EESA06H3 Lecture Notes - Lecture 10: The Maritimes, Canadian Shield, Rodinia

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16 Apr 2011
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EESA06 Lecture 10
Canadas Geologic Journey: The Last 4 Billion Years
1
We will look at how North America was constructed
The process we will follow is one on deconstruction taking
something apart and seeing how it was made with continents
Plate tectonics is the tool of understanding how North America got
built
2
Skipped
3
Continents grow through time by obduction (ex. India colliding
with Asia)
North American continent shows 5 growth stages:
oArctica (2.7 Ga) oldest, Ga means billions of years
oColumbia (1.8 Ga)
oRodinia (1.0 Ga) really IMPORTANT because life evolves
at this time
ARCTICA + COLUMBIA + RODINIA = CANADIAN
SHIELD which is very old and hard rock, later
softer sedimentary rocks were added on eastern and
western Canada
A + b + c = Canadian Shield (the craton the core of north
America and we have added eastern north America (Maritime)
and them we add the west, Maritime is added when Pangea is
forming and then western Canada is added after Pangea breaks
up)
Pangea assembly (350 Ma): Maritime Canada is added during
Appalachians Orogeny (Appalachian Mountains)
Pangea breakup (<200 Ma): British Columbia is added during
Cordilleran Orogeny (e.g. Rockies)
4
Geology of North America known for more than a 100 years
The Canadian shield in the picture are the brown rocks
The shield (A+B+C) is the old core of North America
Then we can see the green and blue rocks which is rapped around
that core
The blue area on the right hand side is the Appalachian Belt
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Only until the plate tectonic theory in the late 60s about the oceans
closing and the continents moving that this complex picture could
begin to be interpreted
5
These are zircons which we use for uranium lead dating
Uranium lead age dating using Zircons is one of the break through
methods
6
Tom Crow didnt invent the technique but he got it to the stage
where it could be used, expert on dating various rocks
Need to find out how much lead is in the crystal because that will
tell us how long that crystal has been sitting there and the radio
active uranium has been breaking down
7
The oldest continental crust found so far in Canada (yellow knife) is
Acasta Gneiss (NWT) that is 4.1 billion years old
However its hard to know the oldest rock in the world because any
country that has a shield have very old rocks
Why is it not correct to say that the Acasta Gneiss is the oldest
rock?
oAns. Because it has been metamorphose so there must
have been a pre-existing rock. And if we look at the Zircons
in here they are what we call Detritals means they are
secondary and they have been eroded from something else
and are old, garbage, they have been derived from
something else
Ex. The Zircons originate in igneous rocks, they then
get weathered and then become a part of a
sedimentary rock and metamorphose to a gneiss
8
The pattern in this map is center, its sort of a Swiss roll pattern
The oldest stuff is in the middle and as you come out it gets younger
and younger
Its not a perfect pattern but the conclusion we can draw is that
continents have grown through time
Old rocks embedded in the interiors of continents
9
DECONSTRUCTING NORTH AMERICA
Province a old term used by geologist to identify rocks in a large
area which are pretty similar in appearance/geology with sharp
boundaries
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Orogen the rocks that results from orogenic activity, zone of folded
and highly deformed rocks
What is the most common rock type that will be found in these
orogen? Gneiss rocks
The provinces are separated from one another by the orogens
The provinces were little micro continents (like india) and the
orogens were all folded rocks when that continent collided with
another continent (think himilayas)
The provines were little continents that moved around and they
collieded with one another they formed orogens when they collided
where you can find Gneiss and Greenstone Belts - these are
metamorphose ocean rocks
The whole growth process started at the Slave Province (top left)
this is the true core of the shield. The yellow knife area/slave
provinces is the oldest part of the North American continent
Greenland is part of the north American continent, it broke off
about 80 million years ago but it didnt develop into a full ocean, its
just separated by a failed rift
Growth Stages
oA 1st Arctica
oB 2nd Nena
oC 3rd Rodinia/Grenville Orogeny the basement rocks got
added at this time
The shield is the Craton the old rocks that make up the ridged
interior of the continent
o4 Appalachian mountain added during the formation of
Pangea
o5 - The Cordilleran Orogeny added after the break up of
Pangea
oTHERE IS 5 PARTS TO NORTH AMERICAN GEOLOGY
10
Zircon dating has been very useful
Lithoprobe means probing rocks, these vehicles are called
dancing elephants’
These vehicles are parked and there is a hydraulic plate which is
pushed down onto the ground and then the plate is subject to very
strong vibration
Why are they making vibration? We are trying to mimic
earthquakes, seismic waves. It enables us to reconstruct whats
down there to about 70 km
Seismic transects
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