake some 5 km
2
in extent, and strati-
graphical and archaeological evidence is used by King
and Vita-Finzi (1981) to argue that local topography is
the result of some 30 similar earthquakes over a period of
1.5
Similarly, Sohn
et al.
(2007) argue that alluvial fan sedi-
mentation in the Black Mountains of southern Death Val-
ley is responding to climatic rather than tectonic forcings.
However, Rockwell, Keller and Johnson (1985) argue that
active folding and faulting of alluvial fans in the Ventura
Valley, California, have produced complex fan morpholo-
gies that are quite distinct from those in less active areas
10
4
yr.
Large river systems tend to be prone to avulsion, which
may or may not be tectonically triggered. Avulsion is
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