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Tectonophysics 369 (2003) 231 – 252 www.elsevier.com/locate/tecto 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)00227-0 232 A. Tremblay et al. / Tectonophysics 369 (2003) 231–252 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 234 A. Tremblay et al. / Tectonophysics 369 (2003) 231–252 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 235 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 236 A. Tremblay et al. / Tectonophysics 369 (2003) 231–252 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. 238 A. Tremblay et al. / Tectonophysics 369 (2003) 231–252 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- 240 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 242 A. Tremblay et al. / Tectonophysics 369 (2003) 231–252 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 244 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 246 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. 248 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 250 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). 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