The Geology of Western Canada & Assembly of the Cordillera
T.S. Hamilton, Camosun College
(Read this while viewing the maps of the Cordillera in the hallway and at the back of F360
so you can see where the regions, the physiography and structures are found.)
Physiographic Belts:
The Canadian Cordillera of British Columbia, the Yukon, and parts of Alberta and the
Northwest Territories is a region that is both more scenic and more complex than the rest
of the North American Craton. Across the Cordillera there are a series of NNW trending
physiographic belts totaling 1000 to 1350 km wide, that have distinctive: elevation, relief,
bedrock geology and structures. The Cordillera is essentially the youngest and deformed
westernmost edge of the North American continent. To understand the geological
architecture of the Cordillera is to understand how continents grow.
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The distinctive physiographic belts (west to east) of the Cordillera are: The Insular Belt,
The Coast Belt, The Intermontane Belt, The Omineca Belt and The Foreland or Hinterland
Belt. East of this is the Great Plains of the North American Craton, which is underlain by
the strata of the Western Canada Sedimentary Basin overlying various Precambrian
basement terranes of the Canadian Shield.
The Geology and Development of the Different Belts from the Coast to the Craton:
The Insular Belt:
The Insular belt includes Vancouver Island, the Gulf Islands, the Straits and western most
slopes of the Coast Mountains, The Queen Charlotte Islands, the island archipelago of the
panhandle of Alaska (including the Wrangell Mountains), the SW corner of the Yukon and
the SE most corner of mainland Alaska including the Yakutat Block. The physiography of
the southern portion of the Insular Belt (South of Dixon Entrance and Prince Rupert)
ranges from 3km tall glaciated mountains through foothills to sea level lowlands and
basins (Dixon Entrance, Hecate Strait, Queen Charlotte Sound, Georgia Strait) underlain
by Quaternary, Tertiary and Cretaceous sediments. The deepest point is at Captain's Island
in lower Jervis Inlet, where a fjord glacier eroded to more than 700 m below sea level.
Further north in Alaska, NW BC and SW Yukon, the Insular Belt is more mountainous and
its physiography resembles the Coast Belt. Because of the convergence across the Queen
Charlotte-Fairweather Transform fault, and the Tertiary through modern accretion of the
Yakutat Block in the corner of Alaska (near 60
is found in that region (also Mt. St. Elias) as well as the highest uplift rates on the planet
(~2 cm/yr). Because of both ongoing uplift and Tertiary through recent subsidence and
glaciation in low lying basins, this belt is the most tectonically active. It includes both
Canada's most active fault, the Queen Charlotte (1949, M=8.1) and a variety of small
Quaternary basaltic volcanoes from Revillagigedo Is. (55.5°N) just north of the Queen
Charlottes to Mt. Edgecumbe, a Quaternary volcano on Kruzof Island (57.1°N) a little
further north in SE Alaska.
