Geology Reference
In-Depth Information
et al
. (2004) used a stream-power-based incision
rule and appealed to a model of tilting of
the range in which a dense keel was allowed
to disconnect from the base of the crust.
The flexural-isostatic rock uplift pattern resulting
from this removal, and flexural rigidity that
decayed strongly toward the Basin and Range,
produced a strong asymmetry in rock uplift,
tilting the range toward the west. The river
responded to this tilting by initiating a knickpoint
at the western edge of the range. This knickpoint
subsequently propagated eastward.
Stock
et al
. (2004) argue that the rapid incision
in the 3-1 Ma interval reflects the translation of the
steep knickpoint past the site of the caves, with
low rates of incision left in the wake of the knick-
point. Note that, in the absence of several dated
caves, one could not tell this story: one long-term
average rate derived from a single dated terrace or
cave is insufficient to discern changes in incision
through time. This wave of rapid incision associ-
ated with the passage of the knickpoint up the
main stem of Kings Canyon in turn sends knick-
points up its tributaries, which show strong con-
vexities in their profiles. Importantly, a large
fraction of the core of the range is effectively
“ignorant” of these events, because the hillslopes
on the interfluves in the large regions between
major drainages have yet to be influenced by the
drop in base level of these major streams or their
tributaries (Stock
et al
., 2005; Clark
et al
., 2005b).
into flat-lying Cenozoic stratigraphy to the north.
The present locations of 37 box-headed canyons,
some with spectacular waterfalls, represent the
target for models of how the fluvial system has
operated (Fig. 11.14). Berlin and Anderson
(2007) proceeded by crafting a generic incision
model in which horizontal incision rate was
governed by the drainage area upstream of the
knickpoint, taken to some power:
d
d
x
(11.12)
c
==
aA
p
where
c
is the celerity or wave speed of the knick-
point in the upstream (
x
) direction,
A
is the
upstream drainage area, and
a
and
p
are empiri-
cally determined constants. The system is initiated
by a base-level drop on the Colorado River at a
time loosely constrained by geological observa-
tions on the southern edge of the plateau. The
single knickpoint works its way upstream to the
first tributary junction, at which point it bifurcates
into two knickpoints. Each of these knickpoints
will proceed more slowly than the first, because
the drainage areas of the tributaries are smaller
than the trunk below the junction. This process of
bifurcation, repeated at each tributary junction,
therefore leads to stepwise slowing of the growing
set of knickpoints (Fig. 8.10A). Any particular
model run is assessed by the misfit between the
final locations of model knickpoints and actual
canyon heads. The model that best explains the
present pattern of canyon heads (Fig. 11.14) is one
in which the power
p
= 0.5 (as found in the Waipaoa
case), meaning that the speed of the knickpoint
goes as the square root of drainage area.
As discussed in Chapter 9, a similar situation
exists along the Yellow River (Fig. 9.19), in which
the propagation of knickpoints through the Yellow
River and its tributaries has served to extract a
large volume of sediment from a series of basins
in the last half-million years (Harkins
et al
., 2007).
Sets of knickpoints
Given the importance of fluvial incision in
landscape evolution, and the fact that, in some
cases, the fluvial response to change in base
level is through propagation of knickpoints, sig-
nificant attention has focused on natural experi-
ments that display numerous knickpoints. In
both New Zealand's Waipaoa catchment (Crosby
and Whipple, 2006; Crosby
et al
., 2007) and in
Colorado's Roan Plateau (Berlin and Anderson,
2007), tens of knickpoints can be attributed to a
single change in the base level of the system.
The Waipaoa case we have already discussed in
Chapter 8 (see Figs 8.7 and 8.11). In the case of
the Roan Plateau, incision of the adjacent
Colorado River has cast off a wave of incision
Mountain range-scale models
At the scale of mountain ranges, numerical
models have evolved significantly over the last
two decades. Tucker and Hancock (2010) provide
