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Tectonophysics 369 (2003) 231 – 252
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Supracrustal faults of the St. Lawrence rift system, Québec:
kinematics and geometry as revealed by field mapping
and marine seismic reflection data
Alain Tremblay *, Bernard Long, Manon Massé1
INRS-Eau, Terre et Environnement, Centre Géoscientifique de Québec, C.P. 7500, Sainte-Foy, QC, Canada G1V-4C7
Received 3 October 2002; accepted 20 May 2003
Abstract
The St. Lawrence rift system from the Laurentian craton core to the offshore St. Lawrence River system is a seismically active
zone in which fault reactivation is believed to occur along late Proterozoic to early Paleozoic normal faults related to the opening
of the Iapetus ocean. The rift-related faults fringe the contact between the Grenvillian basement to the NW and Cambrian –
Ordovician rocks of the St. Lawrence Lowlands to the SE and occur also within the Grenvillian basement. The St. Lawrence rift
system trends NE – SW and represents a SE-dipping half-graben that links the NW – SE-trending Ottawa – Bonnechère and
Saguenay River grabens, both interpreted as Iapetan failed arms. Coastal sections of the St. Lawrence River that expose fault
rocks related to the St. Lawrence rift system have been studied between Québec city and the Saguenay River. Brittle faults
marking the St. Lawrence rift system consist of NE- and NW-trending structures that show mutual crosscutting relationships.
Fault rocks consist of fault breccias, cataclasites and pseudotachylytes. Field relationships suggest that the various types of fault
rocks are associated with the same tectonic event. High-resolution marine seismic reflection data acquired in the St. Lawrence
River estuary, between Rimouski, the Saguenay River and Forestville, identify submarine topographic relief attributed to the St.
Lawrence rift system. Northeast-trending seismic reflection profiles show a basement geometry that agrees with onshore
structural features. Northwest-trending seismic profiles suggest that normal faults fringing the St. Lawrence River are associated
with a major topographic depression in the estuary, the Laurentian Channel trough, with up to 700 m of basement relief. A twoway travel-time to bedrock map, based on seismic data from the St. Lawrence estuary, and comparison with the onshore rift
segment suggest that the Laurentian Channel trough varies from a half-graben to a graben structure from SW to NE. It is
speculated that natural gas occurrences within both the onshore and offshore sequences of unconsolidated Quaternary deposits
are possibly related to degassing processes of basement rocks, and that hydrocarbons were drained upward by the rift faults.
D 2003 Elsevier B.V. All rights reserved.
Keywords: Québec; Rifting; St. Lawrence; Fault rocks; Seismics
* Corresponding author. Present address: Departement des
Sciences de la Terre et de l’Atmosphère, Université du Québec à
Montréal, C.P. 8888, Succursale Centre-Ville, Montreal, QC,
Canada PQ H3C 3P8.
E-mail address: [email protected] (A. Tremblay).
1
Present address: Department of Earth and Planetary Science,
McGill University, Montreal, QC, Canada.
1. Introduction
Continental rifting and breakup leave profound
structural damage in the basement rocks of rifted
0040-1951/03/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0040-1951(03)00227-0
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margins. The rift structures consist of faulted basement blocks with tilted horst and graben systems
developed on both listric and planar normal faults
(Gibbs, 1984). At the scale of the continental crust,
rifted margins are commonly characterized by major
detachment faults, which separate upper crustal rocks
showing steeply dipping brittle faults from middle
crustal rocks in which shallow dipping mylonite zones
are predominant (see Lister et al., 1991). In mature
passive continental margins, the upper crustal rift
faults and structures are rarely exposed at the surface
due to the deposition of extensive sheets of syn- to
postrift sediments, and therefore, their recognition
frequently depends on seismic reflection profiles. In
old rifted continental margins that have suffered
orogenic crustal compression following formation,
the characterization and recognition of rift structures
are even more complicated. In this case, the rift faults
may have been over-printed or obliterated by younger
episodes of ductile deformation and/or reactivated into
reverse or normal faults due to crustal compression,
synorogenic sedimentary loading and subsidence of
foreland areas, or a younger episode of continental
extension following orogenic accretion.
The St. Lawrence rift system of eastern North
America (Fig. 1) is a seismically active zone (Adams
and Basham, 1989) where fault reactivation is believed to occur along late Proterozoic to early Paleozoic normal faults related to the opening of the Iapetus
ocean (Kumarapelli and Saull, 1966; St-Julien and
Hubert, 1975; Kumarapelli, 1985). The rift-related
faults fringe the contact between the Grenvillian
metamorphic basement to the NW and Cambrian–
Ordovician sedimentary rocks of the St. Lawrence
Lowlands platform to the SE (Fig. 1). The St. Lawrence rift system trends NE –SW and represents a halfgraben that links the NW – SE-trending Ottawa – Bonnechère and Saguenay River grabens (Figs. 1 and 2),
both interpreted as Iapetan failed arms (Kumarapelli,
1985). From SW to NE, the St. Lawrence rift faults
follow the north shore of the St. Lawrence River, up to
a point located approximately 100 km to the NE of
Québec city, where they become submarine structures,
continuing NE into the St. Lawrence River estuary
and the Gulf of St. Lawrence. When the actual St.
Lawrence River drainage system was established is
yet unknown, as is the structural control on the
morphology of the gulf. There is compelling stratigraphic (Lavoie, 1994; Lemieux, 2001) and magmatic
(Kumarapelli, 1985) evidence for Cambrian-to-Ordovician growth faulting along the St. Lawrence rift
system, but the lack of isotopic age data and the
Fig. 1. Schematic map of the St. Lawrence rift system showing the location of the Ottawa – Bonnechère and Saguenay grabens. Dark grey is for
the Grenvillian basement and light grey for younger rocks of the St. Lawrence Lowlands and the Appalachians. SLRS, St. Lawrence rift system.
A. Tremblay et al. / Tectonophysics 369 (2003) 231–252
233
Fig. 2. Geological map of the St. Lawrence rift system between Québec city and the St. Lawrence estuary, showing the major rift faults, inferred
limit of the Charlevoix impact crater, and location of high-resolution seismic lines used for this study. CRF, Chateau – Richer fault. See Fig. 1 for
location.
absence of rock strata younger than Ordovician make
it difficult to constrain the timing of younger fault
episodes. The faults crosscut Upper Ordovician strata
of the St. Lawrence Lowlands, hence yielding a lower
age limit for fault reactivation. There is no consensus
for the formation or reactivation of these faults during
the opening of the Atlantic ocean in Mesozoic times
(Kumarapelli, 1985; Hasegawa, 1986; Adams and
Basham, 1989; Du Berger et al., 1991; Lamontagne
et al., 2000; Hayward et al., 2001).