The geology of the Insular Belt is characterized by Wrangellia and Alexander, 2 major
allochthonous tectonostratigraphic terrane. The oldest rocks are marine sediments and
volcanics, Lower Paleozoic sediments as determined from rare conodont microfossils on
Moresby Is. and on Vancouver Is. Both sedimentary and volcanic strata are present in the
Paleozoic (Devonian through Permian). Apparently this crust was part of an intra-oceanic
island arc system from its marine volcanics and sediments (Myra and Nitnat Formations
on Vancouver Island) in the Devonian and again in the Pennsylvanian from the marine
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sediments and sills of the Sicker Group. The marine arc volcanics of the Devonian Myra
formation are the host to volcanogenic massive sulfide deposits involving copper and zinc
like at the Myra Falls mine west of Campbell River. By Permian this twice thickened Island
arc crust was somewhere in more tropical climes from its layers of crinoidal Buttle Lake
limestones (now Mount Mark formation, Cowichan Valley and Strathcona Park). In Upper
Triassic time there was a massive outpouring of marine flood basalts making up to 5 km of
pillow and pillow breccias of the Karmutsen Formation. This section and period is akin to
many of the larger high standing intra oceanic lava plateau (Ontong-Java, Shatsky Rise
etc.). After some more tropical limestone deposition, by middle Jurassic this transitional
crustal fragment was once more over the top of a subduction zone form the calc-alkaline
volcanics (Bonanza, Yakoun) and related plutons (Island Intrusions) present on Vancouver
Island and in the Queen Charlottes. Those intermediate plutons acted like heat engines in
the upper crust and convected enough hot water around to form both disseminated
porphyry copper deposits like that on the north end of Vancouver Island at Island Copper
and copper-iron skarns like at Tasu where the plutons cut the Karmutsen volcanics and
adjacent limestones. Paleomagnetic evidence on these Jurassic rocks (180 to 168 Ma) gives
a paleolatitude of about -30° (southern hemisphere). By about 80 Ma in the Cretaceous, the
microfossils indicate more a northerly latitude and "North American" assemblages. The
inference from the sedimentary rocks of the Nanaimo Basin on Eastern Vancouver Island
is that the Insular Terrane had been emplaced against the cordillera by this time and was
now part of North America. While the exact timing and location for this emplacement is
under discussion, folding of about 95-90 Ma may mark the emplacement of Wrangellia
and the 80-65 Ma rocks of the Nanaimo Group probably mark an overlap assemblage that
covers up the newly accreted real estate. At the onset there is a big erosional unconformity
with the rocks of Wrangellia and some major river valleys that preserve bolder
conglomerates along eastern Vancouver Island (west of Nanaimo and on Mt. Washington).
The lowest strata of the Comox formation are terrestrial and coal bearing in some places.
(This was the economic incentive for immigrants to settle Vancouver Island.) From the
Comox Formation on upsection, the setting then included a broad shallow continental
shelf to bathyal basin, possibly a fore arc basin somewhat wider than Georgia Strait today,
with large free floating ammonites, benthic flat clams like Inoceramus and big predatory
swimming reptiles including: giant turtles, the peg-toothed crocodilian Mosasaur, and
long necked Elasmosaurs like those found near Courtenay along the Puntledge River and
New Island Highway. Because of the repetitive number of massive conglomerate beds,
there was probably active faulting and tectonic activity to the east in the region of the
modern Coast Mountains. During and since the Upper Cretaceous there has been a
succession of subduction zones just offshore including the Farallon Plate since 95 Ma in the
south (Queen Charlottes to Mexico), the Kula Plate from 56 to 10 Ma (SE Alaska) in the
North. Today there is a new 3 part ridge system against the Pacific making the Explorer,
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Juan de Fuca, and Gorda Plates as successors to the Farallon, and currently subducting
beneath Vancouver Island, Washington and Oregon since Upper Miocene (about 16 Ma).
Some of the 56 Ma seafloor basaltic crust and deep water sediments have been accreted to
the coast in westernmost Oregon, Washington and southernmost Vancouver Island (south
of the Leech River Fault) and as the Yakutat Block in SE Alaska. In the south this latest
terrane accretion was complete by 40 Ma (Late Eocene) because conglomerates of the
Sooke Formation overlapping and including rocks fragments from the Sooke Gabbro and
Metchosin and Crescent Basalts together with Leech River Schist and locally derived
pebbles from the older rocks of Wrangellia. The terrane of marine basalts, seamounts and
deep-sea sediments that lies from Sooke to Mendocino California and from the west coast
to the Cascades is called Siletzia. This piece of real estate was accreted by Upper Eocene
and was responsible for the uplift and high-grade metamorphic rocks on southern
Vancouver Island. It is this accretion and uplift event that formed the lode gold deposits at
Gold River and the Manganese deposits of the Cowichan valley, both typical of forearc
environments.
