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C HAPTER
4
CANADA’S
HEARTLAND:
THE SHIELD
“the barren Shield, immortal scrubland of our own
where near the beginning the spasms of lava
settled to bedrock schist,
barbaric land, initial, our,
own, scoured bare under
crush of the glacial recessions …”
T
Dennis Lee, Civil Elegies 1972
he Canadian Shield is the mineral-rich heartland of our country
and a vast storeroom of wealth
for the future, including huge
supplies of freshwater. Its rocky
vastness and solitude have been a source of
inspiration to generations of painters, poets
and writers. When the last ice sheet began to
melt 12,000 years ago, Native peoples began
to wander across its ice-scoured, lake-studded
surface, driven by the seasonal movement of
animals. At first, the Shield’s rocky expanse
was a major obstacle to European settlement,
and the building of a transcontinental railway
across its waterways and muskeg was the first
challenge of the newly formed Dominion of
Canada after 1867. The railway boom of the
1880s fortuitously brought to light the miner-
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al resources of the Shield and promoted systematic investigation of its geologic history
using newly developed techniques. The
world’s oldest rocks have been unearthed
from its far northern reaches. Shield rocks
contain evidence of: the earliest plate tectonic
activity, a gigantic meteorite strike, the largest
rift known on Earth and some of the oldest
fossilized life forms so far discovered anywhere in the world. Evidence has been
brought to light that shows the Shield was
brought together by repeated collisions of
crustal blocks forming a tectonic mélange.
Geologic knowledge gleaned from the
Canadian Shield has profoundly influenced
the way geologists understand the early history of other continents.
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Fig. 4.1 Canada’s roots Highly metamorphosed gneiss, composed of distinct bands of alternating pinkish granitic rock and darker more iron-rich rock, is one
of the most common sights across the Canadian Shield.This highly distinctive rock records intense deformation at great depths when crustal blocks collided to make
up the crustal mosaic that comprises the oldest part of Canada. Now exposed at surface over broad areas of the country, these rocks testify to deep erosion and the
removal of huge volumes of rock over the ensuing millions of years to form the flat Shield surface.These rocks, seen here on Copperhead Island in eastern Georgian
Bay, Ontario, are the deep roots of what were once high mountains.These rocks record deformation of rock softened by high pressure and temperature at depths of
at least 25 kilometres during the Grenville Orogeny when South America and Africa collided with North America (Fig. 4.7D).
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The painter Ed Bartram puts the finishing touches to one of his creations based on meticulous observations of the banded gneisses that occur across
the Canadian Shield; the landscapes, rocks, and solitude have been inspirational to many Canadian artists.
The Canadian Shield is underlain by rocks that span the
immense time interval between the beginning of the Archean
(at 4 Ga) and the end of the Proterozoic (some 540 million
years ago). Geologically complex, the origin of the Shield was
bewildering to early workers; its history only began to be
understood in the light of plate-tectonic models after 1970
coupled with data from geophysical surveys probing deep
below its surface. Today, it is evident that the Shield has
grown in size over the past 4 billion years by the addition of
crustal blocks welded together by plate collisions. This is
referred to as continental accretion.
Continents are migratory in nature, propelled as rafts
around the surface of the Earth, embedded in larger lithospheric plates. Periodically, continents cluster together to form
supercontinents which eventually break up freeing individual
continents to disperse again. In their migrations, these continents collide with and incorporate other crustal blocks, thereby slowly growing in size (Fig. 4.7).
by early geologists), separated from each other by sharply
defined boundaries and belts of younger crust. This complexity bewildered nineteenth-century geologists, who could not
understand how these pieces might have been juxtaposed. We
now know that plate-tectonic processes assembled the various
crustal blocks during a 3-billion-year tectonic construction
project, and that the belts are the product of orogenic events
when blocks collided. Canadian geologist Paul Hoffman
coined the phrase “the United Plates of America.” The main
crustal elements are shown in Fig. 4.2.