Few field studies have been conducted on faults
attributed to the St. Lawrence rift system. The faults
have traditionally been described as normal faults
(e.g. Osborne, 1956; St-Julien and Hubert, 1975), an
interpretation that has been challenged by Shaw
(1993) who applied a large-scale sinistral strike-slip
model to account for the formation of faults dissecting
the platform sequence during the Appalachian foreland oblique compression. Lachapelle (1993) has
investigated some of these faults between Québec city
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and Cap-Tourmente (Figs. 2 and 3). He argued that
fault orientations have been controlled by preexisting
basement structures and that the occurrence of fault
gouge marks the reactivation of Iapetan rift structures
during the Ordovician. Rocher et al. (2000) and
Rocher and Tremblay (2001) have conducted paleostress analyses of these supracrustal faults in the
Montréal and Québec city areas, and have argued
for a complex tectonic history involving faulting
during Iapetus rifting and reactivation during and after
Appalachian compression. North of Québec city (Fig.
2), Rondot (1968, 1971, 1989) has identified a series
of regional NE-striking faults within the Charlevoix
impact crater, which he attributed to Iapetus rifting.
Rondot (1998) stressed that fault breccias and pseudotachylyte occurrences in the Charlevoix area are
basically the result of impact cratering rather than the
product of inherited or neoformed faults. However,
Tremblay and Lemieux (2001) have recently shown
that the comparison of brittle faults related to the St.
Lawrence rift system with those found in the Charlevoix impact structure indicates that brittle fault rocks
and pseudotachylytes are not limited to impact cratering and occur well outside the crater boundaries along
the St. Lawrence rift system.
For this study, we have investigated the field
characteristics of supracrustal faults of the St. Lawrence rift system between Québec city and La Malbaie
Fig. 3. Simplified geologic map showing the trace of faults related to the St. Lawrence rift system between Cap-Tourmente and La Malbaie. See
Fig. 2 for location.
A. Tremblay et al. / Tectonophysics 369 (2003) 231–252
(Figs. 2 and 3), and we present the kinematic, structural and textural characteristics of these faults. The
study area offers excellent shoreline exposures along
the St. Lawrence River. The transition between onshore and offshore rift faulting occurs at LaMalbaie
and, therefore, we also present high-resolution marine
seismic reflection data acquired in the St. Lawrence
estuary (Fig. 2) in order to identify submarine topographic relief that can be attributed to the extension of
the St. Lawrence rift system, and to speculate on the
implications of rift faults for the occurrence of natural
gas in marine Quaternary unconsolidated sediments.
2. Geological setting
The studied onshore segment of the St. Lawrence
rift system occurs on the north shore of the St.
Lawrence River and extends for 120 km NE, from
Québec city to La Malbaie (Fig. 2). The bedrock
predominantly consists of Grenvillian metamorphic
rocks made up of high-grade orthogneisses and paragneisses, charnockites, anorthosites and granitoid
intrusions of the Laurentides Park Complex (Sabourin, 1973; Laurin and Sharma, 1975; Rondot, 1989).
These metamorphic rocks are part of the Jacques –
Cartier tectonic block (Du Berger et al., 1991),
wedged between the Saguenay graben and the St.
Lawrence rift system, and belong to the «Allochthonous Polycyclic Belt» of the Grenville orogen as
defined by Rivers et al. (1989). The metamorphic
rocks now exposed at the surface equilibrated at ca.
20 –25 km crustal depths and show complex overlap
of mantle and crustal processes (Du Berger et al.,
1991). Also, K – Ar ages from the Grenville Province
(Anderson, 1988) suggest that tectonic uplift, denudation and cooling below ca. 300 jC of the metamorphic rocks currently exposed at the surface was well
underway by 850 Ma.
Down-faulted Cambrian –Ordovician rocks of the
St. Lawrence Lowlands platform are found to the
south and SE of the St. Lawrence rift system faults
(Fig. 2). The St. Lawrence Lowlands is an unmetamorphosed and slightly deformed Cambrian to Early
Ordovician rift and passive continental margin succession (Bernstein, 1991) overlain by Middle to Late
Ordovician shallow to deep marine foreland basin
deposits (Lavoie, 1994). Rocks of the St. Lawrence
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Lowlands unconformably overlie the Grenvillian
basement. The sedimentary succession progressively
thickens toward the SW, from the basal unconformity
exposed along its northern limit in the Québec city
area, to exceed 1500 m in the Montreal region. The
basal unconformity with the underlying Grenvillian
basement is exposed in the footwall of the Montmorency fault (Fig. 2) and on both sides of the St.
Laurent fault (Fig. 3). In the Charlevoix area, rocks
of the St. Lawrence Lowlands also occur within two
NW –SE-trending valleys, interpreted as part of the
impact-related annular graben occurring in the footwall of the St-Laurent fault (Rondot, 1989; Lemieux
et al., 2003). The meteoritic origin of the Charlevoix
structure has been established from shatter cone
occurrences, microscopic shock metamorphic features
in breccia dikes and basement gneisses, and remnants
of impact melt (Rondot, 1971, 1998; Robertson,
1975). The crater is believed to be Devonian based
on unpublished K –Ar analysis of impact melt and
breccia dike which yielded ages of 321 and 372 Ma
(Rondot, 1971). The Charlevoix area is part of eastern
Canada’s most seismically active zone, the Charlevoix
seismic zone (Lamontagne, 1999; Lamontagne et al.,
2000), but it is not clear if the current seismicity is
principally related to impact cratering or to the St.
Lawrence rift system (or both).
3. Brittle faults of the St. Lawrence rift system
We have mapped brittle structures and fault rocks
of the St. Lawrence rift system in the Québec city area
and along the shoreline of the St. Lawrence River
from Cap-Tourmente to La Malbaie, the latter segment offering almost continuous rock exposure for a
distance of approximately 60 km (Figs. 2 and 3).
Three major supracrustal faults, known as the Montmorency (Fig. 2), Cap-Tourmente and St-Laurent
faults (Fig. 3), have been investigated. These faults
form regional structures of the St. Lawrence rift
system and expose various types of brittle fault rocks
that can be considered as typical products of the rift
faulting. There are several other rift faults in the
Québec city area, such as the Deschambault, Neuville
and Chateau-Richer faults (Fig. 2), but the Montmorency fault is the best exposed. The Cap-Tourmente
and St-Laurent faults essentially occur within the
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Grenvillian crystalline basement but remnants of the
St. Lawrence Lowlands are locally found, mainly
along the Cap-Tourmente fault. In the following
section, we describe the orientation and the kinematics
of Montmorency, Cap-Tourmente and St-Laurent
faults, and the nature of associated fault rocks. We
use the Sibson’s (1977) classification of fault rocks as
modified in Scholz (1990); the only significant departure from the Sibson’s classification being the addition
of foliated gouge (see Snoke et al., 1998 for details).