In the north of the Insular Belt, in Southeast Alaska, the Yakutat Block is still being
accreted today, leading to much compression and very large uplifts. In between the 2
subduction zones (Cascadia and Aleutians) from Eocene through Upper Miocene (45-5
Ma) there was an extensional basin along the central coast of Queen Charlotte Sound and
the Queen Charlotte Islands. There was up to 7 km of subsidence and up to 4 km of rift
related volcanics and up to 3 km sediments deposited in a whole series of small grabens.
Because of the great sedimentary thickness, high heat flows and good organic source rocks
this is Western Canada's most prospective frontier petroleum basin with many geological
similarities to Cook Inlet SE Alaska. To date there are only natural oil seeps on land and
beneath the sea but so far no productive wells. There has been a defacto moratorium on
exploration since 1969, but recent interest in opening this back up from both industry and
government. Because of ongoing subduction, the western (outermost) side of the Insular
belt is being uplifted, while the inland eastern edge is subsiding. The westerly uplifts are
due to compression and doubling up the crustal thickness. The easterly subsidence
coincides with a steepening in the dip of the subducting slab. This hinge line runs from the
Western side of Puget Sound, along the western edge of Georgia Strait. To the west the
rocks are folded, faulted and uplifted with Mesozoic and older rocks exposed at the
surface. To the east, it is a deep body of water with several hundred metres of glacial
deposits over about 2 km of Tertiary strata having only minor deformation.
The Coast Belt:
The Coast Belt is dominated by the Coast Mountains, which run from Vancouver to
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Alaska. In Canada this is not strictly the west coast, but rather the western side of the inner
coast eastwards through the mountains that adjoin them. Further South in Washington
and Oregon the equivalent mountain belt is called the Cascades. The Coast Mountains and
the Cascades are both underlain by Paleozoic and Mesozoic rocks in a series of eastwards
directed thrust sheets with intervening folds. While you might think of the Cascades
Mountains as being our young continental margin volcanic arc, these subduction related
volcanics represent only a small percentage of the total volume of the Coast Belt
mountains (Coast Mountains and Cascades). These belts are quite distinctive in position,
elevation and geology from the Coast Ranges of Oregon and California which lie along the
west coast further south and are all comprised of Eocene to Oligocene seafloor and
overlying sediments. In the Coast Belt the highest peaks are in the Central Coast at Mt.
Waddington and Silverthrone.
The bedrock geology is dominated by plutonic suites and metamorphic assemblages that
are mainly Mesozoic in age but include Tertiary and younger plutons as well. Much of this
bedrock has been uplifted from the mid to lower crust, although there are a few panels of
low grade rocks and roof pendants in some of the batholiths that are equivalent to rocks of
the Wrangellia allochthon. East of Prince Rupert there is a single Eocene pluton called the
great Tonalite Sill (Diorite) that is 5 km thick. This marks the addition of a magma body 5
km thick to the lower crust here. This is tantamount to increasing the crustal thickness by
10-15% in a single intrusive event. The Cascades volcanic Arc in Oregon and Washington
has a series of major strato-volcanoes up to 3,500 m spaced about every 50 km from Mt
Shasta in northern California through Mt. Baker near Bellingham. While we have
Quaternary to Recent volcanics north of the border too (Garibaldi, Mt. Meagher, Mt.
Cayley, Salal Creek), these are only thin veneers of volcanic deposits over Cretaceous and
older bedrock. This is not due to any great change in arc activity but is mainly due to the
fast uplift and erosion in our mountain belt coupled with intense Quaternary glaciations
that removed all but the feeder dykes for any older volcanic piles. Tertiary basalt dykes
are widespread throughout the Coast Mountains of BC but any volcanic piles which they
fed are long since eroded.
