Geology Reference
In-Depth Information
presence of clear fault scarps, and proximal
depocenters typify regimes of more rapid
faulting (Fig. 10.4). The amount of sediment
accommodation space in the adjacent basin is
highly sensitive to the rate of faulting, given
the tendency of hanging-wall subsidence to
be several-fold greater than footwall uplift
(Fig. 4.25). For any given sediment flux from
the footwall, slower subsidence reduces accom-
modation space and promotes fan prog-
radation. In humid settings associated with
rapid normal-fault slip, short transverse streams
sourced in the footwall can be tributary to
axially flowing master rivers that are located
near the mountain front. In drier climates,
closed-basin lakes could be located at the toes
of the short tributary fans.
Whereas the slopes on short, range-front
fans may approach 15
°
near their apices, in
general, when more than 0.5 km from the fault,
fans typically have slopes of 1-3
°
(Wallace,
1978). If the mean slope of a group of fans or
a bajada is more than 3-4
°
over distances
exceeding 2 km, such slopes suggest that this
portion of the fan is underlain by tilted bed-
rock and that faulting has propagated out into
the basin (Wallace, 1978).
In order to deduce the current state of activity
on a normal fault, several different attributes of
the mountain front and of the associated depo-
sitional and erosional systems can be examined.
The amount of incision at the apex of fans
aligned along the mountain front-piedmont
junction is determined by (i) the balance
between the rock uplift rate in the footwall,
(ii) rates of fluvial erosion in the footwall, and
(iii) variations in sediment supply. If active
uplift is elevating the stream within the moun-
tains more rapidly than it is incising, active
deposition will occur on the apex of the fan
(Bull and McFadden, 1977). To the extent that
incision outpaces footwall uplift, the fanhead
will tend to become entrenched. The degree of
entrenchment and the age of the dissected rem-
nants of the former fan apex provide a general
indicator of the amount and timing of the
change from active to less active footwall uplift.
It must also be realized, however, that sediment
fluxes can vary rapidly as the climate changes.
Mountain-Front Sinuosity
A
uplifted footwall block
mountain-piedmont junction (undissected)
sinuosity
= 1.01
mountain front length
hanging-wall basin
active range front
hangingwall basin
B
mountain-piedmont junction (dissected)
sinuosity
= 1.62
mountain front length
inactive range front
Fig. 10.7
Sinuosity of faulted mountain fronts.
A. Active normal faulting with linear mountain-
piedmont junction leading to low sinuosity.
B. Embayment of mountain front along a less active
fault creates higher sinuosity.
Changes in sediment and water discharge can
lead to fanhead incision or aggradation that
is independent of any tectonic variations.
Therefore, entrenchment that persists through
several climate cycles is more likely to be a
response to tectonic forcing than entrenchment
occurring solely within one cycle.
Several simple numerical measures of the
mountain front and its related fluvial system
have been used to classify the state of long-term
tectonic activity. Because a range front above an
active normal fault is generally straight, whereas
an inactive one becomes increasingly embayed,
the
sinuosity
of the mountain front is a poten-
tial indicator of the level of long-term tectonic
activity (Bull and McFadden, 1977). Sinuosity,
S
, is determined by dividing the length of the
mountain-piedmont junction,
L
mp
, by the length
of the associated range,
L
r
(Fig. 10.7):
S
=
L
mp
/
L
r
(10.1)
A sinuosity near 1 is usually interpreted to char-
acterize an actively deforming range, whereas as
the sinuosity increases to 2 or more, it indicates
a highly embayed range front with relatively little
active faulting. These data can be readily derived
from topographic maps and DEMs, although
some subjectivity is involved when designating
the mountain-piedmont junction. This technique
