Geology Reference
In-Depth Information
zone of decreased
gradient, ponding,
forced deposition
uplifted surface
A
unperturbed
alluvial gradient
fold scarp
(knickpoint)
fixed
elevation
point
first
folding
event
t
0
axial surfaces
B
alluvial fill in
piggyback basin
steepened, but
retreating river profile
knickpoint
migration
3
2
1
0
adjustment
to
folding
original profile
t
1
new space for
sediment accumulation,
decreased gradient
Rivers Crossing Folds
C
deformed river
profile 3
new
knickpoint
Fig. 9.22
Interactions of an
antecedent stream with a
growing fold.
Fold uplift creates accommodation
space for sediment in the piggyback
basin. Deposition in the piggyback
basin allows the stream gradient to
steepen across the fold, but also
raises the river above the surrounding
plain. Such deposition promotes
avulsion, often away from the former
water gap. With each increment of
uplift, a knickpoint forms at the
downstream end of the fold and
propagates upstream. Modified from
Burbank
et al.
(1996c).
second
folding
event
t
2
fold widening & uplift
steepened, but
retreating river profile
gentler
gradient
D
renewed
knickpoint
migration
7
6
5
4
adjustment
to
folding
deposition in
response to 2nd
folding event
t
3
edge of the older fold (Fig. 9.23). As is typical
with fans, a suite of radial channels emanated
from each fan apex. During growth of the new
fold, most of these channels were defeated,
leaving elongate linear valleys and numerous
wind gaps that now decorate the fan crest. Owing
both to the lateral continuity of the new fold and
to the fact that most of the modern fans have bev-
eled laterally several kilometers into the upstream
flank of the fold, most defeated streams do not
appear to have been diverted around the fold tip.
Instead, a few of the antecedent channels have
gathered the flow from defeated ones and
sustained their courses across the new fold
(Fig. 9.23). The relationships between the fans,
channels, and the new fold change along strike.
Beyond the fold tip, the lateral extents of the old
and modern fans are the same. At the tip, the
modern fan margin has been deflected
∼
5 km
away from the tip. Farther along the continuous
fold, modern fans have radii that are 3-5 km
shorter than the older fans that preceded them.
The piggyback basin is aggrading with fan sedi-
ments and, depending on that rate of aggradation