The Coast Belt to the north of Prince Rupert (in SE Alaska) is unique in the whole of the
cordillera, in having some rocks with inherited Precambrian zircons. This is a conundrum
as only rocks adjacent to old continental shields exhibit this. This might be due to an older
age or tectonic imbrication of the crust there or perhaps to its proximity to ancestral North
America (Laurentia). Otherwise there is no continental crustal affinity for this dominantly
Mesozoic welt of crust. Because of the thickness and high metamorphic grade (Garnet +
Sillimanite) particularly in the Coast Belt, it is thought that this crust was enormously thick
in Cretaceous time and represented an Andean type margin. But all of this former
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thickness has been reduced by crustal extension since the end of Farallon Plate subduction
by rifting from there through the Queen Charlotte Basin and profound local erosion that
removed most of the super crustal strata preceding the Miocene to modern uplift of the
Coast Belt which exposes Eocene plutons (Quottoon) and biotite grade stretched pebble
conglomerates both crystallized deep in the crust.
Just the same as magnetic minerals having blocking temperatures below which they retain
their magnetization, so also do datable minerals have specific blocking temperatures for
the retention of their daughter isotopes. Each mineral has a different blocking temperature
eg. Hornblende (450°) > Biotite (235°) > Apatite (~ 100°C) especially for the retention of
fission tracks due to alpha particles from U or Th fission α-decay. By collecting a suite of
minerals form different elevations through a granitic batholith it is possible to date the
uplift age by this method rather than the primary cooling age one would get from
proportions of daughter isotopes. It is also possible to estimate past geothermal gradients
during the time of uplift. This has been done by Randy Parrish and Marcos Zentilli and
their students for a variety of localities. The surprise is that the Coast Mountains are
relatively young and seem to have been formed only in the last 10-12 Ma. This is a factor of
5 or more younger than the Rockies or 100 or more times younger than the Appalachians.
As a consequence we now understand many of the coastal rivers to have been lowered
through the Coast Mountains during their uplift but that the rivers themselves (Stikine,
Iskut, Skeena, Fraser etc.) are appreciably older as physiographic features. Not only do
they cut declivitous canyons (enlarged to fjords) through the Coast Mountains, but these
old rivers carried distinctive lithic sediments into coastal basins like the Queen Charlottes.
As a final indication of the youth of the uplift in the Coast Belt, there are Miocene volcanic
strata on the Eastern side of the Coast Mountains that are tilted down to the East by the
young uplift in the last 10 Ma.
The Intermontane Belt:
The Intermontane belt is a losenge shaped region of sediment and volcanic filled lowlands
and plateaux that makes up about 40% of the interior of BC. It extends from the Stikine in
the north to the Okanagan in the south and from the Eastern slopes of the Coast
Mountains to the Rocky Mountain Trench and the Western slopes and foothills of the
Kootenays and Ominecas. This belt includes the flat lying or low relief forested and ranch
country of the Nechako, The Chilcotin, the Cariboo and the Okanagan regions. The
basement architecture is a series of oceanic tectonostratigraphic terranes, which from west
to east are called: Stikinia, Cache Creek Terrane and Quesnellia. The bedrock geology is
dominantly formed of Upper Paleozoic marine sediments: limestones, cherts, turbidites
and shales, Upper Triassic basaltic volcanics (Nicola, Skolai etc.) and Lower Jurassic arc
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volcanics (Hazelton Group). The Lower Jurassic marine volcanics are particularly
important as they are the host to all of the major volcanogenic massive sulfide (VMS)
deposits in the cordillera (Kutcho, Eskay Creek etc.). This welt of 3 terranes was accreted to
BC in Middle Jurassic time as one large block or allochthon that is called Superterrane I. In
north central BC between Smithers and the Stikine, the large Jurassic Bowser basin formed,
collecting clastic sediments from the first uplift of the Rockies a few hundred Km to the
east (Omineca Mountains). Since the accretion of Superterrane I, there have been a series of
arc related Middle Jurassic and Cretaceous plutons and Cretaceous volcanics that mark the
time of Farallon Plate Subduction, causing crustal thickening and the transformation of
this oceanic material into more Continental style crust. These large batholiths formed the
many porphyry copper and molybdenite deposits of B.C.'s interior. The Cretaceous
volcanics (Tip Top formation) in particular indicate deep mantle sources with Garnet and
Amphibole, implying a very thick, Andean style crust (~80 km or more). This is to be
expected from the incredibly fast Farallon plate subduction with rates up to 28 cm a year
(this is not a typo)! The birth of a new spreading ridge and the Kula Plate offshore
transformed the Farallon subduction margin into a Kula plate subduction and transform
margin by Lower Eocene. This greatly reduced convergent stresses across the northern
half of the cordilleran margin and the crust relaxed and "flowed out" westwards causing
many extensional basins and major right lateral strike slip faulting along the Cordillera.
