Geology Reference
In-Depth Information
Upstream of the knickpoint, the pre-acceleration
gradient will persist, because that part of the
channel will not “feel” the acceleration at the
outlet until its effects are translated upstream
by the knickpoint (Whipple and Tucker, 1999).
One well-documented region where both spatial
and temporal changes in rock uplift rate have
occurred is the northern Californian coastal region.
Here, over the past few million years, the
Mendocino triple junction has swept northward
across the region and created a wave of accelerated
uplift followed by gradual restoration of rates in
the wake of the triple junction (Merritts and Bull,
1989). Studies of coastal terraces, such as that
depicted in Fig. 9.4, reveal more than six-fold
increases in uplift rates over the past 100kyr for
some sites, thereby providing a striking context in
which to examine channel responses to these dif-
ferences in rates. Whereas the southern part of the
region has sustained uplift rates
<
0.5 mm/yr for
the past 300 ka, the northern region accelerated to
3-4 mm/yr at
∼
100ka. Despite these strong con-
trasts in uplift rates, channel concavity,
q
, remains
nearly constant (Fig. 9.12) throughout the study
area (Snyder
et al.
, 2000). Such constancy suggests
that (i) the channels are in rough equilibrium with
Profile Adjustments to Doubled Uplift Rate
final profile adjusted
to uplift rate 2X
R1
R2
unperturbed reach
of initial profile (R2)
knickpoint
log area
upstream migrating knickpoint
initial profile adjusted
to uplift rate X
Distance Downstream
Fig. 9.11
River-channel adjustments to
accelerated uplift.
Channel profile in a transient state due to doubling of
the uplift rate with respect to the outlet. As a knickpoint
sweeps upstream, the lower channel profile (R1)
steepens, whereas the upper profile (R2) retains its
initial gradient, one that was adjusted to the original
uplift rate. Once the knickpoint sweeps to the
headwaters, the entire profile will be steepened.
Inset shows lower and upper reaches in slope-area
space. Note the different steepnesses (vertical
position of the line with respect to area), but same
concavities (slope of the line). Modified after
Whipple and Tucker (1999).
Concavity versus Uplift Rate
5
1.0
uplift
rate
4
0.8
Fig. 9.12
Channel concavity
and steepness in different
uplift regimes.
Rates of rock uplift deduced from
marine terraces define rates range
from
decelerating
u
p
lift rate
3
0.6
m
ean
θ
2
0.4
1
<
concavity
0.5 mm/yr to 4 mm/yr along
the northern California coast
(Merritts and Bull, 1989). (Top)
Concavity is largely insensitive to
uplift-rate variations. Mean
concavity is
∼
0.45, consistent with
expectations for adjusted channels.
(Bottom) Steepness shows strong
contrasts, with higher average
steepness in the high-uplift-rate
zone. Note correlation of steepness
index with uplift rate across the
zone of decelerating rates. Modified
from Snyder
et al.
(2000).
0.2
north
south
0
0.0
Steepness versus Uplift Rate
5
uplift
rate
4
decelerating
uplift rate
100
3
mean ks: 92
north
sout
h
2
mean ks: 53
1
steepness index
50
0
2
4
6
8
10
12
14
16
18
20
Catchment Number
