Download Figure 1) Map of the Saint Lawrence Rift System

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Student H
Professor Mark Castner
PHY 131A
6 October 14
Earthquakes are indispensable. They are present throughout the world, even in Northeast
New York, and Canada. In most of New York, earthquakes aren’t common, except in one area
known as the Saint Lawrence rift system. The Saint Lawrence rift system is in the middle of a
complex zone. Its faults are speculated to come from three major events: The spreading caused
by the Iapetus Ocean, faults formed during early stages of Appalachian compression, and a fault
caused by the impact of the Charlevoix crater. It is bordered by the Lower St. Lawrence Seismic
Zone, and Adirondack Lowlands and Highlands. The Lower St. Lawrence Seismic Zone is
believed to be connected to the faults located in the St. Lawrence rift system. The Adirondack
region was affected by the Grenville orogeny which plays a role in the rift system as well. The
understanding of these areas explains the physical features in the surrounding Canada and New
York. This area is very congested, and contains numerous faults that relate back to the formation
of the continents.
The Saint Lawrence rift system (Figure 1) is a seismically active zone where fault
reactivation is believed to occur (Tremblay 2). A rift is a region where the crust has split apart,
marked by a rift valley (Bolt 368). The Saint Lawrence rift system is believed to be a failed rift.
The reactivation is believed to be a result of normal faults that were related to the opening of the
Iapetus Ocean between the Proterozoic and Paleozoic time (Tremblay 2). These normal faults
where a failed rift, which caused grabens (depressed area surrounded by parallel faults). The St.
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Lawrence rift system is believed to be a half-graben, which links the Ottawa-Bonnechere graben
and the Saguenay River graben (Lemieux 223). These faults border between the Grenvillian
basement and the St. Lawrence Lowlands. The Grenvillian basement is located in southwest
Quebec and it borders the United States, the St. Lawrence river between them (Dufréchou 61).
Studies by M. Rocher show that initially faulting was caused by the Iapetus rifting, and
reactivation occurred during and after the Appalachian Compression (Tremblay 3). The
Appalachian Compression was the colliding of continents about a billion years ago, forming the
Appalachian Mountains (Building). The reactivation of these faults is complicated by
Charlevoix Area. The Charlevoix Area, which was impacted by a meteorite, lies on the border of
Grenvillian and the St. Lawrence River Platform (Lemieux 223). A major normal fault system
extending from northeast to northwest along the Saint Lawrence rift system was marked by
catacylastic, gouge breecias, and rarely pseudotacheylyte. Cataclastic rock is a type of
metamorphic rock that is formed by the progressive fracturing and comminution of existing rock,
and is mainly associated with fault zones (Catalastic). Fault breecias rock results from the
grinding of two faults. Pseudotachylyte is found along faults that are formed by frictional
melting of wall rocks during fault movement (Pseudotachylite). The Charlevoix area is divided
into two main regions. The first would be the internal region, which includes the area impacted
by the meteorite (Lemieux 224). The external area is simply the surrounding area. Detailed
studying of faults both within and outside the Charlevoix internal area suggests that faulting
occurred before and after the impact of the meteorite (Lemieux 221). The relationship between
the Iapetan rifting and younger faults needed to be determined (Tremblay 3).
The faults of the St. Lawrence system between the Cap-Tourmente and Baie Saint Paul
went under study (Tremblay 4). The study went way beyond the Charlevoix area, covering most
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of the Saint Lawrence system. Both the Cap-Tourmente and Baie Saint Paul faults are located
within the Grenvillian basement (Tremblay 4). The Saint Lawrence Fault was found to extend
from Cap-Tourmente to Baie-Saint-Paul, parallel to the St. Lawrence River (Tremblay 4).
Rondot, who mapped the fault, shows that it crosses the Charlevoix crater without any deflection
or fault offsets. The Saint Lawrence Fault had remnants of Ordovician rocks. This fault crosscuts
east to northwest and contains subsidiary structures to the Cap-Tourmente Fault (Tremblay 4).
The Cap-Tourmente Fault runs parallel to the border of the Grenvillian basement and the St.
