Geology Reference
In-Depth Information
A
Upstream of Fold
Within Core of Uplift
2
6
4
1
4
6
2
0
6
4
2
0
0
0
0
2
1
0
0 0
Profile Distance (m)
20
30
40
50
0 10203040 5060
Profile Distance (m)
B
Incision into Fold
fold axis
7
535
channel profile
terrace
profile
incision
proile
channel profile
~ 4x VE
0
500
0
0.4
Downstream Distance (km)
Fig. 8.22
Width changes in
antecedent channels crossing
small growing folds.
A. Surveys of abandoned channels
in the Mackenzie Basin, New
Zealand, reveal three- to eight-fold
decreases in width as channels
traverse a fold. Note that, because
loess and soils have partially filled
the abandoned channels, their
former geometry is reconstructed
using closely spaced soil probes.
B. Channel incision is measured by
comparing the channel bottom to the
uplifted former terrace surface across
the asymmetric fold. C. Compilation
of data from five channels showing
that 1-2 m of uplift causes a five- to
10-fold channel narrowing. Modified
after Amos and Burbank (2007).
C
35
Incision versus Width
30
25
20
Channel
1
23
15
4
5
10
5
0
0123456789
Differential Incision (m)
slope, and incision rates for bedrock rivers in
Taiwan reach very similar conclusions (Yanites
et
al
., 2010): in response to increasing rates of dif-
ferential uplift, rivers become progressively nar-
rower until a width/depth ratio of ~10 is attained,
after which they also become steeper.
The very detailed documentation of rock
uplift rates above the Main Frontal Thrust in
central Nepal provides a robust framework for
examining how the profiles of rivers respond to
uplift gradients. Given that rivers that are not
tectonically perturbed have average concavities
of ~0.5, we can predict three end-members of
concavity for rivers crossing growing anticlines
(Fig. 8.24): (i) rivers originating in zones of high
uplift rates and flowing to low-uplift zones will
have high concavity because their headwaters
will be differentially uplifted with respect to
