Geology Reference
In-Depth Information
Oblique Collision
Taiwanese Fission-Track Ages
100
unreset ages
100 63
44
54
75
reset
ages
apatite FT ages
zircon FT ages
apatite age trend
zircon age trend
unreset ages
8
Relative
plate
motion
6
4
modeled ages as a
function of erosion rate
north
2
T
2
T
1
T
0
south
Time
0
200
250
300
0
50
100 150
Distance (km)
A
B
Fig. 10.23
Exhumational steady state and Taiwanese fission-track ages.
A. Schematic tectonic framework of Taiwan. Oblique collision of the Luzon arc causes a southward migration of the
deformation front over time (
T
0
-
T
1
-
T
2
). Reset fission-track ages (darker shading) delimit regions where more than
4 km of erosion has occurred. B. Fission-track ages projected on a northeast-southwest transect show two nested
zones: reset zircon fission-track ages with their higher closure temperature (
∼
230
°
C) lie farther from the leading edge
of deformation than do reset apatite ages (closure temperature
∼
100-120
°
C). Modeled distribution of ages for
different exhumation rates (dashed lines) are consistent with sustained erosion of at least 4 mm/yr within the reset
zone. Overall, these data are interpreted to indicate an exhumational steady state in a northwest-southeast cross-
section. The steady-state region is expanding toward the southwest as the collision progresses. Modified after Willett
et al
. (2003) and Fuller
et al
. (2006).
then compared with the sample's depositional
age to define the lag time, which is then plotted
versus time. Finally, changes in lag times during
the depositional record are used to assess (i)
when erosion was most rapid in the past (the
shortest lag times) and (ii) whether intervals of
uniform lag times exist, because such intervals
would be consistent with an exhumational
steady state (Bernet
et al
., 2001). This detrital
approach has the advantage of allowing us to go
back in time to see how cooling-age distributions
have changed during growth or decay of an
orogen. One limitation of this approach is that
we never know precisely where the detrital
grains were derived. Consequently, we cannot
assess whether the spatial distribution of cooling
ages was constant, as the formal definition of
exhumational steady state requires.
In a
topographic steady state
, average
topographic characteristics, such as mean ele-
vation and relief, may vary spatially, but are
independent of time, such that the same topo-
graphic pattern would be predicted to persist
through time (Figs 1.8 and 10.1). Perhaps sur-
prisingly, in an orogen in which rock is being
advected laterally, a balance between rock uplift
rates and erosion rates at a point may be insuf-
ficient to define a topographic steady state. In
particular, an imbalance can be created due to
lateral advection of sloping topography that
causes an increase or decrease in surface height
at a point that is independent of rock uplift (
u
),
subsidence (−
u
), erosion (
e
), or deposition (−
e
),
such that:
d
h
/d
t
=
u
−
e
−
v
d
h
/d
x
(10.3)
where
v
equals the velocity of lateral advection
in the plane of a given cross-section, d
h
/d
t
equals the change in height with time
t
, and
d
h
/d
x
equals the average slope of the topogra-
phy that is being laterally advected (Fig. 10.25).
Hence, in order to sustain a steady-state
topography, if the surface slope trends down-
ward with respect to the advection direction,
then not only must erosion balance rock uplift,
but it must also balance
v
d
h
/d
x
.
