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reset due to heating above their closure
temperature. Unreset mineral ages will reveal
pre-orogenic ages. If particle pathways through
a range and the thermal structure of the range
are time-invariant, then an exhumational steady
state likely exists (Fig. 10.22A), such that the
cooling ages at any site within the range will be
constant through time. For a contractional range
in which rock is accreted from the side, numerical
models predict nested zones of cooling ages for
each thermochronometer (Willett and Brandon,
2002). In this case (Fig. 10.22B), the mineral that
has the lowest closure temperature, such that its
cooling age is locked in closest to the surface,
will occupy the broadest zone of reset ages,
and this zone will become increasingly narrow
for minerals with higher closure temperatures.
During growth of a range and prior to steady
state, the zone of reset ages should gradually
expand, but it should reach a persistent extent at
steady state (Fig. 10.22C).
Until we acquire an ability to go backward or
forward in time, a strict test of an exhumational
steady state will remain elusive, because we cannot
assess, for example, what the actual pattern of
cooling ages was two million years ago. Two quite
different approaches, however, can be used to
argue in favor of an exhumational steady state.
First, the presence of nested zones of cooling ages
that have a distribution consistent with the known
kinematics of a range of orogens support an infer-
ence of steady state, because this geometry sug-
gests a sustained and consistent thermal history
for rocks across an orogen. In several well-dated
orogens, such as the Olympic Mountains in
Washington (Batt
et al
., 2001) or the Central Range
of Taiwan (Fuller
et al
., 2006), nested zones of
cooling ages are observed (Fig. 10.23).
An alternative approach to assess an
exhumational steady state relies on changes in
the cooling ages of detrital minerals in well-
dated stratigraphic records (Bernet
et al
., 2001,
2004; Bullen
et al
., 2001) (Fig. 10.24). These ages
record the time it took for each mineral to travel,
first, to the surface from its passage through its
closure temperature at some depth and, second,
from the surface to the basin where it became
part of the depositional record. Typically, this
last transport phase is far shorter than the earlier
Thermochronometers
with different closure
isotherms
Cooling
Trajectories
T
A
T
B
T
C
T
D
Particle Paths
S
Closure Isotherms
Point Where Age Is Locked In
A
D
Reset Age
Zones
C
B
A
Pre-orogenic age
Lo
Closure Temp.
Hi
Steady-State
Distribution of Ages
A
B
C
D
Reset
Ages
B
Distance
Pre-orogenic age
T
1
-T
2
Age-Distance
Evolution for Thermo-
chronometer A
T
3
T
4
T
5
Steady State : T
6
C
Distance
Fig. 10.22
Cooling ages in a steady-state orogen.
A. Kinematic model and particle pathways for a
convergent orogen. The positions of closure isotherms
for thermochronometers with different closure
temperatures (
T
A
<
T
B
<
T
C
<
T
D
) are warped by lateral
and vertical advection of rock within the orogen.
B. Predicted distribution of ages across an orogen at
steady state as a function of closure temperature (lower
closure temperatures are reset across a broader zone).
C. Temporal evolution of the spatial distribution of
cooling ages from initial (
T
1
) to steady state (
T
6
).
Modified after Willett and Brandon (2002).
phase and is ignored. The difference between
the cooling age of the mineral and its depositional
age is defined as the “lag time.” Short lag times
indicate that the depth-to-surface transect was
rapid and imply high rates of erosion at that
time. The typical strategy in stratigraphic lag-
time studies is, first, to date 50-100 individual
grains, such as apatite or zircon, from a
succession of well-dated stratigraphic horizons.
Next, the distribution of ages at each horizon is
analyzed statistically to pick out the youngest
population of ages (Brandon, 2002), which is