The subduction (or over riding) of the easterly directed Kula Farallon Ridge by the
Cordillera, created a series of "no-slab" windows where hot asthenosphere came right up
to the base of the crust. This eroded the crust from below by partial melting and warmed
the crust so that it extended and thinned down to less than 30 km thickness and exposed
the Mesozoic volcanic greenstones and greenschist facies Paleozoic sediments over
widespread areas. The formation of many elongate grabens, half grabens, rhombochasms
and strike slip basins permitted structural lows to collect and preserve mixed extensional
continental volcanic suites and continental sediments including some economic coal
deposits along the length of the belt, and stream gravel hosted "roll front style" Uranium
deposits in the Okanagan Kettle River region. This warmed crust and oblique shear
continued crustal thinning in the Intermontane Belt throughout the Miocene to Recent and
gave rise to large regions of extensional flood basalts of the Chilcotin, Nechako and Stikine
regions and isolated centres of long lived (15 Ma - 1 Ma) alkaline within-plate shield
volcano complexes (Itchas, Ilgachuz, Rainbow Range in the Cariboo and Hoodoo Mtn.,
Edziza, Level Mountain and Heart Peaks in the Iskut and Stikine). To give you an idea of
the size of some of these features, Level Mountain is 100 km across, comparable the
hotspot volcanoes like the big island of Hawaii. There are also many mantle derived small
basaltic lava flows and cinder cone fields sprinkled the length of the belt with more than
178 eruptive centers between the Okanagan and the Stikine many of these erupted in the
last 10,000 years since the end of the last ice age.
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Omineca Belt:
The Omineca Belt exposes metamorphic rocks from greenschist to upper amphibolite
grade in a series of gneiss domes like at Three Valley Gap, Frenchman's Cap etc. The
tectonostratigraphic terranes include the Kootenay Terrane in the south, the Slide
Mountain and the Yukon-Tanana Terrane in the north. The original stratigraphy ranges
from Proterozoic (Late Precambrian) through Mesozoic but much of this has been
metamorphosed and uplifted during the Eocene. Inhereted detrital zircons show clear
affinitiies with the adjacent Precambrian basement rocks of the North American Craton.
Because of this provenance tie to North American basement, this belt is considered to be
peri-cratonic, i.e. next door to the craton. The inference is that while these rocks may be
structurally disrupted and moved tens or even a few hundred kilometers, that they
originated on or close to the continental margin. There are extensive Proterozoic marine
sedimentary rocks like those than that make up the bulk of the Windermere and Purcell
mountains. Three formations in particular, a red argillite (Apekunny), a green argillite
(Grinelle) and a dark limestone (Siyeh with Moyie/Purcell sills) extend for hundreds of
kilometers along the NW trending strike. Particular turbidite horizons are the host to
massive sulfide deposits (Cu, Pb, Zn) like at Sullivan near Trail. This was B.C.'s longest
operating and most productive mine and the site of the only working smelter. A
particularly pronounced valley is called the Rocky Mountain Trench in the south and the
Tintina Trench in the north but its origin is still being debated as to whether it is merely
erosional or partly tectonic. Ore deposits in this belt include metamorphic types ranging
from Uranium in metavolcanics, tungsten skarns, Columbite-Tantalite in alkaline
intrusions in the Wolverine, Beryl (emerald) and sedimentary exhalative barite in the
Yukon and NW Washington State in the Prichard (Proterozoic)..