3.1. Fault orientation
For the statistical analysis of brittle faults attributed
to the St. Lawrence rift system, we only considered
mesoscale fault planes (i.e. a plane where evidence for
frictional sliding is present; Hancock, 1985). Joints,
abundant in basement rocks, were excluded from the
data set because their genetic relationship to the rift
faults is not always obvious. In the field, faults were
recognized as fracture planes that show evidence for
sliding, marked by cataclasis, slickensides, striated
mineral filling (quartz or calcite) and/or displaced
marker horizons.
The schematic structural maps (Figs. 2 and 3)
as well as field measurements show several fault
trends (Fig. 4) that can be grouped into two major
orientations with respect to the rift axis: NE-trending
longitudinal faults subparallel to the rift axis, and
transverse faults striking to the NW or the SE.
Longitudinal faults show three subsets of trends,
N025j, N040j and N070j, which represent more
than 30% of all measurements (Fig. 4A). These faults
generally dip towards the SE (Fig. 4A), but a minor
number ( < than 3%) dip to the NW. Longitudinal
faults extend for tens of kilometres (up to 60 km for
the St-Laurent fault) along strike and are limited by
transverse faults. Transverse faults show two subsets
of orientations: WNW –ESE (N290j) and NW –SE
(N310j) (Fig. 4A). Dip directions of transverse faults
are towards the NE or the SW (Fig. 4A), which is
consistent with the horst-and-graben geometry illustrated by these structures on seismic profiles (see
below). The transverse faults are of limited extent
(less than 10 km) and are bounded by longitudinal
faults. In the field, the longitudinal and transverse
fault sets show mutual crosscutting relationships,
suggesting that they represent conjugate structures
related to the same tectonic event.
Both sets of faults are high-angle faults with dip
angles averaging 75– 80j (Fig. 4B). Approximately
20% of data measurements show striated and/or
slickensided fault planes. The pitch value of fault
lineations is greater than 70j (Fig. 4C) for more than
65% of measurements, indicating that most structures
are dip-slip faults. A minor number of faults (less than
Fig. 4. Rose diagrams for faults of the St. Lawrence rift system in the studied area. (A) Strike orientations for the rift faults. The format of
orientation data follows the right-hand rule convention (i.e. the down-dip direction is clockwise from the strike direction). The rose diagram
indicates that the majority of NE-striking faults are dipping toward the SE. (B) Dip angle values for the rift faults. (C) Pitch angle values of fault
striations. N, number of data.
A. Tremblay et al. / Tectonophysics 369 (2003) 231–252
10% of measurements) are characterized by subhorizontal to slightly plunging lineations (Fig. 4C), which
were mainly observed along transverse faults.
There are no clear indications that preexisting
structures of basement rocks have influenced the
development of the St. Lawrence rift faults, although
it has been locally suggested (Lachapelle, 1993; Shaw,
1993). As in most extensional settings (e.g. Coletta et
al., 1988; Ebinger, 1989; Schlische, 1993), it is likely
the case but the structural pattern of the Grenvillian
basement is not known in enough detail to establish
such relationships. It is also likely that cratering
processes in the Charlevoix area have affected the
orientation of rift faults within the crater boundaries.
Part of the scattering of fault orientations data shown
in Fig. 4A, particularly for NW- and SE-striking
faults, may have been introduced by the existence of
impact-related faults in this segment of the St. Lawrence rift system.
3.2. Nature of fault rocks
Coastal sections of the St. Lawrence River that best
expose the various types of fault rocks related to the
St. Lawrence rift system have been studied along the
Montmorency, Cap-Tourmente and St-Laurent faults
(Figs. 2 and 3). Rocks delineating these faults consist
of cohesive fault rocks of the cataclastic series (Sibson, 1977).
3.2.1. Breccias and cataclastic rocks
Fault rocks of the St. Lawrence rift system more
commonly consist of fault breccias and cataclasites,
but pseudotachylytes and foliated fault gouges are
also found (Fig. 5). Rock units of the St. Lawrence
Lowlands are only locally exposed in the hanging
wall of the St-Laurent fault, in contact with fault
breccias developed in the metamorphic basement.
The St-Laurent fault is well exposed along the coast
of the St. Lawrence River, at approximately 1 km
north of Cap-Tourmente and at Sault-au-Cochon (Fig.
3). At the latter locality, the St-Laurent fault is
associated with a topographic escarpment of ca. 800
m (Fig. 6), which corresponds to a minimum value of
fault throw. Rondot (1989) estimated the vertical
throw of the St-Laurent fault to be ca. 2 km in the
Charlevoix area. The quality of exposure is such that
at Sault-au-Cochon it can be considered as a type
237
locality. The minimum true thickness of fault-related
rocks exposed at Sault-au-Cochon is 20 m (Fig. 6).
Rock fragments in fault breccias essentially consist of
metamorphic rocks; they are angular to subangular
and vary from a few cm to more than 50 cm in
diameter (Fig. 5A). The breccia matrix is made up of
fine-grained rock fragments (i.e. the size of crush and
fine crush breccias) and is commonly chlorite-rich.
Quartz- and/or calcite-rich veinlets, similar to those
described by Carignan et al. (1997), are locally found.
An ill-defined planar fabric, hosting down-dip or
steeply plunging striations, is frequently developed
in fault breccias. This fabric is locally associated with
nascent C/S fabrics and shear band structures that
indicate normal sense faulting toward the SE (Fig.
5B). Veinlets of dark-colored pseudotachylyte (Fig.
5C) are common within the fault breccias and trend
both parallel and at high-angle with the dominant
fabric of the fault. Pseudotachylyte is also locally
found as a fine-grained component in the matrix of
cataclastic breccias. Decimetre-thick horizons of foliated to massive ultracataclasites associated with
foliated fault gouge are locally exposed. The ultracataclasite horizon is characterized by cm- to mmscale fragments of basement rocks in a very-finegrained and chlorite-rich matrix (Fig. 5D). The contact
between the fault rocks and the unfaulted basement
is abrupt, although the basement frequently shows
the progressive development of brittle fractures
(joints) trending parallel to the fault zone that are
pervasively developed (more than 20 joints/m) in
metamorphic rocks adjacent (i.e. within 5 m) to the
fault boundaries (Fig. 6). The exposure at Sault-auCochon suggests that normal fault zones of the rift
system consists of alternating and anastomosing horizons of fault breccias, cataclasites, ultracataclasites
and fault gouges, fault breccia being the far most
abundant type of fault rocks (Fig. 6).
