Geoscience Reference
In-Depth Information
Figure 10.63.
(a) Bouguer
gravity across the
Kapuskasing zone:
observed (solid line) and
calculated (dashed line).
(b) A crustal model based
on geology and gravity
(densities are in kg m
−3
).
(From Percival
et al
.
(1983).)
perhaps an explosive intrusion, though some authors have suggested that it may
be a meteorite-impact structure. A cross section (Fig. 10.62) through the Dome
shows that the sedimentary layers were underlain by a granite-gneiss upper crust
and granulites in the middle crust. Approximately the upper 20 km of the crust
is exposed here.
Another exposure of Archaean crust is in the Superior geological province
of Canada. Figure 10.63 shows a generalized west-east cross section through
the Kapuskasing zone. It appears that, in this case, a major thrust resulted in the
uplifting of the deep crustal rocks. The upper crust is granitoid and the lower crust
gneiss. The total thickness of exposed crust is about 25 km. This interpretation
has been supported by results of deep-seismic-reflection studies.
The total thickness of the continental crust towards the end of the Archaean
was probably about 35 km or more, similar to today's value. Problem 9.3 offers an
insight into the consequences of this conclusion. The radioactive heat generation
in the crust can be estimated by extrapolating backwards in time from the modern
content of radiogenic elements. From these estimates, together with a knowledge
of the metamorphic facies (and hence temperature and pressure) attained by
Archaean rocks, Archaean equilibrium geotherms can be calculated. Some such
models are shown in Fig. 10.64. Geotherm 1 implies that the thermal base of
the lithosphere (1600-1700
◦
C) was at about 80 km. However, in North America
there is strong isotopic evidence that Archaean diamonds existed and they have
even been mined from Archaean conglomerates in South Africa. The stability of
diamonds is a major constraint on the thermal structure beneath the continents
because they crystallize at about 150 km and 1150
◦
C. These comparatively low
temperatures indicate that the assumptions in the calculation of geotherm 1 need
