Geoscience Reference
In-Depth Information
0
0
2
2
Michigan basin
Illinois basin
4
500
400
300
Age (Ma)
500
400
300
Age (Ma)
Figure 10.34.
Depth to particular sedimentary horizons versus age of the sediment.
Data are from two wells drilled in the Illinois basin and one in the Michigan basin.
Solid curves are 50-Ma exponentials. If the subsidence were due to thermal
contraction of the lithosphere, the depths should plot approximately on these
exponential curves. Horizontal dashed lines indicate major unconformities in the
sedimentary sequence. The Cambrian-Carboniferous (530-360 Ma) subsidence of
the Illinois basin and the Ordovician-Carboniferous (460-360 Ma) subsidence of the
Michigan basin could be due to thermal contraction of the lithosphere. (After Sleep
et al
.(1980). Reproduced with permission from the
Annual Reviews of Earth and
Planetary Sciences
, vol. 8,
c
1980 by Annual Reviews Inc.)
other than the observed subsidence (i.e., no volcanic activity or known uplift)
is not altogether satisfactory, and further work on the basement geology and
subsidence must be done before these basins can be proven to be of thermal
origin. Other explanations are possible.
10.3.5 Flexural basins
Flexure of the lithosphere by a load at the surface produces a foreland basin,
of which the Canadian Alberta Basin and the Appalachian Basin are excellent
examples. If a load or thrust sheet is placed on the lithosphere, the lithosphere
subsides. To study problems of this type, it is not enough to assume local iso-
static equilibrium; rather, the lithosphere must be modelled as a thin, elastic or
viscoelastic plate, and regional deformation must be calculated as discussed in
Section 5.7. The width of a flexural basin is a function of the lithospheric thick-
ness. For a given load, a thick lithosphere supports a wider basin than a thin
lithosphere, as one would intuitively expect. Since isostatic balance requires the
mass of mantle material displaced from beneath the lithosphere to equal the mass
of the load, a wide basin on a thick lithosphere is shallower than a narrow basin
on a thin lithosphere (Fig. 10.35(a)).
In the case of a viscoelastic lithosphere, the initial flexure is the same as for an
elastic lithosphere, but the lithosphere relaxes with time. A viscoelastic material
behaves elastically on a short timescale and viscously on a long timescale. The
viscoelastic relaxation time
τ
, the time taken for any stress to relax to 1
/
eof
its original value, is defined by
τ
=
2
η/
E
,where
E
is Young's modulus and
η