Fault rocks that mark the Cap-Tourmente and
Montmorency faults are identical to those of the StLaurent fault. The Montmorency fault is, however,
less well exposed than the other two structures. It is
made up of two fault segments between Québec city
and Cap-Tourmente (Fig. 2), the NE segment is also
known as the Chateau-Richer fault (Nadeau et al.,
1994). The Cap-Tourmente fault is associated with a
steep topographic escarpment of approximately 700
m, corresponding to a minimum fault throw value.
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The Montmorency fault extends NE from Québec city
and cuts the western extremity of the Cap-Tourmente
fault (Fig. 2). Along its strike, the western segment of
the Montmorency fault marks the contact between the
basement and the St. Lawrence Lowlands or crosscuts
limestone rocks of the latter unit (Fig. 5E). Near
Québec city, it is associated with a fault scarp of ca.
80 m. There, rocks units of the St. Lawrence Lowlands are found on both sides of the Montmorency
fault. They occur as flat-lying carbonates unconform-
Fig. 5. Field photographs of fault rocks related to the St. Lawrence rift system. (A) Fault breccia of Grenvillian basement rocks along the StLaurent fault in the Charlevoix area. (B) Foliated gouge of the St-Laurent fault at Sault-au-Cochon (see Fig. 3 for location). Vertical view toward
the southwest. Note nascent shear bands indicating normal-sense faulting. The width of the photographic view is approximately 1 m. (C)
Veinlets of pseudotachylyte in fractured metamorphic rocks at Sault-au-Cochon. Lens cap is approximately 5 cm in diameter. (D) Close view of
ultracataclasite related to the St-Laurent fault at Sault-au-Cochon. Note the faintly developed fault fabric. Coin is 3 cm in diameter. (E) The
Montmorency fault crosscutting bedded limestone of the St. Lawrence Lowlands near Chateau – Richer. (F) Fault breccia related to the CapTourmente fault. Horizontal planar view. G) Fault breccia associated with the Montmorency fault near Chateau – Richer. Horizontal plane view.
Notebook is 20 cm long. (H) Foliated fault gouge and shear fabrics (indicating normal-sense faulting) along the Montmorency fault at the
Montmorency Falls near Québec city. Vertical view toward the northeast. Hammer for scale.
A. Tremblay et al. / Tectonophysics 369 (2003) 231–252
239
Fig. 5 (continued).
ably overlying the Grenvillian basement in the footwall of the fault, but as a SE-dipping, tilted block of
siliciclastic rocks in its hanging wall. The value of
fault throw for the Montmorency fault is not precisely
known but stratigraphical analysis of the St. Lawrence
Lowlands in the area suggests that it is less than 150
m (D. Lavoie, personal communication), which is
much less than the minimum throw values inferred
for the Cap-Tourmente and St-Laurent faults. However, there are several offshore faults subparallel to the
Montmorency fault in the area, and the depth to
basement determined by seismic profiles (Société
québécoise d’initiatives pétrolières, 1974) and crosscorrelations with depth to basement measured in oil
exploration wells of the area (Ministère des Richesses
Naturelles du Québec, 1974) suggest vertical downthrow values that attain up to 1 km (Nadeau et al.,
1994). Fault breccias occurring along both the CapTourmente and Montmorency faults are 5 – 10 m
thick, and are made up of centimetre- to decimetrescale fragments of gneiss, granitoids and marbles
settled in a fine-grained matrix of crush-breccia (Fig.
5F and G). Foliated fault gouge (Fig. 5H) and calcite
cementation of the crush breccia matrix were locally
observed. Slickensides were only rarely found along
these two faults. As for the St-Laurent fault, pseudotachylyte occurs as cm-wide veins or forms a composite matrix with fine-grained rock fragments. Minor
faults trending approximately N120j and steeply
dipping NE or SW occur in the footwall of the CapTourmente fault. These faults are marked by closely
spaced, NW-trending brittle fractures or by 0.5 –1.0m-wide zones of cataclasites. These minor faults are
interpreted as Riedel-type subsidiary structures. Con-
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A. Tremblay et al. / Tectonophysics 369 (2003) 231–252
Fig. 6. NW – SE schematic cross section across the St-Laurent fault at Sault-au-Cochon and schematic illustration showing the distribution of
fault rock facies along the St-Laurent fault. The brick pattern is for carbonate rocks of the St. Lawrence Lowlands. S.L. = sea level. Vertical
exaggeration = 1.5.
tinuous exposure along the St. Lawrence River clearly
indicates that W- to NW-trending faults associated
with the Cap-Tourmente fault and NE-trending structures of the St-Laurent fault are characterized by
mutual crosscutting relationships.
Within the boundaries of the Charlevoix impact
crater, two types of fault-related breccias exist: (1)
coarse-grained cataclastic breccias and (2) polymictic
clastic matrix breccias. Coarse-grained cataclastic
breccias (type 1) are relatively abundant and identical
to those described above for the Montmorency, CapTourmente and St-Laurent faults. Polymictic clastic
matrix breccias (type 2) are exclusively found within
the impact crater (Lemieux et al., 2003), and represent the Rondot’s (1971, 1989, 1998) mylolisthenite.
This type of breccia corresponds to Lambert’s (1981)
Type B polymictic breccias, which have clastic matrices and are either monomictic or polymictic. Within the impact crater, these breccia zones are 30 –50
cm wide, up to tens of metres in lateral extent and in
sharp contact with host rocks. They are medium to
dark greenish grey and have a heterogeneous texture
characterized by matrix-supported angular clasts
ranging from 0.5 to 2.0 cm in diameter within an
aphanitic calcareous matrix. Rock fragments com-
monly constitute 20 – 40% of the breccias and consist
of gneiss and more rarely sandstone, limestone and
shale fragments. These polymictic breccias of the
Charlevoix area are not genetically related to the St.
Lawrence rift system and have been attributed to
impact cratering processes (Lemieux, 2001; Lemieux
et al., 2003). They are similar to breccias described
by Dressler and Sharpton (1997) for the Slate Islands
impact structure.
3.2.2. Pseudotachylytes
Pseudotachylyte formation is usually associated
with seismic faulting, as the product of either, or
both, comminution and shear-related melting of fault
rocks (Spray, 1995; Curewitz and Karson, 1999;
Fabbri et al., 2000). Pseudotachylyte occurs at several
localities along brittle faults of the St. Lawrence rift
system. It commonly makes up the dark-colored,
fine-grained and wispy component of the matrix of
coarse-grained cataclastic breccias (type 1 breccias
described above) and also forms cm-thick veins of
glassy material (Fig. 5C) oriented both parallel and at
high-angle to brittle faults (Tremblay and Lemieux,
2001; Lemieux, 2001). In the Charlevoix area, these
dark-colored pseudotachylytes have been interpreted
A. Tremblay et al. / Tectonophysics 369 (2003) 231–252
as impact-related fault rocks (Rondot, 1998), similar
to the polymictic clastic matrix breccia (mylolisthennite) described above. However, cataclastic breccias
and pseudotachylyte veins of this type occur almost
everywhere along the St. Lawrence rift system
(Rocher et al., 2000; Tremblay and Lemieux, 2001).
