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The Gaspé Belt is the largest middle Paleozoic belt in the Canadian Appalachians. The most complete stratigraphic record of Upper Ordovician to Middle Devonian rocks of this belt occurs in the Gaspé Peninsula, in the northeastern part of the Québec re-entrant. Rocks of the Gaspé Belt rest unconformably on Cambrian to Ordovician rocks of the Humber and Dunnage zones, and are unconformably overlain by Carboniferous rocks. The regional metamorphic grade of the Gaspé Belt rocks is very low, and some units are still in the oil window.
The deformation history of the Gaspé Belt rocks comprises multi-stage deformation events represented by two phases. A Late Silurian to Early Devonian phase, the Salinic disturbance, manifested by an unconformity, NW-trending folding, synsedimentary faulting, and intra-plate volcanism. The late Early to Middle Devonian Acadian phase is recorded by NE-trending folding, cleavage development and both reverse and strike-slip faulting within a transpressive setting during the oblique continental collision between Laurentia and Avalon (a peri-Gondwanan terrane), which took place farther southeast. The tectonic setting during the Salinic disturbance is consistent with continuous plate convergence (without subduction) after the Taconian Orogeny and before the Acadian continental collision.
The paleogeographic maps of the Gaspé Belt during Pridolian, Pragian and Emsian times reflect the tectonic activity south of the Gaspé Peninsula. The switch from sinistral to dextral relative motion between Laurentia and Avalon terranes on the St. Lawrence promontory (Newfoundland) created a transtensional rift in the Québec re-entrant (Gaspé Peninsula). During the Pridolian, normal block-faulting occurred particularly along the NW-trending eastern margin of the basin where reefs developed on the footwall of the faults. Intra-plate volcanism also occurred from Wenlockian to Emsian along the crustal weakness developed parallel to the NW-trending eastern margin of the basin. Pragian paleogeography shows a deep shelf, carbonate-rich, distal foreland basin in the north, and a siliciclastic turbidite proximal foreland basin developed in front of, and adjacent to, a southern land area. This paleogeographic distribution is compatible with the development of a peripheral foreland basin in front of the Acadian orogenic wedge caused by loading of the Laurentian margin during the convergence of the Avalon terrane within the Québec re-entrant. During the Emsian, the northern deep-shelf carbonates were replaced by littoral and continental sands and gravels reflecting the growing Acadian orogenic wedge to the south and its deep erosion.
The Gaspé Peninsula is part of the northern Appalachians, formed during a Paleozoic orogeny that reflects the full spectrum of tectonic processes, from ocean rifting and spreading, to island arc collision and obduction, accretion of microcontinental terranes of peri-Gondwanan affinities (e.g., Avalon terrane), and final continental collision with Gondwana (Williams, 1979, 1995; Robinson et al., 1998; van Staal et al., 1998). Although orogenic processes were active from Ordovician to Carboniferous time, three major pulses are recognized: the Ordovician Taconian, the Devonian Acadian and the Permo–Carboniferous Alleghanian orogenies (Williams, 1979). Although some authors include all pre-Carboniferous deformation features in the Silurian and Devonian rocks within the Acadian Orogeny (e.g., Williams, 1993), distinct Silurian and Devonian tectonic events are locally recognized in the northern Appalachians (Dunning et al., 1990; Cawood et al., 1994; Hibbard, 1994; van Staal and de Roo, 1995; Castonguay et al., 2001). The term Salinic Orogeny (Dunning et al., 1990; or Salinian Orogeny, Cawood et al., 1995), is now used for Silurian deformation events. In the Gaspé Peninsula, evidence of tectonic activity during the Silurian is present locally but relatively minor when compared to deformation features related to the Taconian and Acadian orogenies. Thus the old term “disturbance” is used in the Gaspé Peninsula to designate Salinic tectonic features (see also Boucot, 1962).