Foreland Belt:
The Foreland Belt is the fold and thrust belt of the Rocky Mountains. There is a
dominantly westwards dipping stack of thrusts that involves Proterozoic Rocks in the
Main Ranges and the west side and Paleozoic through Mesozoic rocks on the east side of
the mountains. The physiography is classic for fold and thrust belts with elongate
mountain ridges held up by tough, erosion resistant rocks like limestones and sandstones
separated by valleys underlain by softer more erodable shales. The streams all run either
NW or NE, either along or across strike in a classic trellis drainage pattern. Often the stack
of thrusts superposed 3 sequences of the same stratigraphy. This was accomplished by EW convergence during the Farallon Plate subduction. The mountain building event
certainly spanned the Cretaceous and Lower Eocene. The structural style is more thin
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skinned with listric thrusts in the south and more thick skinned and blocky often involving
basement blocks in the north. Corresponding to the bend in the coastline of Alaska and the
position of the thickened Omineca Belt in northern BC and the Yukon, the Foreland belt
extends across the Mackenzie Mountains into the Northwest Territories nearly to Great
Bear Lake. This is more than 1330 km from the present coastline, with nearly all of the
additional thickness to the Cordilleran Orogen being taken up by the Foreland Belt. There
are abundant structural plays for oil and gas deposits hosted in Paleozoic and Mesozoic
strata. These include both conventional anticline plays, roll overs in thrusts, pinch outs
against faults and ramp style duplexes. Many of these potentially productive structures lie
in Provincial and National Parks. There are also Cretaceous coal deposits below massive
thrusts and in folded sections from Fernie and the Crowsnest Pass in the South to Tumbler
Ridge in the north.
The Craton and Western Canada Sedimentary Basin:
In the basement below the sedimentary cover rocks of the Western Canada Sedimentary
Basin, there are a series of NE striking 200-400 km wide Precambrian basement terranes
that include many Archean domains under Alberta (Rae, Slave, Hearne) and Proterozoic
Domains (Hottah) further North beneath NE BC and the Northwest Territories.
Apparently the North American Continent grew by layers (of accreted terranes) towards
the NW in the Precambrian. By Late Proterozoic the continent was stable, drifting and
apparently far from any disrupting tectonic influences such that by about 880 Ma it had
eroded flat forming a continent wide peneplain. In the far west (now about the Alberta 9
B.C. border) and in the far north against the Yukon, deep water passive margin sequences
were deposited. The continent rode below sea level for much of Paleozoic time and formed
massive limestone and shale basins. The Devonian limestones are the main reef formers
and oil host rocks below the Alberta and Williston Basins. (Things have never been the
same in Alberta since the discovery well at Leduc #1.) These older limestones are also hosts
to Pine Point and Missisippi style Pb-Zn deposits.
Things really picked up by Middle Jurassic with compressional shortening, the beginnings
of the formation of the Rockies and terrane accretion that added most of B.C.. This caused
uplift in the Rockies, shedding sediments into the interior of the Continent from Jurassic
through Eocene. Many of these strata are continental or deposited in a shallow interior
seaway that extended from the Gulf of Mexico to the Arctic on top of North American
Precambrian basement rocks. There are coal basins, sinuous riverine sands that localize
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shallow gas deposits and also migrated and biodegraded heavy oils that make up the Tar
Sands of Northern Alberta. While the Cretaceous Volcanic Arc reached as far east as SW
Montana, the Crowsnest Pass and the Southern Yukon, volcanic ash deposits occur
widespread across much of the western Canada Sedimentary Basin. This is the easterly
evidence of the West coast active tectonics. By contrast, despite looking for distal
equivalents to the Paleozoic and Triassic volcanism of BC, none are found. This
corroborates the terrane accretion story for the Cordillera.
The Craton has thick cold Precambrian lithosphere, like a keel well down into the
mantle. In Eocene while there is abundant volcanism in the Cordillera, there are only
isolated tiny kimberlite pipes in Alberta, Saskatchewan and the Northwest Territories.
While these igneous bodies are both rare and tiny (< 1km across). they come from great
depths, circa 160 km and carry up diamonds as accidental inclusions in the peridotites
carried up from the garnet and diamond stability field deep in the Upper Mantle.