Pseudotachylytes, in this case, cannot be considered
as a diagnostic feature of impact cratering, and are
more likely related to frictional melting associated
with faults of the St. Lawrence rift system.
3.3. Fault evolution
The St. Lawrence rift system consists of steeply
inclined, dip-slip faults characterized by normal-sense
movement along longitudinal faults and a variable
component of strike-slip faulting along transverse
faults (Fig. 4). This is in contrast with Shaw’s
(1993) interpretation of these faults as the result of a
left-lateral basement-involved wrench system. Shaw
(1993) based his tectonic interpretation on the following points: (i) km-scale sinistral offset of facies
boundaries at the NW edge of the St. Lawrence
Lowlands platform; (ii) horizontal slickenlines observed along the Montmorency fault; (iii) bedding
planes dipping away from the fault in the hanging
wall of the Montmorency fault (i.e. instead of dipping
into the fault plane as commonly described in extensional tectonic settings), interpreted as evidence for
flower structures. However, we did not find structural
evidence for major wrench faulting along the St.
Lawrence rift system. Except for the local occurrence
of subhorizontal slickenlines along transverse faults,
fault striations along the rift faults indicate dip-slip
movement (Fig. 4), and at several localities, kinematic
indicators are consistently indicating normal-sense
faulting (Fig. 5). Sinistral offsets of facies boundaries
can be attributed to the fact that the St. Lawrence
Lowlands platform is an overall SW-dipping sequence
(i.e. the direction of increasing thickness) and that
NE-striking normal faults are thus associated with an
apparent left-lateral sense of displacement. Subhorizontal slickenlines along the Montmorency fault are
probably unique to this locality and likely resulted
from an unresolved older or younger increment of
faulting. Finally, hanging wall beds dipping away
from normal fault planes have also been described
in classical extensional rift settings (e.g. Gross et al.,
241
1997) and can be interpreted as the result of monoclinal flexures (drag folding) above a basement normal fault that progressively propagated through the
overlying sedimentary strata.
The different fault sets of the St. Lawrence rift
system share the same type of structures and faultrelated rocks, suggesting that both the longitudinal
(typified by the Montmorency and St-Laurent faults)
and transverse faults (typified by the Cap-Tourmente
fault) are associated with the same event. The Montmorency fault occupies the same structural position as
the St-Laurent fault (Fig. 2), suggesting that both
structures are correlative and offset by a fault relay
represented by the Cap-Tourmente fault. However,
normal faults and rift systems with large normal faults
are commonly segmented (Gibbs, 1984; Rosendahl,
1987; Peacock and Sanderson, 1991) and we suggest
that longitudinal faults of the St. Lawrence rift system
more likely result from the development of «en
échelon» faults trending parallel to the rift axis, and
that transverse structures such as the Cap-Tourmente
fault represent transfer faults (Gibbs, 1984) or accommodation zones (Rosendahl, 1987). During the extensional phase of faulting, normal faults probably
propagated along the entire rift zone, but their longitudinal propagation was stopped and laterally shifted
along transfer faults, producing an oblique relay
between two longitudinal normal faults. This geometry can be attributed to along-strike variations in
displacement along the longitudinal faults, i.e. fault
displacement decreasing toward the ends of these
faults. Such displacement variations have been documented on normal faults ranging in length from a few
cm to tens of km, indicating that this is scale-invariant
feature (e.g. Muraoka and Kamata, 1983; Barnett et
al., 1987; Peacock and Sanderson, 1991; Schlische,
1993). This would explain the inferred decreasing
fault throw between the St-Laurent and the Montmorency faults, the latter fault ending near Québec city
(Fig. 2) where fault throw is relatively small compared
to that of the St-Laurent fault.
Lachapelle (1993) interpreted fault gouge horizons
of the St. Lawrence rift system as the product of upper
crustal reactivation of older brittle faults. We cannot
exclude such an interpretation, but at present, there is
no clear evidence for repeated brittle faulting largely
separated in time along these structures. It is likely
that cataclastic rocks, pseudotachylytes and fault
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gouge occurrences of the St. Lawrence rift system are
the product of a single and progressive faulting event.
Major fault zones of the crust commonly show a large
variety of fault rocks representing more or less pronounced variations of pressure, temperature, strain
rate, and fluid conditions that prevailed during incremental faulting. For large-displacement normal-sense
faults, it is common to juxtapose ductile fault rocks in
the footwall against brittle fault rocks in the hanging
wall due to progressive exhumation (e.g. Snoke et al.,
1998). This is obviously not the case for the St.
Lawrence rift system, where ductile fault rocks
(mylonites) that would be related to the rift faults
are lacking. However, in the upper crust, contrasting
types of brittle fault rocks can be juxtaposed, and this
commonly records changes of deformation mechanisms during progressive and incremental deformation (Magloughlin, 1992; Swanson, 1992; Snoke et
al., 1998; Fabbri et al., 2000).
4. Seismic signature of the St. Lawrence rift system
In order to extend our structural interpretation of
the St. Lawrence rift system beyond field data, we
used high-resolution seismic reflection data acquired
in the St. Lawrence estuary (Fig. 2). The aims of the
following analysis of seismic profiles are mainly to
localize and to characterize major submarine topographic relief corresponding to fault scarps, which can
be attributed to the offshore extent of the rift system.
The St. Lawrence estuary extends NE – SW for
more than 400 km, from the vicinity of Québec city
to Anticosti Island. Bathymetric charts, seismic profiles and geophysical data (Lamontagne et al., 2003)
from the estuary show a complex submarine morphology. Massé (2001) conducted a detailed study of the
seismostratigraphy of unconsolidated sediments of the
St. Lawrence estuary. In order to precisely determine
the thickness of the different seismostratigraphic units,
from the base to the top of marine Quaternary sediments, and to map their lateral extension, Massé
(2001) analyzed more than 700 km of high-resolution
seismic reflection profiles (along a 200-km-long segment of the St. Lawrence River). These were acquired
in the St. Lawrence estuary between the Charlevoix,
Forestville and Rimouski areas, on the north and south
shores of the St. Lawrence River, respectively (Fig. 2).
