Geology Reference
In-Depth Information
Models of impact of Effective Elastic Thickness on Deformation
T
e
T
e
= 16 km
= 8 km
T
e
= 4 km
T
e
= 2 km
range
height (km)
2.5
2.5
2.5
2.5
cumulative
coseismic
after 5 km
reverse slip
60 km
60
thrust
fault
2.0 km
maxim
um
ra
nge heigh
t
1.5 km
earthquake cycle
plus flexure due to load;
no erosion or deposition
1.5 km
max
imum
basin dep
th
2 km
profile without
erosion
final
topography
erosion & resulting
geologic structure
with erosion &
deposition
basin fill
width = 110 km
60 km
35 km
20 km
Flexural wavelength
Range & basin width
Effective thickness (T
e
)
increasing
Fig. 11.15
Matrix of mountain range-scale models in which the effective elastic thickness (
T
e
) is varied.
(Top row) The pattern of vertical displacements associated with a prescribed total slip of 5 km on the thrust fault is
shown. All patterns are the same and show 2.5 km of maximum rock uplift on the hanging-wall block, and <1 km of
maximum subsidence on the footwall block. Note the clear coseismic asymmetry in which hanging-wall uplift far
exceeds footwall subsidence. (Middle row) The pattern is now allowed to flex in response to relaxation at depth: the
resulting sum of dislocation plus isostatically driven flexure is shown. The difference in the plots reflects the decline
in the assumed effective elastic thickness from 16 km to 2 km from left to right. The width scale of the resulting
mountain range-basin pair declines dramatically from 100 to 20 km. (Bottom row) A crude redistribution of mass by
erosion of the mountain crest and deposition in the adjacent basin. The scale and the asymmetry of the geological
structure vary significantly with the flexural rigidity. Modified after King
et al.
(1988).
generated in the bounding basin (e.g., Flemings
and Jordan, 1989, 1990). Again, no attention was
paid to the details of the processes generating
the debris, nor of the delivery mechanisms of
that debris to the basin edge. The focus was
on the details of the stratigraphic package, and
how these reflected either episodes of rapid
motion along the fault or climate variations.
The first numerical model both to move to a
two-dimensional planform and to incorporate
channels as critical elements in the evolving
landscape was that of Koons (1989), who
focused on the Southern Alps of New Zealand.
Koons incorporated a simple, geometrically
reasonable, and geologically defensible rock
uplift pattern. He inserted a set of streams that
lived in specific places within this uplift pattern,
that did not move, and that draped smoothly
and logarithmically from a channel head near
the uplift maximum to the base level of the sea,
and he dictated that the surface processes that
operated upon the interfluves between these
stream channels could be approximated by
diffusion that was allowed to vary spatially. In
particular, in an effort to mimic the strong
orographic forcing caused by the fact that the
Southern Alps are embedded in the westerlies,
he tied the landscape diffusivity to the rainfall
rate. As shown in Fig. 11.16, the essence of the
ridge topography is simulated well using this
simple set of model rules. Initially, the ridgeline
is highest near the coast on an interfluve that is
