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the uplift rate and (ii) channel-profile adjustments
to the increased rates have occurred in less
than 100 kyr, the time when rates accelerated.
In contrast to the constant concavity, channel
steepness shows strong regional variability
(Fig. 9.12). The steepness index,
k
s
, is nearly twice
as large in the high-uplift-rate region as in the
low-rate region. Notably, the uplift rate and steep-
ness index co-vary in the zone of changing uplift
rates (catchments 12 to 16; Fig. 9.12). In the north-
ernmost catchments, however, the steepness index
decreases despite the rather high uplift rate.
Although the cause for this decoupling is not
known, orographic rainfall is about twice as great
in the north than in the south (Snyder
et al.
, 2000).
To the extent that the efficiency of erosion is
linked to the mean annual discharge, higher rain-
fall could drive more erosion and promote less
steep channels for a given erosion or uplift rate.
L
an
dsc
ape Indicators of a Blind Thrust
100
140
120
100
80
60
40
20
steepness index
mean relief
80
60
40
20
exponential increase
in steepness index
0
0
0
5
10
15
20
25
A
Distance South (km)
Rod
gers
Cre
ek
north
Hayward
Mt. Tam
San Andreas
Mt. Tam
Blind Thrust
B
From landscapes to faults
Fig. 9.13
Topographic indices related to differential
uplift above a blind thrust.
A. Normalized steepness index (reference
concavity = 0.45) and relief in the inner gorge as a
function of distance. Both parameters increase towards
the south to where Mount Tamalpais is located (
∼
24 km)
and suggest a concomitant increase in rock uplift rates.
B. Schematic model for an uplift gradient above
a listric blind thrust. Vertically exaggerated topography
and major dextral strike-slip faults are shown. Modified
after Kirby
et al.
(2007).
Sometimes one of the biggest challenges for
field geologists is simply recognizing the
existence of major faults. Such faults may be
blind, they may rupture the surface where few
geomorphic markers exist with which to
recognize differential displacement, or dense
vegetation may obscure clear views of offset
features (e.g., Fig. 1.5). The identification of
faults under these conditions commonly requires
a quantification of landscape attributes that
respond to the uplift or subsidence caused by
active faults.
A recent study of the region surrounding
Mount Tamalpais along the northern San
Andreas Fault (Kirby
et al.
, 2007) exploits
changes in mean topography, channel steepness
indices, hillslope angles, and topographic relief
to delineate a spatial gradient that is interpreted
as a response to a blind thrust (Fig. 9.13). Along
a transect southwards toward Mount Tamalpais,
both the mean and maximum elevations rise, as
does the average steepness of hillslopes and
the fraction of slopes considered to be at a
threshold angle for failure. Steepness indices
for a suite of similarly sized catchments along
this north-south transect show an exponential
increase toward the south (Fig. 9.13A). Channels
in all these catchments display inner gorges
whose walls are at threshold angles, suggesting
that these walls are eroding by bedrock
landsliding, presumably because the channels
are incising at rates faster than soil-mantled
hillslopes can erode. The relief from the channel
bottom to the top of the inner gorge also
progressively increases toward the south
(Fig. 9.13A) and is interpreted to indicate
increasingly rapid rates of channel incision.
The presence of an inner gorge, rather than
a gradually steepening hillslope rising above
each channel, suggests that this landscape is
in a transient state. This suite of observations
suggests that rock uplift rates increase
