Geology Reference
In-Depth Information
Figure 12.7
The Canadian sector of
the Grenville orogenic belt. Simplified
map (
A
) and cross-section (
B
) of the
Canadian Grenville Province showing
the outcrops of the Laurentian foreland,
outer autochthon, inner allochthon and
exotic terranes of the orogen 'lid'. Inset
map shows the whole North American
Grenville belt stretching from Labrador
to Texas, bounded to the SE by the
younger Appalachian belt. GFT, Grenville
Front thrust; ABT, Allochthon Boundary
thrust; LO, Lake Ontario; Ant, Anticosti
Island; inset: Gd, Greenland; BI, Baffin
Island; Nfl, Newfoundland; HB, Hudson
Bay; GM, Gulf of Mexico. Based on
Rivers (2009).
BI
Gd
1000km
Atlantic
Ocean
HB
Nfl
GFT
ABT
North
American
continent
FIG. A
ABT
foreland
allochthon
Florida
Texas
GM
Ant
Newfound-
land
foreland
major
thrusts
Lake
Huron
autochthon
line of section (B)
exotic
terranes
allochthon
exotic
terranes
LO
500km
A
NW
SE
GFT
ABT
?
B
base of crust
100km
palaeomagnetic data for Amazonia
during the period in question is not
well established, and an alternative
possibility is that the south-east side
of the Grenville belt was formed by a
series of displaced terranes border-
ing a subduction zone. After the end
of the Grenville orogeny, a number of
extensional basins were formed in the
region south-east of the present Gren-
ville belt margin, culminating in the
creation of an ocean during the Cam-
brian period, thus moving any missing
Grenville-age crust to its far side.
Structure of the Canadian sector
This part of the belt (Figure 12.7) con-
sists of two main zones, an outer zone,
bounded by a major thrust zone known
as the
Grenville Front thrust
(GFT),
and an inner zone bounded by a wide
shear zone, named the
Allochthon
Boundary thrust
(ABT). The name
'
allochthon
' is applied to crust that has
travelled some distance from its origin,
in distinction to the '
autochthon
', which
consists of locally derived crust. The
foreland consists of Archaean to early
Proterozoic rocks making up the core
of the Laurentia craton. These rocks are
reworked but still recognisable in the
outer (autochthonous) zone. The GFT
descends to a depth of between 25 and
40 km, reaching the base of the crust,
so that rocks originating there are now
exposed at the surface. NW-SE elonga-
tion lineations on GFT shear zones indi-
cate that thrusting was at right angles to
the trend of the belt. Later extensional
movements have reversed the initial
thrust-sense movements on the ABT.
The Archaean and early Proterozoic
rocks forming the main part of the
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