Rates of erosion and uplift - Tectonic Geomorphology - page 226

calculating erosion rates, the estimated rates
would vary from about 5 mm/yr in the valley bot-
toms to about 1 mm/yr at the summits. Whereas
these rates clearly bracket the actual erosion rate
(2 mm/yr), the erosion rate associated with any
individual age is likely to be misleading.
Given such uncertainties, why bother with this
thermochronological approach? The strongest
argument is that cooling histories provide a
useful long-term perspective that is related to
denudation. Such a perspective is difficult to
achieve without a means of measuring time over
millions of years. Second, the example given
here is perhaps not typical. If mean erosion rates
or the topography relief were lower, the pertur-
bation of the isotherms and apparent rate
variations would be less. Third, even with the val-
ley-to-summit age differences, the data define the
limits of past cooling rates and provide useful
brackets on acceptable long-term erosion rates.
Finally, as long as a reasonable kinematic geom-
etry is known for the deformation, numerical
modeling (e.g., Braun, 2003) can now be used to
predict both the topography of subsurface iso-
therms and the three-dimensional array of cool-
ing ages at the surface that would be expected
for a given erosion rate (Plate 4). Hence, the
effects on ages of the competition of erosion
with the lateral and vertical advection of rocks,
heat, and fluids in the crust can be modeled
(Bollinger et al. , 2006; Ehlers and Farley, 2003;
Whipp and Ehlers, 2007). Even the effects of
changes in relief or erosion rate on the array of
surface ages can be explored (Braun, 2002;
Braun et al. , 2006; Ehlers et al. , 2006). Such
modeling efforts provide an increasingly robust
context for the interpretation of the implications
and uncertainties of cooling ages for reconstruct-
ing the erosion histories of mountain belts.
To create the best estimates of the past erosion
rates and changes in topography, extensive
arrays of radiometric ages, reliable kinematic
geometries, and considerable computational anal-
ysis are needed. Kinematics become increas-
ingly important as the rate of lateral advection
increases with respect to rock uplift. The more
sophisticated numerical models can incorporate
measurable variables, such as heat production
and conductivity, as well as computed variables,

such as topographically driven fluid flow. All of
these factors affect the thermal structure of an
orogen and the resultant cooling ages.
In recent decades, tectonic-geomorphic studies
have benefited from efforts to develop high-
precision thermochronological analyses that are
sensitive to increasingly lower temperatures.
Because such temperatures occur in the shallow
subsurface, these thermochronometers provide
rather direct insights on rates of erosion during
the most recent intervals of time. For many
decades, apatite fission-track dating was the ther-
mochronometer that probed the lowest closure
temperature (110 ° C). Drawbacks with the apatite
fission-track approach include the typical depth
of the closure isotherm (3-5 km) and its large
uncertainties: typically ∼ 10%, but commonly
exceeding 50% in regimes of very rapid erosion
(Blythe et al. , 2007). More recently, the develop-
ment of the (U-Th)/He thermochronometer has
enabled dating of apatite and zircon with closure
temperatures of ∼ 60-70 ° C and ∼ 180 ° C, respec-
tively, and with typical precisions of < 10%, even
for young ages (Reiners et al. , 2004; Wolf et al. ,
1996). Thermal histories down to temperatures of
30 ° C or less have been achieved by stepwise heat-
ing of apatite and modeling of concentration gra-
dient of 4 He due to thermal diffusion near the
outer margins of a grain (Shuster and Farley,
2005). New OSL applications to bedrock samples
are interpreted to record closure temperatures of
30-40 ° C (Herman et al. , 2010). Overall, expand-
ing use of these low-temperature thermochro-
nometers will permit much more detailed and
more reliable reconstruction of recent erosion at
time scales of 10 4 -10 6 years.
Wherever you are standing today, somewhere
beneath your feet (probably within a few
kilometers of the surface) rocks at temperatures
above their closure temperature would reveal
ages of 0 Ma, if you could date them. Although
closure temperatures have been defined for
many minerals (Table 7.2), cooling across these
temperatures does not represent an on/off
switch. For several thermochronological systems,
a well-defined thermal zone exists in which
some, but not all, of the products of radiometric
decay are retained. For apatite fission-track, for
example, tracks are annealed at temperatures