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indicate that the basalt deposits are very thick and that the crust is about 50 km
thick beneath the rift, compared with a more typical 35-45 km for neighbouring
regions.
A 24-30-fold, deep-seismic-reflection line across this rift in Lake Superior is
shown in Fig. 10.61. The rift is very clear indeed. On its northern and southern
margins, the rift is bounded by normal faults. The major basin reflectors are
believed to be lavas with some interlayered sediments; they extend downwards to
almost 10 s two-way time (about 30 km depth). These reflections may originate
either from the sediment-lava contacts or from the contacts between lavas of
differing compositions. Similar strong reflections (SDR) observed on Atlantic
continental margins are thought to be associated with basaltic lavas that were
erupted, at elevated temperature or near a hot spot, when the continent split apart.
The reflections labelled Ba are interpreted as the pre-rift basement and those
labelled M as the crust-mantle boundary. These mantle reflections, occurring at
13-15 s, indicate that the crust in this region is nearly 50 km thick. In the central
part of the rift, the present thickness of crust between the rift deposits and the
Moho gives only about 4 s two-way time. This corresponds to a thickness of about
12-14 km, which is about one-third of the normal crustal thickness and therefore
about one-third of the pre-rifting crustal thickness. Thus, if the simple stretching
model in Section 10.3.610.3.6 is assumed valid, the crust was extended by a factor
β
=
3 during the rifting. Such a value implies that complete separation of the
crust may have occurred. The assumption that the present M reflector was the
ancient as well as the present Moho is, of course, open to debate. The lowermost
crust beneath the rift could easily be intrusive material, and the M reflection could
be a new post-rifting Moho. In that case, the value of
β
would be considerably
greater than 3; so we can take 3 as a minimum value.
Whatever the final interpretation of the details of the Keweenawan structure,
it is a major, old intercontinental rift filled with an incredibly thick sequence of
lavas and sediments.
10.5 The Archaean
The Earth's history has four aeons. The Hadean is the time from accretion until
about 4 Ga ago (this boundary is as yet undefined). The Archaean is from around
4Gaago to 2.5 Ga ago, comprising about one third of the Earth's lifetime. One of
the problems facing geologists and geophysicists who are studying the Archaean
is that the
uniformitarian
assumption, loosely stated as 'the present holds the key
to the past', may be only partly correct.
Aktualism
, 'the present is the same as
the past', is certainly not true. The Earth may have behaved very differently in
the beginning, with a different tectonic style, so our interpretations of structures,
rocks and chemistry may be ambiguous.
When did the continents form? As was mentioned in Section 6.10, the old-
est rocks are about 3.8-4.0 Ga old. The oldest known terrestrial material is some
