Geology Reference
In-Depth Information
Impact of Regional Tilting on Gradients
Fig. 9.8
Gradient analysis of
tilted stream segments.
A. Stream network is divided into
segments based on positions of
stream junctions. The model
assumes that, if tilting is occurring,
streams flowing in the direction
of tilting will have increased
gradients, whereas those flowing
in the opposite direction will have
gentler gradients. B. Depiction of
stream gradients. Each stream
segment is plotted according to
its length and to the elevation of
the upper and lower ends of the
segment. The upstream end of
the segment (on the
x
axis) is
determined by the length of
segments that are upstream of it
and tributary to it. Thus, segments
1 and 2 flow into segment 6,
segments 3 and 4 flow into
segment 5, and segments 5 and 6
flow into segment 7. Segments
1 to 4 all represent first-order
tributaries that have no upstream
segments - their upper ends plot
on the left-hand margin. The sum
of the lengths of segments 1 and 2
determines the horizontal position
of the upstream end of segment 6
into which they flow. C. Departure
from ideal gradient. When
compared to the idealized gradient,
steeper segments suggest tilting in
the downstream direction. Compare
the relative sense of tilting with the
hypothesized fold orientation in the
top panel. Modified after Merritts
and Hesterberg (1994).
Map of Segments & Effect of Tilt on Slope
anticlinal
crest
4
5
3
2
7
6
stream segments
A
1
4
elevation of segment 5
at junction with 3 & 4
3
elevation of segment 7
at junction with 5 & 6
1
5
2
7
6
elevation of segment 6
at junction with 1 & 2
length of upstream segments and of segment
B
1
gentler gradients
3
predicted undisturbed
profile
2
6
4
5
steeper gradients
7
C
length of upstream segments and of segment
that they will approach a graded profile rather
rapidly (Snow and Slingerland, 1987) once the
perturbation ceases. Thus, anomalously steep
or gentle river profiles, especially when not
correlated to lithologic contrasts, may be
interpreted as responses to ongoing tectonism.
Several approaches use data extracted from
topographic maps to identify areas of active
deformation, such as zones where stream
gradients depart from expected longitudinal
profiles. Consider a region experiencing tilting
toward the south. Rivers flowing south will
tend to have their gradients steepened, whereas
rivers flowing north will have their gradient
lessened with respect to untilted regions. One
approach to the analysis of tilted channel
networks (Merritts and Hesterberg, 1994)
begins by segmenting a drainage network into
its component tributaries (Fig. 9.8A) and
measuring (i) the length of each segment and
