Geology Reference
In-Depth Information
basement
thrust front
Higher Himalaya
Lesser Himalaya
5
.21
Sutlej
1.1
MCT?
3.2
T
MBT
9.7
3.9
P
48
5
5.1
1.6
0
T
MCT
.29
Bheri
Fig. 9.9
Longitudinal profiles
and stream-gradient indices for
Himalayan rivers.
Thicker segments of the profile
indicate reaches where the local
gradient index (SL) is more than
twice the index (
k
) for the entire
profile: SL/
k
≥
2. The steepest
gradients are not associated with the
Main Boundary Thrust or active
deformation to the south. Rather,
they occur near the Main Central
Thrust and appear to result from
upward ramping of the overthrusting
Himalaya above a deep-seated
basement thrust. Modified after
Seeber and Gornitz (1983).
Himalayan
Stream Gradient
Indices
.95
T
MBT
T
i
15
2.5
0
5
1.4
.63
Kali Gandaki
MCT
i
.56
6.6
3.3
MBT
gradient index > 2
P
0
12
2.6
1.0
.62
5
gradient index
<
2
.48
MCT
Trisuli
2.7
MCT: Main Central Thrust
MBT: Main Boundary Thrust
MBT
14
2.2
10
2.4
0
.91
5
T
5
.38
.96
Sun Kosi
MCT
6.0
15
vertical exaggeration = 16x
Arun
0
3.2
basement
thrust front
0
0
50
100
150
200
Distance (km)
(ii) the elevation of its upper and lower ends.
The elevation range of each segment (
y
axis)
can then be plotted against the sum of the
lengths of the upstream segments and the
length of the segment itself (
x
axis) (Fig. 9.8B).
The lower elevation of each feeder or tributary
segment is matched to the upper elevation of
each segment into which it flows. Subsequently,
using the
x
axis position as dictated by the length
of the upstream segments, each segment is com-
pared with an idealized logarithmic longitudinal
profile (Fig. 9.8C). Significant departures from
the ideal profile serve to identify segments that
could be interpreted to indicate increased or
decreased gradients over time due to tilting.
When coherent areas are located in which all or
most of the streams flowing in a given direction
show the same tendency toward steepening or
flattening, regional patterns of warping can be
deduced (Merritts and Hesterberg, 1994). One
must, however, exercise considerable caution in
the application of such a technique, as it rests on
the assumption of an “ideal” profile, and all
effects of the variations in lithology or grain size
of the material involved in the fluvial system
must be assumed to be small compared to the
tectonically induced changes in slope.
Before digital topographic data became
widely available, departures from expected
channel gradients were sometimes identified on
the basis of changes in the stream-gradient
index (SL), which compares the slope of a local
reach with the distance to the drainage divide
(Hack, 1973). For a short reach, the stream-
gradient index can usually be approximated by
SL = (
Δ
H
/
Δ
L
)
L
(9.1)
where
L
is the distance measured from the
drainage divide to the mid-point of the reach, and
the slope of the short reach (
Δ
H
/
Δ
L
) is considered
constant. For a well-adjusted channel profile, the
stream-gradient index will remain nearly con-
stant or change only slowly. Abrupt increases in
the index typify oversteepened reaches.
A pioneering study of major Himalayan rivers
by Seeber and Gornitz (1983) used stream-
gradient indices to identify those river reaches
that were anomalously steep (Fig. 9.9). Their
analysis clearly showed that the steep reaches
were not associated with what were considered
the younger, active faults, such as the Main
Boundary Thrust (MBT; Fig. 9.9), but instead
were localized either above a deeply buried
thrust ramp in the basement or near the trace