The contrasting Taconian and Acadian structural styles in the Gaspé Peninsula have always been noted in the past (Malo and Béland, 1989). Northwest-verging folds and large-scale thrusts in the Cambro–Ordovician rocks of the Humber Zone in northern Gaspé Peninsula are associated with the emplacement of Taconian thrust sheets (St-Julien and Hubert, 1975). South of these Taconian allochthons, NE-trending, open, and upright folds with major ENE- to E-trending dextral strike-slip faults in Silurian–Devonian rocks characterize the Acadian deformation. In the Gaspé Peninsula, the Taconian Orogeny is interpreted as the result of a collision between the lower Paleozoic passive continental margin of Laurentia (Humber Zone) and peri-Laurentian island arc terrane, or a continental magmatic arc (Dunnage Zone) above an east-dipping subduction zone, and the obduction of ophiolitic nappes (St-Julien and Hubert, 1975; De Broucker, 1987; Tremblay et al., 1995). The Acadian Orogeny is interpreted in terms of a continental collision of Avalon terrane along the irregular margin of Laurentia and its Taconian accreted terranes (Malo et al., 1995). Most of the previous tectonic studies in the Gaspé Peninsula dealt with the Taconian Orogeny (St-Julien and Hubert, 1975; De Broucker, 1987; Slivitzky et al., 1991) or the Acadian Orogeny (Malo and Béland, 1989; Kirkwood and Malo, 1993; Kirkwood, 1995, 1999). More recently (Malo and Bourque, 1993; Malo and Kirkwood, 1995; Bourque et al., 2000), synsedimentary faulting recognized in the Upper Silurian–Lower Devonian rocks of the northeastern part of the Gaspé Peninsula has been associated with an unconformity, i.e., the Salinic unconformity of Boucot (1962). The regional tectonic significance of this unconformity and the synsedimentary faulting were not addressed in the past. In this paper, we describe the Salinic tectonic features, and discuss their geodynamic setting in view of existing plate tectonic models for the northern Appalachians during the Late Silurian–Early Devonian.
The Gaspe Belt
The Gaspé Belt is the largest depositional belt of the middle Paleozoic rock assemblage of the Canadian Appalachians and extends well into New England (Figs. 1⇓, 2⇓). It shows the most complete stratigraphic record, consisting of four broad temporal and lithological assemblages (Bourque et al., 2001, this issue). From the base to the top (Fig. 3⇓), these are: 1) the uppermost Ordovician–lowermost Silurian deep water, fine-grained, carbonate-siliciclastic facies of the Honorat and Matapédia groups (HO-MA); 2) the Silurian–lowermost Devonian shallow to deep shelf facies of the Chaleurs Group (CH); 3) the Lower Devonian mixed siliciclastic and carbonate fine-grained deep shelf to basin facies of the Upper Gaspé Limestone and Fortin groups (UGL-FO); and 4) the Lower to Middle Devonian nearshore to terrestrial coarse-grained facies of the Gaspé Sandstone Group (GS). Upper Silurian and Lower Devonian continental tholeiitic basalts and andesites are present in the upper Chaleurs, upper Gaspé Limestone and lower Gaspé Sandstone groups. The Gaspé Belt is located in the Québec re-entrant (Fig. 1⇓) where the metamorphic grade and the intensity of the deformation of the middle Paleozoic rocks are lower than in Newfoundland, to the northeast, and in southern Québec and New England, to the southeast.
In the Gaspé Peninsula (Fig. 2⇑), rocks of the Gaspé Belt are deposited unconformably on, or are in fault contact with, rocks of the Humber and Dunnage zones (Malo and Bourque, 1993). In southern Gaspé Peninsula, rocks of the Gaspé Belt, as well as those of the Humber and Dunnage zones, are unconformably overlain by flat-lying Carboniferous strata (Fig. 2⇑). The two angular unconformities that bound the Gaspé Belt sequence, the sub-Caradoc and the sub-Carboniferous, are used to constrain the timing of deformation within the Late Ordovician to Middle Devonian. Folding and faulting are clearly dated as Middle to Late Devonian (Rodgers, 1967; Malo and Bourque, 1993; Malo and Kirkwood, 1995) and the main deformation within the belt is ascribed to the Acadian Orogeny. The Gaspé Belt rocks were affected by regional anchimetamorphism to low-grade metamorphism usually associated with burial metamorphism (Nowlan and Barnes, 1987; Chagnon, 1988; Hesse and Dalton, 1991). Older rocks are generally more thermally mature and occur in anticlines (Chagnon, 1988; Hesse and Dalton, 1991; Bertrand and Malo, 2001, this issue). Illite crystallinity and organic-matter reflectance studies indicate anchimetamorphism grade for rocks of the Honorat Group (Chagnon, 1988), and the intensity of metamorphism decreases toward the top, where some rocks of the Upper Gaspé Limestone and Gaspé Sandstone groups are still in the oil window (Bertrand and Malo, 2001, this issue). The intensity of metamorphism also decreases regionally eastward. Local zones of high-grade metamorphic rocks are spatially associated with Devonian intrusions (e.g., the Murdochville area, Bertrand and Malo, 2001, this issue) and dyke swarms along major Acadian faults (Malo et al., 2000). Intrusions are restricted to the northern part of the peninsula (e.g., Mont McGerrigle plutonic complex, Dôme Lemieux area, and the Copper and Porphyry mountains in the Murdochville area; Fig. 2⇑). Several stocks and dyke swarms intrude the Gaspé Belt rocks farther south (Doyon and Berger, 1997).
