Tectonic frameworks - Arid Zone Geomorphology - page 19

Table 2.3 Estimates of tectonic uplift rates as a function of
tectonic setting.

5. Deflection of the sedimentary systems where tectoni-
cally controlled morphological changes act as barriers
to sediment transfer both into the basin from the hin-
terland and between various areas within the basin.

Recent vertical
crustal
movements

Neotectonic
warping

6. Changes in the rate of sediment supply uplift or erosion
of the supplying hinterland.

Tectonic setting

1. Cratons Less than 1 mm/

yr

One key characteristic of the supply of water and sediment
to arid zone lake basins, and therefore of arid zone flu-
vial activity, is the coexistence of far-travelled perennial
rivers with catchments outside the desert with ephemeral
desert streams (Currey, 1994a). The latter have a capac-
ity for transporting sediment during flood events that far
exceeds that of similar-size perennial channels. One ex-
planation for this high rate of bedload transport is that
armouring of the channel bed has no time to become
established in ephemeral bedload streams (Laronne and
Reid, 1993). These differences between ephemeral and
perennial streams have implications for both the rate
and the nature of fluvial and lacustrine sedimentation in
arid zone depocentres.

2. Active
continental
margins

Up to 20 mm/yr Up to 10 mm/yr

3.Older
orogenic belts

Up to 5 mm/yr Up to 1 mm/yr

4. Interorogenic,
intercratonic

Up to 10 mm/yr Up to 5 mm/yr

5. Passive
continental
margins

Up to 10 mm/yr Up to 1 mm/yr

Source : Fairbridge (1981).

and fission track thermochronology (timescale 10 6 -10 7
yr) for particular regions.
The study of neotectonics has included the documenta-
tion of recent vertical crustal movements (RVCMs), which
can be determined by instrumentation and measurement,
and which are therefore limited to observations made dur-
ing the last century (Fairbridge, 1981). The order of mag-
nitude of these very recent movements and that of neo-
tectonic measurements determined over longer timescales
are summarised in Table 2.3 for the various arid zone tec-
tonic settings identified in Table 2.1. Again there appears
to be a scale dependency, with the RVCMs exceeding the
neotectonic estimates by up to an order of magnitude.
Depositional sequences may also reflect tectonic influ-
ences. Frostick and Steel (1993a, p. 2) identify six ways in
which fluvial or lacustrine sedimentation may be affected
by tectonic controls:

2.4 Lengths of record

From the point of view of process geomorphology, the
different tectonic settings are associated with different
potentials for sediment generation and sediment supply,
and for the disruption of sedimentary sequences in partic-
ular climatic contexts. The record of change can be read
in both erosional landforms and sequences of sedimen-
tary deposits. The status of different variables changes
as a function of the timescales considered. Prior to the
development of radiometric dating techniques, the evolu-
tionary state of a particular landscape was assessed with
reference to Davisian or Penckian models (Summerfield,
2000; Burbank and Anderson, 2001, Figure 1.2), and this
analysis was used to provide the relative age of that land-
scape (Thornes and Brunsden, 1977). The development
of radiometric dating has shifted the burden of proof of
antiquity from the landscape itself, but successful dating
depends not only on the presence of suitable materials
but also on the existence of techniques with appropriate
time ranges of application. Until relatively recently most
dating in arid zones was based on 14 C dating, which can
only be used for materials containing organic or inorganic
carbon, within the age range 0-45 ka. All 14 C dates also
require calibration in order to take account of variable
rates of production of 14 C in the upper atmosphere. With
the development of other dating techniques, particularly

1. Changes in the overall accommodation space available
for filling within the basin.

2. Changes in the direction of tilt and the location and
size of the depocentre.

3. Changes in the orientation and character of basin mar-
gins and in overall basin size and shape (e.g. by back-
stepping of marginal faults).

4. Changes in the gradient both at the margins of the basin,
through faulting and within the basin (e.g. through tilt-