Geology Reference
In-Depth Information
Arid Zone, Uplift-Induced Planform Changes
Planform Changes
A
B
footwall
braided
1
alluvial
plain
terrace
plunges
beneath
aggrading
footwall
M
meandering
Terrace Age
M
1
2
3
Modern
Young
to
Old
zone of
uplift
straight
2
C
D
3
2
3
1
meand-
ering
hanging
wall
zone of
uplift
1 km
M
straight
Fig. 8.12
Planform river changes due to differential
uplift.
Irishman Creek in New Zealand's Southern Alps changes
from braided to anastomosed as it aggrades in the realm
of active footwall subsidence, as evidenced by the
terrace that plunges beneath the active floodplain. The
active floodplain narrows, and the channel becomes
single-threaded, deeper, and more sinuous as it crosses
the hanging-wall fold. Modified after Amos
et al.
(2007).
Fig. 8.13
Planform changes related to active uplifts,
western Pamir foreland.
Satellite imagery and interpretations of rivers and
structures along the Amu Darya (Oxus River) bounding
the arid, Afghanistan-Tadzhikistan border. See Plate 3
for color imagery. Flow is toward the lower right in all
images. Changes in planform river patterns appear
intimately associated with tectonic controls exerted by
folding, tilting, and possibly faulting. A, B. Image and
interpretation of upstream reach of the Amu Darya
showing transitions from braided to meandering to
straight channels as an active uplift is approached.
C, D. Image and interpretation of downstream reach
of the Amu Darya. Note the abrupt narrowing of the
right-hand tributary channel (C) as it crosses the zone
of uplift.
Most of these characteristics are seen in the
Mackenzie basin, New Zealand, where Irishman
Creek crosses a thrust fault and its associated
hanging-wall anticline (Fig. 8.12). Active footwall
aggradation is denoted by a change from braided
to anastomosed channels. Late Pleistocene river
terraces on the footwall plunge gently toward the
hanging wall and then disappear beneath the
active floodplain nearly a kilometer from the fault
itself. Presumably, footwall subsidence, driven by
the load of the hanging wall, caused downward
deflection of the river terrace. As the river crosses
into the hanging wall, it becomes single-threaded,
narrower, more incised, and more sinuous.
Second, in satellite images of the foreland fold
belt to the west of the Pamir Range (Fig. 8.13
and Plate 3), a repetitive planform pattern
is apparent. Upstream of channel constrictions,
broad meander belts are present and are
typically associated with braided channels
farther upstream. This transition from braided to
meandering in the middle of large basins
suggests that fluvial gradients progressively
