Geoscience Reference
In-Depth Information
arches between the Appalachian and Illinois basins and between the Illinois and
Michigan basins are shown in Fig. 10.38. Thrusting and emplacement of loads
on the Appalachian mountain system took place from about 470 until 200 Ma.
The modellers used the observed subsidence of the neighbouring Michigan and
Illinois basins, as shown in the previous section, and investigated the effect that
their presence has on the intervening arches as a result of the Appalachian loading.
Figures 10.38(c) and (d) show the cross sections calculated for the same model
loads for an elastic lithosphere and a viscoelastic lithosphere. A model with an
elastic lithosphere gives too little uplift over the arches whereas a model with a
viscoelastic lithosphere gives far too much relaxation and thus too much uplift
and erosion. The best-fitting model has a temperature-dependent viscoelastic
lithosphere. This study of the Appalachian basin has shown that the subsidence-
time plots for the adjacent Michigan and Illinois basins (see Fig. 10.34) should
ideally be corrected to remove the loading effects of the Appalachians.
Foreland basins such as the Appalachian basin, the Alberta basin and the
northern Alpine molasse basins are generally linear two-dimensional features,
unlike many intercratonic basins, which are roughly circular. It is clear that, since
subsidence-time plots for foreland basins are controlled by factors external to
the basin and lithosphere, they might not be useful except insofar as they can
indicate the possible timing and magnitude of the imposed loads.
The lithosphere can be loaded from below as well as from above. An example
of an intercratonic basin that may have formed as a result of loading in or beneath
the lithosphere is the Williston basin, which straddles the U.S.-Canadian border
just east of the Alberta basin (Fig. 10.39). Prior to the initiation of subsidence
during the Cambrian, there had been no tectonic activity in the region since
a probable continental collision at about 1800 Ma in the Hudsonian event and
some late-Proterozoic events further west. The basin appears to have subsided
continuously at a slow, fairly constant rate for most of the Phanerozoic (over
400 Ma). A remarkable feature of the subsidence is that the centre of depression
of the basin remained almost in the same place throughout that time (Fig. 10.40).
One simple model for the subsidence of the basin involves a steadily increasing
load hung under the centre of the basin. This raises the obvious question 'What is
the load?' One possibility is that, by some means, a region deep in the lithosphere
was heated and then slowly cooled and contracted, becoming denser. The problem
with this model is that this intrusive body must account for the subsidence without
leaving surface volcanism - perhaps a very large, cool intrusion. An alternative
explanation is that some part of the lithosphere is undergoing phase changes or
metamorphic reaction. Geologically, the most probable reactions are those which
involve the growth of high-pressure assemblages in the deep crust or upper mantle.
As an example, the complete transformation from gabbro to eclogite at the base
of the crust would increase the density from about 3.0
10
3
kg m
−
3
.
This change in density, about thirty times greater in magnitude than that caused
by the cooling of gabbro through 150
◦
C, means that the region undergoing the
×
10
3
to 3.4
×
