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north by a major thrust fault system and to the south by
basically an unfaulted but formerly pedimented margin
(Figure 14.13(a)). The structure is essentially a passive
control over alluvial fan location and style, with sim-
ple mountain-front fans to the north backfilling slightly
into the mountain catchments (Figure 14.2(c)), but with
a complex of pediment-burying irregular mountain-front
and intramontane fans to the south. However, the faults
are not totally inactive; mid-Pleistocene fan sediments on
the northern mountain front are faulted and the older fan
segments have probably been tilted, steepening the upper
fan gradients (Silva
et al
., 1992). On the south there are
minor faults crossing the piedmont area (Somoza
et al.
,
1989), but these have little or no discernible effect on the
fan geomorphology.
Death Valley, California (see Box 14.1), is a half-
graben bounded to the west by the uplifting Panamint
Range and to the east by the normal-fault-bounded mar-
gin of the Black Mountains (Hunt and Mabey, 1966; Hunt,
1975). This passive structural setting results in the con-
trast in fan style between the large, telescopic west-side
fans and the smaller stacked east-side fans (Denny, 1965).
The Panamint mountain front is essentially a nonfaulted
uplifted block, affected only by small antithetic faults.
The fans backfill into the mountain catchments. The nor-
mal fault along the Black Mountains mountain front is
highly active, resulting in a straight fault-bounded moun-
tain front, simple mountain-front fans, aggrading from the
apices down, essentially because accommodation space
is increased as the floor of the valley subsides. A similar
complex pattern of different styles, dependent on local ac-
tive tectonics, including sustained aggradation, has been
identified on downfaulted fans in the Dixie Valley, Nevada
(Bell and Katzer, 1987; Harvey, 2005).
The response of fans to tectonically induced base-level
change can be illustrated by the fans in the Tabernas basin,
southeast Spain (Figure 14.13(b)). The basin is bounded
to the north by the tilted and uplifting basement schists
of the Sierra de los Filabres and to the south by more
schists of the the Sierra de Alhamilla thrusting forward
along a frontal reverse fault system. To the north of the
Sierra de Alhamilla is the Serrata del Marchante, an anti-
cline in Neogene conglomerates forming a growth fold
over a blind basement fault also thrusting northwards
(Mather
et al
., 2003). Quaternary alluvial fans formed
along these three mountain fronts (Harvey
et al.
, 2003),
as huge backfilled fans on the Filabres mountain front,
simple mountain-front fans along the Marchante front and
a confined fan complex where the Rambla de la Sierra is-
sues from the Sierra de Alhamilla (Harvey, 1978, 1984b;
Delgardo-Castilla, 1993). During the Late Pleistocene,
the drainage, causing a lacustrine or palustrine environ-
ment to form to the west of the fans (Harvey
et al.
, 2003).
During the Late Pleistocene the barrier caused by the
growth fold was breached and the drainage rapidly incised,
creating a spectacular badland landscape in the western
part of the basin, but also cutting back into the alluvial fan
zone in the eastern part of the basin. At present the incision
wave has cut back into the Sierra de Alhamilla, dissect-
ing the Sierra fan; dissection of the Marchante fans is,
at least in geomorphological timescales, imminent, but is
nowhere near dissecting the Filabres fans. This illustrates
an important point related to coupling through drainage
systems (Harvey, 2002c). The tectonic signal, which may
be synchronous only in the vicinity of the structure them-
selves, is otherwise variable and time transgressive, as the
effects are propagated through the system. A similar inci-
sion wave, but related to Mid Pleistocene deformation has
been identified by Garcia
et al
. (2004) in the Alpujarras,
west of the Tabernas basin, whereby tectonically induced
dissection has propagated through the system, eventually
causing fan entrenchment at the head of the system.
14.3.2.2 Climatic change
Although alluvial fans are not exclusively dry-region
forms, they are particularly well developed in arid moun-
tain areas. What is important is not climate itself, but cli-
matic change (Bull, 1991). Fan surface form adjusts to the
prevailing flood and sediment regime (see above, Section
14.2.4, Figure 14.7(b)), and if a climatic change causes
a change in this regime, the fan will adjust accordingly.
Indeed, in a whole wealth of studies of Quaternary dry-
region fans, the climatic signal seems to dominate, and
even in areas of considerable tectonic activity or base-
level change the climatic signal is only modified by the
tectonic or base-level signal (e.g. Frostick and Reid, 1989;
Roberts, 1995; Ritter
et al
., 1995; Bowman, 1988; Harvey,
2002a, 2003; Pope and Wilkinson, 2005).
Three examples illustrate the alluvial fan response to
Quaternary climatic change. On many Spanish fans, al-
though the fan catchments themselves were not glaciated
during Pleistocene cold phases, the cold dry climates gen-
erated large volumes of sediment, which resulted in sus-
tained periods of regional aggradation affecting fans and
river valleys (Harvey, 1987, 2003). During the intervening
interglacials the fans were largely sediment starved and
underwent fanhead trenching and limited progradation.
During the current interglacial period, the Holocene, this
has resulted in many fans undergoing incision and headcut
formation at or immediately downfan from the intersec-
tion point (Figure 14.14; compare Figure 14.15(a) and