The seismic profiles indicate that the St. Lawrence
estuary is characterized by longitudinal axes of basement rock and by numerous perpendicular faults
defining a complex network of sedimentary basins
and subbasins, such as the Laurentian Channel (Massé
and Long, 2001). The latter appears as a partially
filled basin, deeply incised into basement rocks,
which has been considered to be the most important
ice conduit of the Laurentian Ice Sheet during the
Quaternary (Syvitski and Praeg, 1989). The St. Lawrence Channel is located at a water depth of over
300 – 400 m, while its width varies from 15 to 25 km
(Drapeau, 1992). The analysis of seismic profiles
reveals numerous slope instability features in unconsolidated sediments, such as failure canyons, failure
scars, submarine landslides, active faults and gas
horizons (Massé and Long, 2001). Gas horizons are,
however, only present in the deepest part/water of the
Laurentian Channel, where they can laterally extend
over a few tens of metres (Massé and Long, 2001).
These gas horizons were identified on cores and
seismic profiles acquired during the MD-99 Cruise
(Long and Cagnat, 2000) and most likely represent
gas hydrates. Their occurrences in the Laurentian
Channel are of particular interest here and a possible
genetic link with rift faults will be discussed below.
The high-resolution seismic profiles were first used to
delineate the contact between the unconsolidated
sedimentary sequence and the bedrock basement over
which it accumulated in the St. Lawrence estuary and,
second, to identify topographic relief that can be
reasonably attributed to fault scarps.
4.1. High-resolution seismic data acquisition and
processing
The high-resolution, single channel seismic reflection profiles were collected during three cruises (BS97, ACH-97 and DR-98). The seismic lines were shot
with a 1 and 4 Kj sparker providing an average
penetration of 1 s (approximately 900 m). The frequency content of the seismic data, centered on 400
Hz, provided a resolution of 2.5 m. In the following
presentation of data and discussion, all values in
milliseconds were converted to metres using an average velocity in water of 1500 m/s. Therefore, all depth
values are approximate and represent minimal thicknesses or depths in unconsolidated sediments and
A. Tremblay et al. / Tectonophysics 369 (2003) 231–252
especially in bedrock, where the velocity is considerably higher than in water. For example, MST (GEOTEK Multi-Sensor Core Logger System)
measurements made during the MD-9922 cruise of
the IMAGES project on core #99-2220 (Fig. 2)
indicate that velocity in Quaternary sediments
increases from 2000 to 2300 m/s between 0 and 15
m depth, and reaches 2450 m/s at 50 m depth (Cagnat,
in preparation). For the purpose of this study, we use
three representative seismic profiles (Fig. 2): BS-16/
09/97, BS-18/09/97 and ACH-11-12/10/97. Two of
these profiles (BS-16/09/07 and BS-18/09/97) trend
NW – SE, perpendicular to the St. Lawrence estuary
and the inferred orientation of the St. Lawrence rift
system, whereas profile ACH-11-12/10/97 trends parallel to the estuary and extends for approximately 140
km along the strike of the rift system. In the profile
displays shown here, one horizontal km is equal to
about 75 m of water depth, which equates to a vertical
exaggeration of about 13:1 with respect to the seafloor, and about 10:1 with respect to the top-ofbasement.
4.2. Interpretation of seismic profiles
Profiles BS-16/09/07 and BS-18/09/97 are shown
in Figs. 7 and 8. Profile BS-16/09/97 (Fig. 7) extends
between Rimouski and Forestville (Fig. 2), on the
south and north shore of the St. Lawrence River,
respectively, for a total distance of approximately 30
km. Profile BS-18/09/97 shows a ca. 20-km-long
section of a NW –SE seismic line extending from a
mid-point located between the mouth of the Saguenay
River and Forestville on the north shore and a point
located 40 km to the SW of Rimouski on the south
shore (Fig. 2). The unconsolidated seismostratigraphic
units shown on profile BS-16/09/97 (Fig. 7) are from
Massé (2001).
Massé (2001) recognized seven seismostratigraphic units of irregular thicknesses within the unconsolidated sedimentary sequence. The contact between
the unconsolidated Quaternary sediments and basement rocks is a highly reflective surface, which has
been recognized on most seismic profiles of the area.
Mapping of the top-of-basement reflection on seismic
profiles BS-16/09/97 and BS-18/09/97 (Figs. 7 and 8)
clearly shows that the Laurentian Channel forms a
partially filled sedimentary trough under a water depth
243
of more than 300 m. To the NW, the Laurentian
Channel trough is bounded by an apparently steep
slope corresponding to a single topographic scarp
approximately 225 m high (corresponding to ca. 300
ms TWTT) on profile BS-16/09/97 (Fig. 7). The SE
boundary of the Laurentian Channel trough also
corresponds to apparently steep topographic slopes
(ca. 200 ms TWTT, or approximately 150 m on the
profile in Fig. 7), although the geometry is more
complicated than on the NW side of the channel. In
reality, if these slopes are corrected for the vertical
exaggeration and then migrated to their proper geometries, the faults indicated here have true dips of 40 –
60j and the shoulder topography is dipping gently
toward the NW. The SE shoulder geometry of top-ofbasement reflection suggests the presence of smallscale, totally or partially filled subbasins under water
depths of 0 –150 m. The thickness of the unconsolidated sedimentary sequence is less than 75 m (i.e. 100
ms TWTT) on the shoulders of the Laurentian Channel trough, but varies between minimal values of 225
and 350 m (300 –450 ms TWTT) within the channel.
These thicknesses increase, however, to 300 and 450
m if one uses an average minimal velocity of 2000 m/
s in unconsolidated sediments. Thicknesses of this
order have been measured for distances of more than
50 km along the strike of the Laurentian Channel
(Massé, 2001).
The geometry of the top-of-basement reflections
along the NW – SE-trending seismic lines across the
Laurentian Channel are suggestive of a rift graben
structure. The submarine topographic slopes that
bound the channel trough are interpreted as fault
scarps that have been erosionally bevelled. They
likely correspond to the same system of longitudinal
faults described above for the onshore segment of the
St. Lawrence rift system. As for the onshore rift faults,
we interpret the submarine topographic scarps as
corresponding to minimum values of fault throw.
Offsets of top-of-basement reflections on both sides
of these faults suggest throws of more than 300 –500
m (i.e. 450 –700 ms TWTT), which compares well
with minimum fault throw values inferred for the St.
Laurent fault (see above).
Due to weak penetration for values >900 ms
TWTT and to the presence of multiples, high-resolution seismic data cannot be used to typify the seismic
signature of basement rocks, and it is thus difficult to
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A. Tremblay et al. / Tectonophysics 369 (2003) 231–252
Fig. 7. NW – SE-trending high-resolution seismic reflection profile BS-16/09/97 crossing the St. Lawrence Channel. The top-of-basement reflector (TBR) defines a well-developed
graben bounded by NE-striking fault escarpments. The seismostratigraphy of unconsolidated deposits is from Massé (2001). See Fig. 2 for location. TWTT = two-way travel time.
