Page 131 - Fundamentals of Geomorphology

114 STRUCTURE

a
() 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
b
() strength of the surrounding crust supports them. How-
Upthrust
block 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
Down-sagging
pond 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-
Figure 4.13 Landforms produced by anastomosing
faults. (a) Anastomosing faults before movement. old angle of stability. In consequence, an increase in the
(b) Anastomosing faults after movement with upthrust tectonicupliftrateproducesaspeedyresponseindenuda-
blocks and down-sagging ponds. tion rate as river channels cut down and trigger landslides
Source: Adapted from Kingma (1958) on adjacent slopes (Montgomery and Brandon 2002).
Where changes in tectonic uplift rate are (geologically
speaking) rapidly matched by adjustments in denudation
landforms (Figure 1.1). Plate tectonics explains some rates, orogens seem to maintain a roughly steady-state
major features of the Earth’s topography. An exam- topography (Summerfield 2007). The actual steady-
ple is the striking connection between mountain belts state elevation is a function of climatic and lithological
and processes of tectonic plate convergence. However, factors, higher overall elevations being attained where
the nature of the relationship between mountain belts rocks are resistant and where dry climates produce lit-
(orogens) and plate tectonics is far from clear, with sev- tle runoff. Such orogens never achieve a perfect steady
eral questions remaining unsettled (Summerfield 2007). state because there is always a delay in the response of
What factors, for example, control the elevation of topography to changing controlling variables such as cli-
orogens? Why do the world’s two highest orogens – mate, and especially to changing tectonic uplift rates
the Himalaya–Tibetan Plateau and the Andes – include because the resulting fall in baselevel must be propa-
large plateaux with extensive areas of internal drainage? gated along drainage systems to the axis of the range.
Does denudation shape mountain belts at the large scale, Work with simulation models suggests that variations
and are its effects more fundamental than the minor in denudation rates across orogens appear to affect pat-
modification of landforms that are essentially a product terns of crustal deformation (Beaumont et al. 2000;
of tectonic processes? Since the 1990s, researchers have Willett 1999). For relatively simple orogens, the pre-
addressed such questions as these by treating orogens, vailing direction of rain-bearing winds seems significant.