Lawrence Lowlands. This fault merges within the Montmorency Falls Fault, which is another
fault contained in the St. Lawrence rift system (Tremblay 7). The Fault rocks in the CapTourmente Fault are identical to those of the St. Lawrence Fault. These rocks are mainly fault
breccia, cataclastic and pseudotachylyte. Examining the fault rock, and structure it seems that the
Cap-Tourmente and the St. Lawrence are of the same faulting event; however it’s not like that.
The Cap-Tourmente crosses the St. Lawrence, concluding that the Cap-Tourmente is a transfer
fault. Lachapelle in 1993 suggested that these transfer faults possibly could have been formed by
older faults from the pre-rift basement (Tremblay 7). Lachapelle continued study of fault gouge
around the St. Lawrence rift lead to the conclusion that the older faults that have become more
active are products of single and progressive faulting (Tremblay 8). From the Cap-Tourmente
fault to the La Malbaie, the St. Lawrence fault continues northeast and does not any effect of the
Charlevoix impact (Tremblay 8). This lead to the conclusion that the reactivation of the Saint
Lawrence rift system and related faults came after the Charlevoix impact occurred (Lemieux
221). The pseudotachylyte and brittle fault rocks of the castaclastic series are not only in the St.
Lawrence rift system, but also in the Charlevoix area. The similarities between these rocks
structures support the idea that the Charlevoix area predates the St. Lawrence Fault (Tremblay
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12). The impact is best believe to happen in the Devonian area, making the St. Lawrence fault
reactivation more plausible during Mesozoic times, when the rifting of the Atlantic Ocean was
occurring.
The Lower St. Lawrence Seismic Zone (Figure 2) is just upstream of the Saint Lawrence
rift system. The Lower St. Lawrence Zone, LSZ, is located between Baie-Comeau and Sept-Tles
(Lamontagne 317). This LSZ has a usual range of about 50-100 earthquakes yearly, averaging
about 60. These earthquakes are common, but these earthquakes don’t get above 5.0 magnitudes
(Lamontagne 319). The earthquakes in the LSZ are usually connected with the St. Lawrence rift
system. These earthquakes are described as part of reactivations of the Iapetan rift faults, which
is what makes up a major part of the rift system. The epicenters in the LSZ are very similar to
those in Charlevoix (Lamontagne 322). This connection is vital for the Canadian coast along the
Saint Lawrence River. This connects creates seismic provisions for the building codes in Canada
(Lamontagne 322). The LSZ is made up of five main geological regions: The Precambrian
Shield of Grenvillian age, the Cambrian-Silurian Anticosti playform, the Appalachian nappes,
the Carboniferous sediments, and the Quaternary deposits. The Precambrian Shield consists of
Mesoproterozoic supracrustal and mafic gneissic complexes, migmatit, and Anorthositemangerite-charnockite-granite suits (Lamontagne 324). This is the bottom layer, or basement.
The Cambrian-Silurian Anticosti platform is composed up limestones, dolomites, and sand
stones (Lamontagne 324). This layer covers the bottom layer, or the Precambrian Shield. The
Appalachians are made up of sedimentary rock from Cambrian Ordovician (Lamontagne 324).
This layer of sedimentary rock covers the Anticosti platform. The Carboniferous sediments also
consist of sedimentary rocks that were deposited in the Magdalen basin (Lamontagne 324).
Second to the top the Carboniferous sediments like over the Appalachians. Lastly on top, the
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Quarternary deposits are the top layer that include marine clays and glacial till (Lamontagne
324). As seen with the Saint Lawrence rift system, many different reasons caused faults in the
area. In the LSZ, Grenvilian collision, Iapetus Ocean rifting, Taconian reactivation, and the
Acadian orogeny are the factors of faults in this area. Although earthquakes are common in this
region, most earthquakes aren’t mapped. The majority of seismic activity in the LSZ occurs
under the Saint Lawrence River, which would lead to that the epicenters are located in the
Precambrian basement (Lamontagne 324). This supports the hypothesis that the faults dealt with
today are originated from the faults created by the initial rifting of this Iapetus Ocean. The faults
in the LSZ are high stressed. The reasoning for the high stress is the fact that this region has
numerous intersecting faults (Lamontagne 334). These intersections can sometimes cause up to
eight times the stress level. This high stress can favor ruptures (Lamontagne 334). Earthquakes
particularly in this area aren’t much of a risk, seeing that most seismic activity is below 4.
