Geology Reference
In-Depth Information
Reconstructing Relative Uplift & Incision From Relict Channel Profiles
relict
landscape
2.8
reconstructed
stream profile
3
relict
landscape
reconstructed
stream profile
intermediate
landscape
2.4
relict
2
relict
2.0
total
incision
total
incision
modern
intermediate
1
1.6
modern
0
4
8
12
0
10
20
30
40
Distance (km)
Distance (km)
A: One-Stage
B: Two-Stage
Fig. 10.55
Relict channels, knickpoints, and evidence for pulsed uplift.
Relict landscapes lie above knickpoints in these topographic profiles from the Sierra Nevada, California. Projections of
the relict profiles based on the channel concavity serve to define the channel incision, as well as the relief in the relict
landscapes. A. Single knickpoint and one inferred phase of rock uplift and incision. B. Two knickpoints and two
pulses of inferred rock uplift. Modified after Clark
et al
. (2005b).
the glaciers and the landscape likely depends on
the underlying rate of rock uplift. High rates will
encourage high relief, rapid erosion both
beneath and adjacent to the glacier, and
abundant down-glacier transport of supraglacial
debris, whereas low rates will lead to lower
relief, lower glacial erosion rates, and smaller
supraglacial loads (Fig. 10.54).
The existence of high mountain ranges or
plateaus prompts questions about how and
when they attained their current height. Did
they rise steadily over time, pop up in a single
interval of rapid uplift, or experience several
punctuated periods of rapid rock uplift? Both
thermochronology (Reiners, 2005) and river
profiles (Harkins
et al
., 2007) can be used to
reconstruct intervals of relative rock uplift or
base-level lowering. When used in combination,
these approaches sometimes allow both the
magnitude and timing of events of rock uplift to
be deduced.
One such study in the Sierra Nevada of
California uses relict landscapes, modern
river profiles, and (U-Th)/He cooling ages to
delineate a pulsed uplift history (Clark
et al
.,
2005b). Whereas the cooling ages have specific
relevance directly to the Sierra, the techniques
used to indentify relict landscapes and pulsed
uplift could be widely applicable. Topographic
profiles across the Sierran landscape reveal
multiple knickpoints that separate regions of
contrasting steepness (Fig. 10.55). Clearly, the
presence of knickpoints that are not litho-
logically controlled indicates that this landscape
is in a transient state. When plotted in log
area-log slope space (Figs 8.4 and 8.8), data
from reaches above the knickpoints define the
concavities of relict channels. When this con-
cavity is used to project the relict channel pro-
file laterally to a position above a current local
base level (as defined by a tributary-main stem
junction or a basin floor), the difference in
elevation between the projected profile and the
current elevation of the local base level provides
an estimate both of the magnitude of incision
since that reconstructed profile was abandoned
and of the paleorelief of the relict channel
(Fig. 10.54A). Where more than one knickpoint
occurs along a profile, several stages of incision
can be identified (Fig. 10.54B). The presence of
relict landscapes and multiple knickpoints in
several different locations supports interpreta-
tions of pulsed uplift (Clark
et al
., 2005b). In the
Sierra, the most recent pulse appears to be
younger than
∼
3.5 Ma and is consistent with the
removal of a dense, lithospheric root from
beneath the range (Fig. 10.45) (Saleeby and
Foster, 2004). An older interval of rock uplift is
interpreted to have begun sometime after 32 Ma,
but currently awaits better temporal constraints
(Clark
et al
., 2005b). Despite the somewhat
weak time constraints, the geomorphic analyses
