Geology Reference
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and landscapes more generally, as products of a coupled
tectonic-climatic system with the potential for feedbacks
between climatically influenced surface processes and
crustal deformation (Beaumont
et al
. 2000; Pinter and
Brandon 1997; Willett 1999).
The
elevation of orogens
appears crucially to depend
upon the crustal strength of rocks. Where crustal con-
vergence rates are high, surface uplift soon creates (in
geological terms) an elevation of around 6 to 7 km that
the crustal strength of rocks cannot sustain, although
individual mountain peaks may stand higher where the
strength of the surrounding crust supports them. How-
ever, in most mountain belts, the effects of denudation
prevent elevations from attaining this upper ceiling. As
tectonic uplift occurs and elevation increases, river gra-
dients become steeper, so raising denudation rates. The
growth of topography is also likely to increase precipita-
tion (through the orographic effect) and therefore runoff,
which will also tend to enhance denudation (Summer-
field and Hulton 1994). In parts of such highly active
mountain ranges as the Southern Alps of New Zealand,
rivers actively incise and maintain, through frequent
landslides, the adjacent valley-side slopes at their thresh-
old angle of stability. In consequence, an increase in the
tectonic uplift rate produces a speedy response in denuda-
tion rate as river channels cut down and trigger landslides
on adjacent slopes (Montgomery and Brandon 2002).
Where changes in tectonic uplift rate are (geologically
speaking) rapidly matched by adjustments in denudation
rates, orogens seem to maintain a roughly
steady-state
topography
(Summerfield 2007). The actual steady-
state elevation is a function of climatic and lithological
factors, higher overall elevations being attained where
rocks are resistant and where dry climates produce lit-
tle runoff. Such orogens never achieve a perfect steady
state because there is always a delay in the response of
topography to changing controlling variables such as cli-
mate, and especially to changing tectonic uplift rates
because the resulting fall in baselevel must be propa-
gated along drainage systems to the axis of the range.
Work with simulation models suggests that variations
in denudation rates across orogens appear to affect pat-
terns of crustal deformation (Beaumont
et al
. 2000;
Willett 1999). For relatively simple orogens, the pre-
vailing direction of rain-bearing winds seems significant.
()
a
()
b
Upthrust
block
Down-sagging
pond
Figure 4.13
Landforms produced by anastomosing
faults. (a) Anastomosing faults before movement.
(b) Anastomosing faults after movement with upthrust
blocks and down-sagging ponds.
Source:
Adapted from Kingma (1958)
landforms (Figure 1.1). Plate tectonics explains some
major features of the Earth's topography. An exam-
ple is the striking connection between mountain belts
and processes of tectonic plate convergence. However,
the nature of the relationship between mountain belts
(orogens) and plate tectonics is far from clear, with sev-
eral questions remaining unsettled (Summerfield 2007).
What factors, for example, control the elevation of
orogens? Why do the world's two highest orogens -
the Himalaya-Tibetan Plateau and the Andes - include
large plateaux with extensive areas of internal drainage?
Does denudation shape mountain belts at the large scale,
and are its effects more fundamental than the minor
modification of landforms that are essentially a product
of tectonic processes? Since the 1990s, researchers have
addressed such questions as these by treating orogens,
