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spatially variable subsidence in the fan accumu-
lation area can go some way to controlling fan
thickness and morphology (Whipple & Trayler
1996). Viseras et al. (2003) found that in the
Betic Cordillera of southern Spain in areas where
the accommodation space for alluvial fan devel-
opment is driven by relatively high tectonic sub-
sidence (at
such as river capture. This situation tends to occur
where the development of regional gradients can
increase the stream power, and thus erosivity,
of one stream in relation to its neighbours. The
headward erosion associated with these pheno-
mena may be exacerbated where the geological
structure generated by the tectonics also exposes
sediments of higher erodibility. Combined with
the runoff-dominated hydrology, high-intensity
rainstorm events and minimal protection from
vegetation cover associated with arid envir-
onments, and river capture becomes a prolific
modifier of drainage networks. As such river
capture can play a major role in sediment pro-
duction, delivery and routing in tectonically
active arid landscapes. Details of such an exam-
ple are highlighted in Case Study 5.1.
1mmyr
−1
) the resultant alluvial fans
tended to aggrade vertically and were not incised.
Where the accommodation space was controlled
by lower rates of subsidence (
>
1mmyr
−1
), how-
ever, the resultant fans were narrower and pro-
graded out into the basin with some incision.
Indirect effects of tectonics include situations
where regional tectonics, such as epeirogenic
uplift, create the necessary conditions for a
secondary geomorphological process to operate
<
Case study 5.1 Influence of tectonics on rates of sediment production, delivery and routing,
the Sorbas Basin, south-east Spain
The Sorbas Basin is located within a basin and range topography within the Betic Cordillera
of southern Spain. Southern Spain represents part of the plate boundary between the African
and Iberian plates, which ceased subduction in the late Neogene. Since this time compression
has dominated with tectonic movement being expressed through differential uplift within and
between the sedimentary basins and the mountain ranges. Pliocene to recent average uplift rates
are calculated to be in excess of 160 m Myr
−1
for the Sierras Alhamilla and Cabrera, but are
typically much lower for the basins (80 m Myr
−1
for the Sorbas Basin; 11-21 m Myr
−1
for the
Vera Basin; Mather et al. 2002). This deformation has been significant in generating regional
topographic gradients. Combined with the geological structure this has led to the develop-
ment of abundant river captures as a direct function of related increases in stream power and
accelerated headward erosion. These river captures are significant in affecting the sediment
flux, routing and delivery within and between basins (Mather et al. 2002; Stokes et al. 2002).
One such capture occurred in the upper Pleistocene and forms the focus of this case study.
The modern topography of the Sorbas Basin is dominated by an incising drainage network
(the Rio Aguas), which is associated with many landscape instabilities such as extensive badland
terrain and landslides (Case Fig. 5.1a). Landscape erosion is thus locally severe (Harvey 2001),
with some areas of abandoned agricultural land undergoing gully headcut retreat of several metres
in one rainstorm event. Erosion-pin experiments indicate localized surface lowering of several
millimetres per year. In some lithologies piping is abundant and forms a pseudokarst system,
which supplements and overprints surface runoff features in terms of sediment and water
discharge and routing. Localized piping features may be as much as 15 m deep and 1-2 m in
diameter. These badland areas are associated with material of low residual strength such as marls
and silts. The main landslides, in contrast, tend to dominate in areas of stronger lithologies with
higher unconfined compressive strengths such as limestones, or in pervious material, which in
