Geology Reference
In-Depth Information
Thermal Processes in Mountain Ranges
A
Normal faulting
Erosion
Fluid flow
Sedimentation
and compaction
Fig. 11.27
Processes governing thermal
evolution in faulted mountain ranges.
In both normal-faulted cases (A) and
thrust-faulted cases (B), heat moves by both
conduction and advection. The advection
occurs both because the rock and
groundwater are in motion and because the
surface itself is evolving due to geomorphic
processes. Isotherms are warped by these
processes, and they intersect the evolving
surface at different elevations. The data that
we use to interpret long-term rates of fault
motion in these settings are the times at
which a rock sampled at the surface crossed
its particular closure temperatures (
T
1
,
T
2
,
T
3
).
Careful interpretation of these ages requires
models of the evolving thermal structure of
the mountain mass. Modified after Ehlers
(2005).
Advection
Advection
Piggy-back basin
sedimentation
B
Thrust faulting
Erosion
Foreland basin
sedimentation
and compaction
Erosion
Fluid flow
Modeling the thermal field
assigned, and it steadily erodes, retaining its shape.
This uniformity is a good place to start, of course,
although, as we shall see, this set of assumptions
does not allow assessment of several important
scenarios. At the surface, the temperatures are
controlled by the atmospheric processes that are
commonly summarized with a lapse rate, declining
with elevation at a rate of, say, 6
One of the most common applications of
landscape evolution models has been in the
interpretation of thermochronometry data. The
thermochronometers being used have been
pushed to lower and lower closure temperatures,
so that they constrain mean exhumation rates of
less and less rock. The closure temperatures of
the systems have become low enough that the
rocks are recording temperatures that occur
close enough to the Earth's surface for the iso-
therms to be modified significantly by the pres-
ence of the surface. This warping of the thermal
structure must be modeled in order to interpret
the depth at which the rocks start their thermal
clocks (Fig. 7.20). In such cases, we may not
assume that isotherms are horizontal, planar sur-
faces unaffected by topography (Fig. 11.27 and
Plate 4).
Early models solved for the expected thermal
structure beneath a uniformly eroding landscape
under steady conditions: a topographic form is
C/km. Because
this gradient is much smaller than that within the
Earth (nominally 25
°
C/km), the thermal structure
is nearly uniform at the surface, so that the
topographic surface is almost an isotherm. At great
depths, the isotherms are flat: the Earth at these
depths knows nothing of thermal variations at
the surface. Between the topographic surface
and these depths, the isotherms must warp
from something closely mimicking the surface
to something that is flat. The isotherms are com-
pressed beneath valleys, spread apart beneath
ridgetops, and deformed near faults that juxta-
pose rocks with contrasting thermal states. The
spatial pattern of closure ages for particular
thermochronometers is predictable (Fig. 7.20), and
°
