Geology Reference
In-Depth Information
propagate up the fluvial system. As this channel
erosion passes the base of an adjacent hillslope,
it causes the hillslope gradient to increase and will
initiate a change in the rate of hillslope processes.
These hillslope processes could eventually affect
the interfluves and finally the shape of the entire
catchment. Large geomorphic elements, such as
the catchment shape, are unlikely to be affected
by individual seismic events. Thus, a clear
hierarchy of response times exists (rivers
respond sooner than hillslopes, etc.), and an
analogous hierarchy of topographic inertia is
evident (catchment shape is resistant to change,
whereas river gradients are susceptible to small
perturbations).
Consideration of the scaling of elements in a
given geomorphic system, of response times or
inertia of those elements with respect to imposed
changes, and of the rates, magnitude, and dura-
tion of different styles of tectonic forcing suggests
a way in which the apparently incompatible
landscape evolution concepts of Davis, Penck,
and Hack can be reconciled. In fact, recent
numerical models of tectonically perturbed land-
scapes have explicitly addressed this problem.
Using a surface-process model that links channel
incision, sediment transport, and hillslope ero-
sion, Kooi and Beaumont (1996) developed a
model that predicts a lag between the onset of
deformation and the response of the geomorphic
system to that deformation (Fig. 1.4). The overall
response of the geomorphic system and the mag-
nitude of the lag depend strongly on the nature
of the tectonic forcing. If the forcing is impulsive
(
à la
Davis), then the topography is rapidly
created and simply degrades through time. If the
deformation increases to a maximum through
time and then wanes (
à la
Penck), topography
gradually builds in the face of progressively
increasing rates of erosion. The maximum topo-
graphic expression occurs slightly after the rate
of deformation begins to wane, because the rock
uplift still outpaces the rate of erosion. Finally, in
the latter half of the cycle, the topography wanes,
despite gradually diminishing rates of erosion.
If the tectonic forcing is continuously sustained,
then the Kooi and Beaumont model predicts
that, after an initial interval of building of
topography, rates of rock uplift and erosion will
Impulsive
Process-
Response
Models
1.0
tectonic
flux
lag
0.5
sediment
flux
0.0
0
10
20
30
40
50
Time (Myr)
Varying
waxing
waning
1.0
sediment
flux
tectonic
flux
lag
0.5
0.0
0
10
20
30
40
50
Time (Myr)
Sustained
lag
1.0
sediment
flux
tectonic
flux
0.5
0.0
0
10
20
30
40
50
Time (Myr)
Fig. 1.4
Tectonic versus sediment flux process-
response models.
Duration and magnitude of rock uplift (the vertical
tectonic flux) are compared with the erosional sediment
flux from uplifted mountains. The three scenarios
(impulsive, varying, sustained) are analogous to the
models of Davis, Penck, and Hack (Fig. 1.2). Note that the
time lag between tectonic forcing and sediment response
is variable. Modified after Kooi and Beaumont (1996).
become balanced (
à la
Hack), and the topogra-
phy will attain a persistent dynamic equilibrium
(Fig. 1.4). A change in the rate of tectonic forcing
would push the system toward a new equilib-
rium, whereas cessation of deformation would
return the system to an almost Davisian state in
which the topography is systematically degraded.
