Geoscience Reference
In-Depth Information
Tectonic frameworks
Helen Rendell
2.1
Introduction
The history of global climate change during the Qua-
ternary is now well documented as a result of the de-
tailed analysis of ocean sediment cores, spanning the last
2.7 million years (Myr), with ice cores providing a high
precision record for the past 0.7 Myr. In common with
other environments, dryland landscapes and sediments
can be interpreted in the context of these changes (see
Chapter 3). Dryland landscapes and sediments can also
be used to establish or refine local sequences of climate
change. In addition, since landforms may be used to pro-
vide insights into recent crustal movements (Burbank and
Anderson, 2001), unravelling climatic and tectonic influ-
ences remains a key problem (Frostick and Steel, 1993b).
Timescales are of critical importance in attempting to re-
solve the relative dominance of these influences and recent
developments in dating both rock surfaces and sediments
using cosmogenic nuclides, together with results from
fission track thermochronology, have greatly aided reso-
lution (Cockburn
et al.
, 2000).
A major problem is that tectonic influences at the
Earth's surface have operated more or less continuously
over the last 3 Myr whereas, over the same timescale,
climatic conditions have changed in a cyclic manner. The
loci of tectonic activity are well specified at the global
level and are predominantly, but not exclusively, associ-
ated with active plate boundaries. Tectonic activity may
appear episodic in the short term (10
1
-10
3
yr) (McCalpin,
1993), but is effectively continuous in many areas when
viewed over the longer term (10
4
-10
7
yr). Rates of tec-
tonic uplift are normally expressed in units of mm/yr or
m/1000 yr, even though vertical displacements during in-
dividual events may be of the order of 10
2
-10
3
mm. Many
contemporary dryland areas have only been arid or semi-
arid since some time during the Late Tertiary. The major
exception may be the Namib Desert, which is thought to
The tectonic setting of dryland areas has received com-
paratively little attention in the literature on arid zone
geomorphology. References to tectonics are often limited
to discussions of alluvial fans or of pans and playas, where
active tectonics may have played some part in the devel-
opment of these landforms. However, tectonic activity,
in its broadest sense, provides the backdrop, in terms of
both absolute and relative relief, against which dryland
processes operate. Tectonic controls operate on sediment
sources and sediment sinks and are, in turn, influenced by
both erosion and deposition (Beaumont, Kooi and Willett,
2000). Tectonic settings also influence the timescales over
which relative stability or instability have dominated par-
ticular areas, and thereby help to determine the continuity
or discontinuity of sedimentary records.
The relationship between tectonics and landforms is
complex (Summerfield, 1985). Until recently much of
tectonic geomorphology has been concerned with using
evidence from landforms to infer rates of operation of tec-
tonic processes (Ollier, 1981; Mayer, 1985; Morisawa and
Hack, 1985). Conversely, some geomorphological studies
have invoked tectonics in order to explain landform evolu-
tion (e.g. Currey, 1994b). Scales of investigation are also
changing:
Now that the plate tectonics paradigm has matured,
attention has been refocused on mesoscale prob-
lems. Among these problems are the more detailed
tectonic and geomorphic evolution of plate bound-
aries and the feedback effects of surface processes,
such as denudation and sedimentation, on the pri-
mary tectonic processes active at these boundaries
(Beaumont, Kooi and Willett, 2000, p. 29).
