Geology Reference
In-Depth Information
Summary
Age Probability and Relief
Erosion Rate (km/Myr)
Relief
(km)
The balance between erosion and rock uplift
lies at the core of tectonic geomorphology.
Most landforms in actively deforming areas
result from this interplay. New geochronologi-
cal tools have greatly enhanced our ability to
date surfaces, landforms, and events with
increasing precision and accuracy. Underpinning
any rate calculations, this new temporal control
permits both an assessment of conceptual mod-
els, such as that of a topographic steady state,
and a calibration of rates of erosion, rock and
surface uplift, deposition, and deformation -
those factors that modulate the evolution of
landscapes.
A key realization of recent studies is that
mechanical erosion can proceed at rates as high
as 5-10 mm/yr. Although the possibility must be
assessed in each situation, it is no longer neces-
sary to attribute high rates of cooling (
0.5
1
.
0
1
.
5
2.0
6.0
4.0
2.0
0
50
0
50 0
50 0
50
Age (Ma)
Fig. 7.31
Cooling ages as a function of changes in
relief and erosion rate.
Probability distributions of cooling ages for a mineral
with a closure temperature of 350
°
C. A Gaussian-shaped
hypsometry is assumed. For any given erosion rate, the
span of ages increases with increasing relief, although
the mean age remains constant. At increasingly higher
erosion rates, the mean age gets systematically younger,
whereas the span of ages decreases for any fixed
amount of relief. Modified after Brewer
et al
. (2003).
>
°
100
C/
>
Myr) or rapid decompression (
1 kbar/Myr) pri-
marily to tectonic denudation via normal fault-
ing. In some mountain belts, erosion by glaciers,
rivers, or landslides has now been measured at
rates rapid enough to accommodate nearly all of
the documented rates of unloading.
The increasingly widespread availability of
digitized topography is permitting rapid
characterization of landscapes and facilitates
easy calculation of both key topographic vari-
ables, such as relief and slope, and geophysical
variables, such as mean surface elevation or
eroded volumes of rock. Although some of
these variables may be statistically tied to
erosion, by themselves, they tell us little about
the mechanisms of erosion. Key frontiers
related to erosion and rock uplift in tectonic-
geomorphic studies include the documentation
and quantification of erosion processes,
discovery of new ways to define key variables,
such as paleoaltitude or geothermal gradients,
and a fuller exploration of the interplay between
deformation and erosion in contrasting climatic
and tectonic regimes.
change the expected distribution of ages (Fig.
7.31). The range of ages (oldest-youngest)
increases as relief grows, whereas the mean cool-
ing age gets younger as erosion rates increase.
Such changes could underpin a research strategy
for sampling the stratigraphic record in order to
deduce changes in both paleorelief and erosion
rate through time. The challenges in applying
such a technique, however, are many (Stock and
Montgomery, 1996): sampled sediment must
have been derived from the same catchment over
time; the mineral used for dating must consist-
ently be present at both the top and bottom of
the catchment; and any changes in erosion rate
must have happened sufficiently long ago and
then be sustained at a constant new rate such
that the age-elevation relationship in the moun-
tains has been reset throughout the eroding
topography. Despite these caveats, in carefully
chosen settings, paleorelief can be estimated
with more clarity than at any time in the past.
