Geoscience Reference
In-Depth Information
2.2
Tectonic setting of drylands
date from the Cretaceous (80 million years before present
(80 Ma)), but both the Atacama and Australian deserts are
considered to date from some point during the Miocene
(22-5 Ma). The onset of aridity is recognised to have
occurred at a global level c. 3 Ma (Williams, 1994). In ad-
dition, all desert areas have experienced climate change
during the Late Quaternary, with some changes involving
a change to humid or subhumid conditions (Chapter 3).
Although Late Quaternary glacial/interglacial cycles have
a frequency of 1
Tectonic settings of contemporary dryland areas control
the development of landforms and of sedimentary se-
quences by influencing factors such as sediment supply. It
is possible to identify five types of tectonic setting: cratons
(shield and platform areas); active continental margins,
associated with Cenozoic orogenic belts; older, Phanero-
zoic, orogenic belts; interorogenic basin and range and
inter-cratonic rift zones; and passive continental margins.
Examples of each of these are given in Table 2.1.
The currently accepted view of what is 'tectonically
stable' has undergone a radical transformation since the
late 1960s. However, it is possible to identify particular
tectonic settings on the basis of relative stability or in-
stability and the term 'craton' is still used to describe
the 'central stable portion of a continent' (Ollier, 1981,
p. 75).
The Earth's crust and upper mantle together comprise
interlocking lithospheric plates, which move relative to
each other. New oceanic crust is continuously created at
spreading centres (mid-ocean ridges and rifts), and this
increase in crustal material is accommodated by 'loss' of
crust by subduction, by folding and thrust faulting, or by
movement along transcurrent faults. The most spectacular
tectonic activity is associated with active plate margins,
and with earthquakes and volcanic activity. Passive (or
trailing-edge) continental margins are not without interest,
with many of them characterised by the presence of so-
called 'great escarpments' (Ollier, 1985a, 1985b). Not all
crustal movements are of course compressional, and the
development of major rift systems (grabens) and so-called
'basin and range' provinces are products of either mantle-
generated or lithosphere-generated rifting (Frostick and
Steel, 1993b).
10
5
yr, some extremely rapid changes
have been identified within these periods, at frequencies of
10
2
-10
3
yr (Taylor
et al.
, 1993; Bond
et al.
, 1997, 2001).
The emphasis in this chapter is on the tectonic set-
ting of contemporary dryland areas, but given the issue
of timescales mentioned above, problems arise from the
fact that the various landforms considered may be out
of phase with contemporary climatic conditions. This is
a particular problem in the case of erosional landforms
such as pediments, for example, which may have evolved
over a much longer period of time than that over which
arid or semi-arid conditions are thought to have persisted
in a particular area (Dohrenwend, 1994).
Arid zones are in many ways ideal places for the study
of tectonics (see Burbank and Anderson, 2001). Large
structures, fold and fault zones are clearly visible on
remotely sensed images of arid zones. Lack of or lim-
ited vegetation leads to enhanced visibility of surface
expressions of tectonic activity like fault scarps and, in
hyper-arid areas such as the Atacama or southern Negev,
scarps cutting alluvial fans may remain undegraded for
substantial periods of time. Visibility is particularly im-
portant given the transient nature of many small features,
which may result from seismic disturbance of the ground
surface.
Although the scale at which tectonics operate may al-
low correlations at the macro- or megageomorphological
scale of landform evolution, even at a mesoscale, tec-
tonic influences may have considerable significance in
arid zones (Beaumont, Kooi and Willett, 2000). In an
arid context, for example, fault-related spring lines have
a much greater significance than within environments in
which water supply is far more plentiful. Also, salt diapirs
provide one source of the salts that play a key role in many
arid zone rock weathering processes.
This chapter will consider the tectonic setting of con-
temporary drylands, the controls that tectonics may have
in terms of uplift and erosion and subsidence and sedi-
mentation, the issues surrounding the development of a
chronological framework for dryland landforms and sed-
iments, the existing record of erosion and sedimentation
and, finally, the interaction of active tectonics and con-
×
2.3
Uplift and erosion, subsidence
and sedimentation
Tectonically driven changes can involve both uplift and
subsidence. Whether these changes achieve topographic
expression is a function of the degree to which uplift
exceeds erosion and to which subsidence exceeds sedi-
mentation. According to Brookfield (1993, p. 16):
...
uplift is positive vertical elevation with respect to
the geoid (basically mean sea level). This normally
refers to net uplift, combining the effects of gross (or
total) uplift minus the effects of erosion. Thus: net