Geology Reference
In-Depth Information
-300 m
2
/yr
Taiwanese Flux
Steady State
erosional
efflux
accretionary
influx
3 mm/yr
foreland
basin
Backstop
(reference)
3 mm/
yr
no internal
shortening
2-3°
0 mm/yr
frontal
accretion
~300 m
2
/yr
km
+300 m
2
/yr
17°
22
km
+920 m
2
/yr
20°
-920 m
2
/yr
Fig. 10.21
Flux steady state for the Central Range of Taiwan.
The accretionary influx is divided between the upper 7 km, which is underplated (and later eroded from the Central
Range), and the lower 22 km, which is subducted. The accretionary flux equals the layer thickness times the
convergence rate. Note that the 300 m
2
/yr flux is underplated beneath the Central Range and then is balanced by
erosion from the range. The crustal subduction is not balanced within the Central Range, but, because it subducts,
it does not formally contribute to the mass of the orogen. Modified after Simoes and Avouac (2006).
In a
flux steady state
, the influx of rock into a
range is balanced by the efflux out of the
range (Fig. 10.21). Consequently, the volume or
cross-sectional area of the range remains
constant through time. Most commonly, the
efflux is simply due to geomorphic erosion, but
any type of tectonic removal of rock from the
range will also add to the efflux. An assessment
of a flux steady state requires both geophysical
determinations of the rate of tectonic additions
and losses from a range, as well as geomorphic
measures of volumetric erosion rates.
A
thermal steady state
exists when the spatial
distribution of rock temperatures with respect
to the surface is unchanging. Such a state occurs
when the conduction of heat and the advection
of rocks (which carry their heat with them)
balance the cooling at the surface that occurs
due to erosion (or heating in response to
deposition), such that the depth-dependent
thermal structure of a range is time-invariant. In
order to attain a thermal steady state, the velocity
field must also be in steady state, such that heat
advection is time-invariant. At present, few data
exist to assess a thermal steady state: present-
day subsurface temperatures are poorly defined,
and reconstruction of past thermal patterns is
equally fragmentary.
A thermal steady state is a necessary
precondition for an
exhumational steady state
,
whereby rates of erosion are spatially invariant.
Commonly, long-term erosion rates are assessed
using low-temperature thermochronometric
techniques, such as fission-track or (U-Th)/He
dating, in which a cooling age, a known
closure temperature, and a geothermal gradient
are combined to estimate the average erosion
rate since a mineral passed through its closure
temperature. Minerals used to assess an exhu-
mational steady state must have had their ages