Vertical exaggeration = 13:1 relative to water column. The labeled dashed line indicates where ACH-11-12/10/97 ties to profile. The horizontal scale is approximative.
A. Tremblay et al. / Tectonophysics 369 (2003) 231–252
245
Fig. 8. (A) NW – SE-trending high-resolution seismic reflection profile BS-18/09/97. (B) Interpretation showing the location of the top-ofbasement reflectors (TBR) and major normal faults. See Fig. 2 for location. Other symbols as in Fig. 7. Vertical exaggeration = 13:1 relative to
water column. The labeled dashed line indicates where ACH-11-12/10/97 ties to profile. The horizontal scale is approximative.
firmly determine the nature of the bedrock in the St.
Lawrence estuary and the Laurentian Channel. However, basement rocks of the NW shoulder of the
channel are likely similar to those exposed along the
shore of the St. Lawrence River, which consist of
predominantly Grenvillian metamorphic rocks, with
minor and isolated occurrences of platformal units of
the St. Lawrence Lowlands. Similarly, the bedrock of
the SE shoulder is likely dominated by Appalachian
rock units, which crop out onshore in Gaspé Peninsula. On the basis of rock exposures characterizing the
St. Lawrence rift system in the Québec city area, we
speculate that the bedrock of the intervening Laurentian Channel trough consists of either St. Lawrence
Lowlands or Appalachian units unconformably overlying Grenvillian metamorphic rocks. Sandford and
Grant (1990) have suggested that part of the Laurentian Channel is underlain by Carboniferous rocks of
the Maritimes Basin (Williams, 1974), which may
imply larger values of vertical fault throw than those
inferred above on the basis of submarine topography.
This also supports the hypothesis of Mesozoic or
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A. Tremblay et al. / Tectonophysics 369 (2003) 231–252
Fig. 9. NE – SW-trending high-resolution seismic reflection profile ACH-11-12/10/97. The seismostratigraphy and interpretation of unconsolidated deposits are from Massé (2001).
The mouth of the Saguenay River is located for reference. See Fig. 2 for location. Other symbols as in Fig. 7. Vertical exaggeration = 13:1 relative to water column. Labeled dashed
lines indicates where BS-16/09/97 and BS-18/09/97 tie to profile. The horizontal scale is approximative.
A. Tremblay et al. / Tectonophysics 369 (2003) 231–252
younger faulting increments along the St. Lawrence
rift system (see below).
Profile ACH-11-12/10-97 (Fig. 9) is approximately
140 km long and extends NE from the vicinity of La
Malbaie, following the trend of the Laurentian Channel toward the Gulf of St. Lawrence (Fig. 2). The topof-basement reflections can be followed along much
of the profile. However, at the mouth of the Saguenay
River, the basement is somewhat obcurred by abundant multiples. Along the SW part of the line, the
water depth is < 150 m (100 – 200 ms TWTT) whereas the thickness of unconsolidated sediments varies
from 75 to 300 m (100 – 400 ms TWTT; Fig. 9). The
NE two thirds of the profile approximately follows the
axis of the Laurentian Channel trough. Here, the water
depth increases to more than 200 m (300 ms TWTT)
and the thickness of the unconsolidated sedimentary
sequence is up to approximately 500 m (i.e. 400 – 650
247
ms TWTT), suggesting minimum values of depths to
basement on the order of 700 – 800 m.
North of the Saguenay River mouth, the geometry
of the top-of-basement reflections is typical of rift
graben strike lines. The basement topography is
irregular but relatively gentle, punctuated by faults
of various apparent dips with opposing dip directions.
Some of these faults are likely longitudinal fractures
crossed highly obliquely; others are probably transfer
faults. None of these faults offset the unconsolidated
sedimentary sequence. At the mouth of the Saguenay
River, the seismic profile suggests the existence of a
major submarine landslide extending over more than
20 km and affecting the unconsolidated sedimentary
sequence over a thickness of more than 175 m (Massé
and Long, 2001). This slump could have been triggered by seismic activity along the axis of the St.
Lawrence rift system.
Fig. 10. (A) Two-way travel time (TWTT (ms)) to bedrock map for the St. Lawrence estuary. Modified from Massé (2001). (B) Map showing
the trace of offshore normal faults of the St. Lawrence rift system in the St. Lawrence estuary, based on the interpretation of high-resolution
seismic data. Patterns and other symbols as in Fig. 2.
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A. Tremblay et al. / Tectonophysics 369 (2003) 231–252
The dip profiles in Figs. 7 and 8 and the strike
profile in Fig. 9 provide a vague sense of basinal
asymmetry and longitudinal segmentation. Both of the
dip lines (BS-16/09/97 and BS-18/09/97) hint at a
slight down-to-the-southeast asymmetry for the
trough basement, and nothing shown on the strike
line (ACII-11-12/10/97) between the tie points argues
against this. It also could be significant that as the SW
end of the strike line approaches the shoreline south of
the Saguenay River (Fig. 2), the basement horizon
increases in depth (Fig. 9). This is suggestive of a
change in basinal polarity to either side of the basement high (accommodation zone?) located at time
00:00 on the profile.
The marine seismic data have been used to construct a map of two-way travel time to bedrock for the
St. Lawrence estuary (Fig. 10A) in order to better
ascertain basinal geometries. The Laurentian Channel
forms an elongated sedimentary trough that trends
parallel to the axis of the St. Lawrence rift system.
The depth of the Laurentian Channel trough decreases
at the mouth of the Saguenay River, as noted above.
South of the mouth, the Laurentian Channel deepens
to values up to 350 – 400 m (ca. 500 ms TWTT) and
its trend is almost perfectly coincident with the
onshore segment of the St. Lawrence rift system
(Fig. 10A). A summary fault map is shown in Fig.
10B.
4.3. Summary
High-resolution seismic data from the St. Lawrence estuary suggest the occurrence of two types of
basement faults expressed as prominent submarine
topographic features that controlled the development
of Quaternary sedimentary basins. The presence of
steeply dipping NE-trending faults bounding the Laurentian Channel trough is clearly indicated by NW –
SE-trending seismic profiles (Figs. 7 and 8), whereas
basement faults dipping moderately towards the SW
and the NE, and which probably strike toward the
west and/or the NW, are shown on line ACH-11-12/
10/97 (Fig. 9).
5. Discussion
Field characteristics and offshore seismic data for
the St. Lawrence rift system permitted the alongstrike reconstruction of rift faults from Québec city
to the NE part of the St. Lawrence estuary, a distance
of approximately 300 km. Fig. 10B shows the trace
and kinematics of the major rift faults of the area.