4.1.1 PROVINCES AND CRATONS
The term province was first used in the mid-nineteenth century (by Logan) and then had no direct plate tectonic connotation other than referring to an extensive region characterized
by a similar geologic history. Today, a terrane refers to a faultbounded crustal block typically hundreds to thousands of km2
in extent. The term superterrane is used for several terranes
that have joined together to form an even larger block. These
larger blocks are broadly equivalent to the provinces recognized in the past. The term craton is also used for these blocks.
For example, the Superior and Slave provinces (Figs. 4.2, 4.6)
are also referred to as cratons. Province, superterrane or cra-
4.1 A CRUSTAL COLLAGE BUILT BY
PLATE TECTONICS
The Canadian Shield is essentially a patchwork quilt comprised of a large number of crustal pieces (named “provinces”
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Innuitian Orogen
A
Nagssugtoqidian
Orogen
Baffin
Orogen
NT
Tr
TE
CT
ON
IC
an
O s-H
ro u
ge ds
n o
n
New
Quebec
Orogen
FRO
SUPERIOR
PROVINCE
FA
UL
T
2
Or
og
en
Appalachian
Mountains
Fig. 4.2 Canada deconstructed A The North American continent consists of five large crustal blocks that took some 3 billion years to
be brought together. Blocks 1, 2 and 3 form the North American Craton
and consist of very old Archean and Proterozoic rocks much older than
600 million years. Some parts of Block 1 were part of an early continent
called Arctica (Fig. 4.7). Block 2 was added during formation of the continent Nena and Block 3 was added when the continent Rodinia formed.
Younger strata of Blocks 4 and 5 were added during plate tectonic collisions after 600 Ma (Chapters 6 and 8 respectively). Much of this story
would have been impossible to unravel without the ability to age date
each part. B A freight train loaded with containers is a good model for
blocks of crust (terranes) being carried by the lithosphere.
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B
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3
TORONTO
4
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M az e n
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C A N A D A
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TERRANE ACCRETION DIAGRAMS: STONE JIGSAW PUZZLES
sing all the tools available, geologists have now identified most of the terranes that make up North America. The next step
is to construct a terrane accretion diagram. The notional continent (Leaflandia) is made up of four superterranes (Oileria,
Ottavia, Habsia, and Canuckia). Ottavia and Habsia are in turn, made up of smaller terranes (1, 2, 3, 4). A terrane accretion diagram is simply a “family tree” of a continent that shows the timing of the different events that brought these terranes together.
The geology of superterranes Oileria and Canuckia is very similar insofar as they have both been in existence for some considerable time and are composed of strata that range in age from the Cambrian to the Cretaceous. In contrast, terranes 3 and 4
have separate histories and were brought together to form the superterrane Habsia in the Early Jurassic (orogenic event A) recorded by a large granite pluton that stitched them together. These were covered by sediments that accumulated in what is called a
successor basin and referred to as an overlap sequence. The next orogenic event (B) was the docking of Habsia against Canuckia
as recorded by rocks of the small successor basin that straddles the suture between the two.
Terranes 1 and 2 also have different geologic histories having been brought together toward the end of the Jurassic docking
(event C), forming the superterrane Ottavia covered by another successor basin.
The next major orogenic event (D) brought Ottavia into contact with the Habsia-Canuckia superterrane in the Cretaceous as
shown by another successor basin fill that
straddles these. The
final assembly (E) of
the complete continent
Leaflandia occurred in
the Cenozoic by the
addition of Oileria.
The sedimentary rocks
of the youngest successor basin extend
across all terranes.
A quiet day looking at rocks in Canada’s far north.
