Geology Reference
In-Depth Information
toe. But, because pristine material continuously
migrates through the axial surface at the toe
and, thereby, adds new uneroded material to
the base of the limb, the maximum amount
of net incision occurs closer to the top of the
fold (Fig. 9.18C). In contrast, a limb that is
lengthening through the fold crest or that has a
constant length but is steepening via rotation is
predicted to have the most channel incision
near the lower part of the fold limb. Whereas
the boundary conditions for these models will
certainly not all be matched by many field
settings, the modeled patterns of incision pro-
vide a means to infer fold kinematics when
other means may be lacking.
can serve to highlight the nature and rates of
landscape change and perhaps can provide
quantitative insights on controlling variables.
Consider the impact of a knickpoint on a
trunk stream as it sweeps progressively upstream
past numerous tributary channels. Each tributary
will experience a relative drop in base level at
the junction with the main stem. This drop is
equivalent to increasing the rock uplift rate of
the tributary catchment, such that we expect a
knickpoint then to migrate up each tributary as
well (Fig. 9.11). The Yellow River on the northern
Tibetan Plateau provides a spectacular example
of the impact of knickpoint migration on tribu-
tary channels and on rates of erosion (Harkins
et al.
, 2007). In the upstream end of the study
area, the Yellow River progressively steepens,
producing an upward convexity, as it descends
∼
900 m over the next 350 km (Fig. 9.19A). Two
basins at the downstream end of the study area
contain Plio-Pleistocene fills that rise
∼
600 m
above the current level of the Yellow River.
The tops of these fills are aligned and project
upstream approximately to the level of the
top of the modern Yellow River knickpoint
(Fig. 9.19A). Along this projected gradient, sev-
eral strath terraces are preserved, suggesting
that the Yellow River previously flowed along
this more even, gentler gradient. Recent magne-
tostratigraphic studies (Craddock
et al.
, 2010)
show that the Tongde Basin was actively
aggrading until
∼
0.5 Ma. Hence, if we assume
that the accelerated incision of the Yellow River
began at 0.5 Ma, the knickpoint migrated at a
mean rate of
∼
50 cm/yr as it swept more than
250 km upstream! Along dozens of tributary
channels, Harkins
et al.
(2007) used DEM
analysis to document pronounced knickpoints
that separate steep lower reaches from gentler
upper reaches (Fig. 9.19B). In order to estimate
the pre-knickpoint gradient of each tributary,
Harkins
et al.
(2007) calculated the normalized
steepness index,
k
sn
, (Eqn 9.2) for the river reach
above the knickpoint. They then projected a
channel with the same steepness downstream
in order to estimate the former elevation of
the tributary mouth (Fig. 9.19B). When the
reconstructed elevations of the former mouths
of numerous tributaries are plotted along the
Transient landscapes
Because climate changes occur at rates that
commonly outpace a landscape's tendency to
reach equilibrium with the current climate, most
landscapes are technically in a transient state all
the time. Here, transient landscapes refer instead
to landscape changes that occur, for example, in
response to changes in the rate of base-level fall,
rock uplift, or precipitation. Stepwise changes in
any of these boundary conditions will force a
landscape toward a new configuration. When a
landscape is captured in this transitional interval,
insights on both the nature and rates of change,
as well as on key controlling variables, can
commonly be obtained.
Knickpoints
Landscapes that are in transition from one state
to another can provide insights on how surface
and tectonic processes respond to changing
boundary conditions. The migration of a knick-
point, a deceleration in the rate of rock uplift, a
change in climate, or the growth of a fold, will
each induce changes that will affect the rates
of surface processes and ultimately modify
the shape of the landscape. Such transitional
landscapes commonly provide opportunities
to examine landscape regions that have been
affected by the change or ones that have not yet
been affected. The contrasts between such areas