Fig. 11 illustrates a schematic tridimensional interpretation of the St. Lawrence rift system. We suggest
that normal faults propagated in an «en échelon»
pattern along the rift axis with relay structures
Fig. 11. Schematic 3D interpretation of the St. Lawrence rift system between Québec city and the St. Lawrence estuary.
A. Tremblay et al. / Tectonophysics 369 (2003) 231–252
between two segments of normal faults developed as
transverse faults. Some of these transverse faults
probably cut across one or more longitudinal normal
faults, and may possibly crosscut the entire graben in
at least one location. Seismic data from the St.
Lawrence estuary, to the north of the Saguenay
River, indicate the existence of a major graben along
the St. Lawrence rift system, the St. Lawrence
Estuary graben (Fig. 10B).
We speculate that the formation and/or reactivation of the St. Lawrence rift system is post-Ordovician and that the rift faults crosscut preexisting
Appalachian structures. Although absolute age data,
which would support this interpretation, are currently
lacking, the hypothesis is supported by the following
points:
(a) Structural analysis (Lemieux, 2001) and isotopic
characterization (C, O, Sr and Pb; Carignan et al.,
1997) of the St-Laurent fault and related structures
in the Charlevoix area suggest that rift structures
are younger than the Devonian impact cratering
event.
(b) Preliminary results of fission-track dating suggest
that some of the fault throw and shoulder uplift of
the rift system are related to the opening of the
Central Atlantic in the Jurassic, and/or to extension and seafloor spreading of the North Atlantic
during the Cretaceous (Glasmacher et al., 1998;
Zentilli et al., 2001). Moreover, the occurrence of
Jurassic tholeiites on Anticosti Island require
significant crustal extension (Bédard, 1992) and
is consistent with the formation of sedimentary
basins of that age.
(c) A redbed-rift tholeiite sequence of Carboniferous
age is well known in Gaspé Peninsula and the
Magdalen Basin, and Carboniferous bedrock strata
have been compiled by Sandford and Grant (1990)
in the St. Lawrence Channel.
(d) The youngest Paleozoic platform strata crosscut by
the rift faults onshore are Upper Ordovician (i.e.
the St. Lawrence Lowlands), but xenoliths of
Devonian carbonates (Clark, 1972) occur in
Monteregian diatremes (ca. 124 Ma). Coupled
with organic material maturation data (Héroux and
Bertrand, 1991), this demonstrates the prior
existence of a thicker cover sequence and implies
more than 5 km of post-Cretaceous erosion.
249
(e) The current seismicity that characterizes the St.
Lawrence rift system (Adams and Basham, 1989)
is consistent with the idea that it is a major crustal
structure that probably suffered a long-lived and
incremental history of reactivated faulting events.
We believe that the analysis of the onshore and
offshore extension of the St. Lawrence rift system
may have significant implications for gas exploration
in the area. In the following, we discuss some of these
implications with respect to the age of rift faulting and
to the textural characteristics of fault rocks.
St-Antoine and Héroux (1993) argued that such
faults represented an access for the migration of
thermogenic gas into the overlying Pleistocene sediments. In the Charlevoix area, soil gas hydrocarbon
measurements and analyses locally yielded thermogenic type fluids spatially related to the St-Laurent
fault, and the efflux of hydrocarbons has been tentatively attributed to deep-seated structures of both the
impact crater and the St. Lawrence rift faults (GHK,
1988).
At present, there is, however, no indication on the
relative proportion of biogenic versus thermogenic
gas contributions. We speculate that degassing processes of the underlying bedrock and the focussing
of methane fluxes along the St. Lawrence rift faults
may have contributed to the formation of hydrates in
the Laurentian Channel. Potential sources of methane are the Paleozoic platform and younger rock
strata that may be present and even the deeper rock
units of the Grenvillian metamorphic basement.
Gases or gas-rich fluids would have migrated upward along high permeability zones, enhanced by
brittle faulting, until gas/fluids entered the hydrate
stability zone and formed hydrates. These filled
sediment fractures and pore spaces at the base of
this zone, or met a porous unconsolidated sediment
in which methane laterally moved and was trapped
to form hydrates. This interpretation is speculative
but ongoing studies will hopefully furnish more
arguments.
6. Conclusion
The investigation of fault rocks of onshore segments of the St. Lawrence rift system indicates that
they are characterized by fault textures and fabrics
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A. Tremblay et al. / Tectonophysics 369 (2003) 231–252
belonging to the elastico-frictional regime and are
typical of faulting conditions in the upper crust. Fault
rocks of the cataclastic series generally coexist with
fault gouge, foliated gouge and pseudotachylyte along
most parts of the rift faults. The presence of localized
gouge horizons does not mark the reactivation of rift
faults but rather represents a primary textural characteristic of these supracrustal faults. Field relations
suggest that the various types of fault rocks are
associated with the same tectonic event. Northeasttrending normal faults of the St. Lawrence rift system
are interpreted as longitudinal faults genetically associated with E –W- and NW – SE-trending faults interpreted as transfer faults.
High-resolution marine seismic reflection data
acquired in the St. Lawrence River estuary successfully identify submarine topographic reliefs that can
be attributed to faults of the St. Lawrence rift
system. Northeast-trending seismic profiles, subparallel to longitudinal rift faults, show a geometry that
agrees with onshore structural features mapped as
transverse faults. Northwest-trending seismic profiles
suggest that normal faults fringing the St. Lawrence
River are associated with a major topographic depression in the estuary, the Laurentian Channel
trough, with more than 500 m of basement topographic relief. A two-way travel time to bedrock
map based on seismic data from the St. Lawrence
estuary suggests that the Laurentian Channel trough
varies from a half-graben to a graben structure from
SW to NE.
Acknowledgements
The Natural Science and Engineering Council
(NSERC) of Canada has provided research grants to
A. Tremblay (PG105669) and B. Long (NSERCStrategic grant). The Department of Fisheries and
Oceans of Canada is acknowledged for shiptime. This
study has been partly supported by the IMAGES
Program for the Marion-Dufresne 99 Cruise and by
the Appalachian Platform and Foreland Architecture
NATMAP project of the Geological Survey of
Canada. Crew members of the Marion-Dufresne,
Alcide-Horth and Denis-Riverain ships are thanked
for support and technical services during the different
cruises. Journal reviewers Terry Engelder and Nathan
Hayward are acknowledged for the comments, which
helped us to produce a much improved final product.
L. Dubé and M. Laithier are acknowledged for
drawing the figures. We thank B.R. Rosendahl for
comments and suggestions on an earlier version.
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