Although these transfer faults located along the Saint Lawrence River could build up enough
stress to release a massive rupture, causing damage to the area. The exact fault conditions
underneath the St. Lawrence River is unknown, but they seem closely linked to the St. Lawrence
platform to the southwest (Lamontagne 334). The connection between the two regions could help
researchers predict the future of these fault lines.
To the south of the Saint Lawrence River are the Adirondack Mountains. The Adirondack
region consists of two major parts, the Highlands and the Lowlands (Figure 3). The boundary
between them is called the Carthage-Colton Shear Zone, CCSZ (Streepey 479). The Lowlands
seemed to have missed the Ottawan metamorphism, which is when the Grenville province had a
continent-continent collision, suggests that the Lowlands were separated laterally from the
Highlands by a small ocean basin, or the two lands came in place by a strike-slip fault (Streepey
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482). This metamorphism predates the rifting, which would put the sides into their current
positions after the Ottawan Orogeny (Streepey 482). The Lowlands consist of carbonates and
siliciclastic sediments that underwent metamorphism (Streepey 482). Metamorphism peaked at
the time of the Elzevirian Orogeny, which was the first part of the Grenville orogenic cycle
(Streepey 482). The Highlands contain metaigneous rocks metamorphosed to granulite-facies
conditions (Streepey 482). Geochronologic studies combined with structural information show
that the CCSZ has to have some sort of normal displacement to explain why the Lowlands and
Highlands are at their current levels (Streepey 486). After comparing geological data two
hypotheses rose about the CCSZ. The first believe that the CCSZ represents the trace of a
reverse/normal fault (Streepey 486). The second is that the CCSZ is just a boundary between the
two lands that were once separated by a small ocean basin (Streepey 486). The CCSZ is a very
old complicated area that is hard to study, which explains why researches do not know much
about it.
The entire area where Northeast New York and New England meets the Southeast
Canada is a very complex region. The Saint Lawrence rift system is one of the more complicated
areas, being affected by numerous natural events that led to faulting. Although, all faults in that
area are somewhat connected to the original fault caused by the spreading of the Iapetus Ocean.
These original faults are mostly unknown to scientists because they are covered by many layers.
This old fault seems to stretch from Lake Ontario to the Lower Saint Lawrence seismic zone,
which explains the earthquakes along these regions. The Charlevoix did affect some of the faults,
and perhaps reactivated some parts of the old fault. These old faults should be studied more
intensely, and carefully. We can draw hypothesis by studying the younger faults in this area, but
we don’t know exactly how the old fault is. The area should be re-examined and studied more in
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depth so we can figure out the exact specs of this original fault, and predict if it will be a serious
concern in the future. Another subject that requires re-examination is the Carthage-Colton Shear
Zone. This shear zone could be a major fault zone, and we don’t even know it. Research should
be put in all the different hypotheses of this area to accurately determine exactly what this area
that separates the Lowlands and Highlands. These two studies are important because if these
areas ever do have a sudden slip or rupture, this region of North America would be caught flat
footed. This area is very complicated and a mystery, a surprise slip could do extensive damage to
the surroundings area of Canada and New York.
Figure 1) Map of the Saint Lawrence Rift System
Source: Tremblay, A. and Lemieux, Y. Supracrustal faults of the St. Lawrence riftsystem
between Cap-Tourmente and Baie-Saint-Paul, Quebec; Geological Survey of Canada, Current
Research 2001-D15. Print.
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Figure 2) Map of the Lower Saint Lawrence Seismic Zone
Source: Lamontagne, Maurice, Pierre Keating, and Serge Perreault. "Seismotectonic
Characteristics Of The Lower St. Lawrence Seismic Zone, Quebec: Insights From Geology,
Magnetics, Gravity, And Seismics." Canadian Journal Of Earth Sciences 40.2 (2003): 317.
Academic Search Premier. Web. 8 Oct. 2014.
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Figure 3) Map of New York State regions, CCMZ shown is known as the Carthage-Colton Shear
Zone which divides the Adirondack regions into the Lowlands to the northwest and the
Highlands to the southeast.
Source: Bailey, David G. "Geology Of New York: The Empire State." Rocks & Minerals 82.6
(2007): 464-471. Academic Search Premier. Web. 9 Oct. 2014.
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