Geoscience Reference
In-Depth Information
10.3.4 Basins of thermal origin
A thermal event probably provides the simplest illustration of the formation of a
sedimentary basin. If a region of crust and lithosphere is heated, there is expan-
sion, the density changes and then uplift takes place. Erosion follows, reducing
the crustal thickness. Subsequent cooling and contraction of the lithosphere forms
a basin. (If no erosion takes place, the surface simply contracts back to its pre-
heating level, and no basin forms; nothing changes.) Isostatic calculations based
on reasonable values of the increase in temperature during heating give a maxi-
mum value for the initial uplift of about 2 km. This would result in a maximum
erosion of 4 km, allowing for the isostatic amplification as material is removed
(assuming that the erosional time constant is of similar magnitude to the thermal
time constant), and therefore would result in a maximum of perhaps 3 km of sedi-
ment. This is significantly less than the thickness of sediments on the continental
margins, where 10 km of sediment is often present (see Fig. 9.4), and somewhat
less than the thickness of sediment in many of the intra-continental sedimentary
basins. To deposit 10 km of sediment as a result of heating the lithosphere, an
increase in temperature of more than 1000 Cisrequired.
The subsidence due to thermal contraction is easily modelled mathematically.
It is similar to the plate model for formation of the oceanic lithosphere (see
Section 7.5.2). To first order, the depth to a particular bed (of age t )inahole
drilled into sediments can be written as
d 0 (e t / t 0
d
=
1)
(10.5)
where d 0 is a slowly varying function of the position of the well in the basin. The
thermal time constant of the lithosphere, t 0 ,isgivenby
L 2
π
t 0 =
(10.6)
2
κ
where L is the thickness of the lithosphere and
is the thermal diffusivity. The
thermal time constant for a 125-km-thick lithosphere with thermal diffusivity
of 10 6 m 2 s 1 is 50 Ma. The Michigan and Illinois basins have been modelled
on the basis of thermal contraction of the lithosphere (Fig. 10.34). The subsi-
dence of the Illinois basin between 530 and 360 Ma can be explained by one
thermal event or by thermal relaxation following an initial extensional event with
β =
κ
1.1-1.2. However, a second thermal event is required in order to account
for the rapid Carboniferous subsidence (
360 Ma onwards). The neighbouring
Michigan basin has a somewhat different subsidence history. The Cambrian-
to-Middle Ordovician (530-470 Ma) subsidence appears to be separate from the
Middle Ordovician-to-Carboniferous (460-360 Ma) subsidence, which approxi-
mates a thermal-subsidence curve. Some evidence exists for igneous activity at
the time of the proposed Cambrian (530 Ma) heating event but not for the others.
Invoking a heating event for which there is at present no direct external evidence
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