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(b)
(a)
0.1
Load, 1 km high
50 km wide
D = 10
24
N m
D = 10
25
N m
D = 10
26
N m
0
1
2
3
0
100
200
300 km
−
0.1
Figure 10.35.
(a) Cross sections of sedimentary basins formed on an elastic
lithosphere due to a 1-km-high, 50-km-wide two-dimensional load. Load density and
sediment density are 2.4 × 10
3
kg m
−3
. The three curves show the difference in basin
cross sections for a lithosphere with flexural rigidity,
D
,of10
24
,10
25
and
10
26
Nm. (From Beaumont (1981).) (b) Deformation of the surface of a viscoelastic
lithosphere by a surface load. The initial deformation is the same as for an elastic
lithosphere (a). However, with time, the viscoelastic lithosphere allows the stress to
relax; so the deformation evolves to curves 2 and 3. The final stage, if the load is left
in place, would be local isostatic equilibrium. (From Quinlan and Beaumont
(1984).)
the dynamic viscosity. This means that the basin becomes deeper and narrower
with time, evolving towards local isostatic equilibrium (Fig. 10.35(b)). The rate
at which this relaxation occurs is characterized by the viscoelastic relaxation
time
τ
.
Figure 10.36 shows how loading can produce a foreland basin. Figure 10.37
shows a cross section of the best-fitting flexural model for the Alberta basin.
This Canadian basin was formed on the North American plate east of the Rocky
Mountains as a consequence of thrusting and emplacement from the west of the
Rocky Mountains. Thrusting in the Rocky Mountains began in the late Jurassic
(approximately 140 Ma) and lasted about 100 Ma. From 35 Ma to the present,
massive erosion (and hence isostatic uplift) has taken place across the region
represented by the model. Almost 10 km of material has been eroded from the
centre of the thrust pile and about 3 km from its western edge. Most of the
Paskapoo formation (dense stipple) has been eroded and many of the older units
outcrop away from the Rocky Mountains. The model shown in Figure 10.37
was calculated for a thin viscoelastic plate with flexural rigidity 10
25
Nm and a
viscous relaxation time of 27.5 Ma overlying the asthenosphere.
The Appalachian basin has been modelled similarly, but, in this case, the
best fit is obtained with a temperature-dependent viscoelastic model that has an
approximately elastic thickness for the lithosphere of 80-90 km. Sections across