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A Acasta
Gneiss
4.0 Ga
Greenland
Aldan Shield
Baltic Shield
North
American
Craton
Lewisian
Wyoming
Finland
Eurasian
Craton
Hebei Province
Superior
Province
5000 km
South
American
Craton
Guiana
Shield
Brazilian
Shield
African
Craton
Zimbabwe
Craton
Indian
Shield
Pilbara
Kaapvaal
Craton
Yilgarn
Australian
Craton
Proterozoic Provinces
(2.5–0.6 Ga)
Archean Provinces
Napier Complex
3.5–2.5 Ga
> 3.5 Ga
Antarctic Shield
B
Fig. 4.3 Chips of the old block A Continents are built of Archean and Proterozoic continental crust brought
together as part of a process called cratonization (parts 1, 2 and 3 on Fig. 4.2). Addition of much younger crust in the
Paleozoic and Mesozoic completed the construction of the continents. B Seen from the air in summer, rafts of pack ice
drifting along Canada’s Arctic coasts provide a good analogy for the growth and structure of continents. Rafts of old
darker-coloured ice from previous winters have been frozen into younger ice that, in turn, broke up and then drifted
apart only to be incorporated into new sheets of ice the next winter.
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ton are simply large jigsaw pieces
of old crust. Finally, a tectonic
event such as when terranes collide, is called an orogeny and the
belt of deformed rock that results is
called an orogen.
A terrane can be thought of as
a container being carried by rail on
a freight line. The container contains freight distinct from that of
neighbouring rail cars and is being
carried along on the flat bed below.
Now imagine that the flat bed is
actually a piece of the lithosphere
undergoing subduction; each container (a terrane) will be scraped off
to accumulate as a terrane wreck.
Terrane analysis refers to the
many different techniques used to
identify exotic terranes and to
understand where they came from,
and when and how they were
brought together. Terranes can be
identified by reference to the type
of fossils and rocks present. They
may, for example, carry sedimentary rocks containing displaced faunas, such as equatorial molluscs
that were subsequently moved to
northern latitudes where they now
occur, in marked contrast to the
fossil record of surrounding terranes. This is of great help in deciphering the relatively young terranes that now make up Atlantic
Canada (Fig. 3.6; Chapter 6) and
Western Canada (Fig. 3.11B;
Chapter 8) but cannot be used in
the absence of fossils in the very
old rocks of the Shield.
Nonetheless, one moving south
from cold high latitudes may carry
glacial rocks with it and one moving in the other direction may carry
rocks recording warm climates.
Whereas the precise timing of terrane accretion cannot often be
determined directly, important
clues can be derived from sediments shed during terrane collision. Collision results in thickening of the crust, uplift and erosion.
In this way, gravel, sand and mud
are shed from uplifted areas and
accumulate in small so-called successor basins where they can be
age dated by various means. This
process is referred to as depositional overlap. The use of detrital
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zircons provides important clues as to
location and type of source areas.
Typically, newly joined terranes are then
pierced by granite intrusions called plutons. In this case the age of the plutons
yields minimum ages on the collisional
event that stitched the terranes together.
Analysis of the zircons in granites yields
high-precision age dating of such events.
Recent understanding of the structure
and crustal growth of ancestral North
America has largely been the result of
geologists probing the Earth and its surface with several tools. Today, scientists
are able to age date rocks using the U/Pb
dating method on zircon, a very resistant
mineral capable of surviving multiple
phases of metamorphism (Chapter 1). This
work differentiates old crustal blocks from
the narrower belts of younger strata (juvenile crust) and records tectonic processes
and magmatic activity that welded the
blocks together. New developments in the
chemical analysis of rocks using rare elements such as samarium and neodymium
have contributed greatly to understanding
of the origins of ancient crust. The ratios
of these elements aid geologists in mapping old crustal blocks from the younger
belts of juvenile crust that surround them.
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A
B
Fig. 4.4 The world’s oldest landform? A The Canadian Shield is the exposed portion of the North American Craton and is an ancient landform of moderate to low relief formed sometime after 800 Ma. Deep weathering and glacial scour has subsequently shaped the Shield to a low-relief surface (B).
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