Geology Reference
In-Depth Information
Channel Responses to Differential Uplift
Main
Boundary
Thrust
5 km
Main
Frontal
Thrust
z one of rapid rock uplif t
Geologic
Setting
foreland strata
foreland strata
A
Bagmati River
Bakeya River
200
280
no steepening
steepening
160
240
120
200
Proile
Proile
80
160
1000
1000
narrowing
narrowing
Width
Width
100
100
0.25
0.25
zone of
rapid uplift
12
12
0.20
0.20
Incision
Incision
0.15
0.15
8
8
incision
rate
incision
rate
excess
shear
stress
0.10
0.10
excess
shear
stress
4
4
0.05
0.05
zone of
rapid uplift
0.00
0.00
0
0
-10
0
10
20
-5
0
5
10
B
Distance upstream of MFT (km)
Distance upstream of MFT (km)
Fig. 8.21 Antecedent channel responses to rapid, differential uplift.
A. Channel setting. Slip along the Main Frontal Thrust (MFT) in the Himalayan foreland of central Nepal drives
hanging-wall uplift at rates exceeding 10 mm/yr (Fig. 7.25). B. Channel profile, floodplain width, incision rate,
and calculated excess shear stress for the Bagmati and Bakeya Rivers as they cross the MFT hanging wall. The
Bagmati narrows without steepening. In the zone of the highest incision rate, the Bakeya both narrows and steepens.
Patterns of excess shear stress are closely coupled to channel narrowing. Modified after Lavé and Avouac (2001).
sequencing is also borne out by small fluvial
channels that were cut across short-wavelength
folds in New Zealand (Amos and Burbank, 2007).
Detailed geodetic surveys document channel
morphology across and along channels that cross
late Quaternary folds with wavelengths of hun-
dreds of meters and differential uplift of a few
meters (Fig. 8.22A and B). These data reveal
abrupt channel narrowing as  the magnitude
of  uplift and incision increases (Fig. 8.22C).
Moreover, the data suggest that narrowing
typically occurs earlier and can occur indepen-
dently of any channel steepening (Fig. 8.23). In
particular, if the magnitude of differential uplift is
sufficiently small, narrowing appears adequate to
increase the channel erosion rate in order to bal-
ance uplift rates. At higher rates or magnitudes of
uplift, channels are interpreted to initially narrow
and then to steepen in order to achieve a greater
overall rate of incision. Studies of river width,
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