Apparently the source for all the productive diamonds is in the subducted roots or former
Archean oceanic plates that build the lithosphere below the North American Continent
(Laurentia).
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Tectonic architecture of an arc-arc
collision zone, Newfoundland Appalachiansspecialpapers.gsapubs.org650 × 790Search by
imageFigure 3. Tectono-stratigraphic subdivisions of Newfoundland (after Williams,
1995a) and northern Appalachians (from Hibbard et al., 2004).
Tectonostratigraphic Terranes and Terrane Accretion:
While the rest of the geological research world studied the ocean basins or other regions of
continental cratonic geology, a revolution in thought was happening in western Canada.
The geologists who worked in the Cordillera were forced to understand and assemble
maps where one side of the map had totally different stratigraphy and structural
sequences than the other. It made for complicated correlations or no correlations at all.
None of the familiar stratigraphy of North America was present but new oceanic
successions of volcanics and sediments were described. Large regions of batholiths like the
Andes or Indonesia were found and studied. Soon paleontologists started turning up asian
and tethyan faunas instead of North American ones. The paleomagnetists like Ted Irving,
boldly started talking in terms of displacements of thousands of kilometers. Jim Monger
systematically went through everybody's stratigraphy and started redefining it in terms of
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correlatable and non-correlatable tectonostratigraphic terranes. The US geologists in
Alaska and California and workers in western Mexico were coming to grips with many of
the same problems. A new paradigm was born that explained much of the disparate
geologic history and sequences in terms of accreted terranes on active tectonic margins.
This new theory seemed to fit so well that Precambrian geologists working in shield areas
reexamined their own geology in terms of different terranes. This is now the new
paradigm for how continents grow by continentalizing added pieces of oceanic crust,
island arcs, oceanic plateau and rifted microcontinental fragments. This same concept has
been applied to the mountain building in the Appalachians (by Researchers like Hank
Williams at Memorial University) and Caledonides of the Atlantic margins. In the final
analysis the main thing that is “continental” about continental crust, is that it is thicker and
more buoyant pieces than the ocean crust nearby which got subducted. Continental crust
has grown throughout geological time by terrane accretion of island arcs, older continental
fragments, overthrusting of the upper crust during collisions & underplating during arcs.
Changing Plate Configurations and Building North America:
While there are older microcontinental fragments well back through the Archean and
remnant zircon cores form even older > 4 Ga because of plate tectonic processes which
rifted prior continents and subducted ocean basins which are no longer available to
analyse ridge positions, magnetic stripes, hotspot tracks etc. We can be fairly certain of
plate motions and configurations back through Early Cretaceous as we still have ~2/3 of
that age seafloor remaining. Pushing reconstructions back further requires studying the
geology or different terranes in continental margin (and older now-interior) mobile belts
and trying to match them up to their long ago rifted cousins on other continents. The main
tools for this are: good old stratigraphic correlations, faunal assemblages, paleo-latitude
estimates from paleomagnetic inclinations and matching up the geological map units of
different far-travelled basement terranes and their zircon igneous and metamorphic ages.
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The Grenville age collisional orogens (see brown on map above) which occurred in Middle
Proterozoic chiefly along the SE margin of Laurentia which reached from the St. Laurence
Seaway to about Sedona Arizona. This was actually comprised of 3-4 mountain building
collisional events from about 1250 Ga to about 900 Ga. This was of course a vast amount of
geological time and comparable to everything from about Silurian to now! As the rocks are
all metamorphic and igneous and Earth held only microbes at that time there is no
convenient way to internally subdivide it. At the end of this time a big supercontinent
called Rodinia was built and sat astride the South Pole. This led to Snowball Earth Times
with Pole to pole ice. In diverse regions of Canada the rocks record these tectonic events
along with the Ediacaran fauna of Mistaken Point NF, the Middle Cambrian Burgess Shale
of Mt. Stephen BC, the Devonian Reef and the oil deposits of the Western Canada.
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