Acadian Deformation Features
The major Acadian structural trend of the Gaspé Belt rocks is oriented NE-SW. The Gaspé Belt is divided into three major structural zones following this regional trend, from northwest to southeast: 1) the Connecticut Valley–Gaspé Synclinorium; 2) the Aroostook–Percé Anticlinorium; and 3) the Chaleurs Bay Synclinorium (Fig. 2⇑). Regional Acadian folds are generally open and upright. Near major faults, folds are tight, steeply plunging and inclined to the NW or the SE. A regional NE-trending cleavage is well developed throughout the Gaspé Belt. In the Connecticut Valley–Gaspé Synclinorium, the intensity of Acadian deformation increases from northwest to southeast.
Major faults are NE-trending in the west, E-trending in the southeast and NW-trending in the northeast (Fig. 2⇑). The E-trending faults of the Grand Pabos fault system (Grande Rivière, Grand Pabos, Rivière Garin faults, Fig. 2⇑), in the southeastern part of the Gaspé Peninsula, as well as the ENE-trending Shickshock Sud Fault in northern Gaspé Peninsula, are dextral strike-slip faults (Malo and Béland, 1989; Sacks and Malo, 1995). In the Aroostook–Percé Anticlinorium (Fig. 2⇑), deformation features related to major strike-slip faults, such as subsidiary faults, low-angle synthetic and high-angle antithetic faults, rotated oblique folding, and cleavage, are compatible with a classic strike-slip tectonics model (Malo and Béland, 1989). The NE-trending faults, subparallel to the regional folding, in the Chaleurs Bay Synclinorium, as well as those in the Connecticut Valley–Gaspé Synclinorium (e.g. Marcil Fault; Fig. 2⇑) are high-angle reverse faults. In western Gaspé Peninsula, the Restigouche Fault is an oblique-slip fault representing the western extension of the Grand Pabos Fault (Trudel and Malo, 1993). Farther north, kinematic analysis along the Sainte-Florence Fault and the strain history of rocks of the Fortin Group show that vertical extension was followed by fold-axis parallel extension during the slaty cleavage development, suggesting that the NW-directed reverse movement was followed by a late dextral strike-slip movement (Kirkwood et al., 1995). In southwestern Gaspé Peninsula, the Sellarsville Fault, bounding the Restigouche Syncline (Fig. 2⇑), is a SE-directed high-angle reverse fault (Trudel and Malo, 1993). Both NW- and SE-directed high-angle reverse faults in southern Gaspé Peninsula were related to the dextral transpressive regime of deformation along the Grand Pabos Fault (Malo and Kirkwood, 1995). In the northeastern part of the Gaspé Peninsula, the NW-trending faults reflect an Acadian dextral strike-slip motion (Fig. 2⇑).
The northeastern region of the Gaspé Belt is characterized by regional NW-trending faults and the local presence of NW-trending folds (Figs. 2⇑, 4⇓). Structural features of the Silurian–Devonian rocks in the Gaspé Peninsula have always been assigned to the Acadian deformation. However, some of the structural features in the northeastern region of the Gaspé Belt do not seem compatible with the proposed strike-slip model for Acadian deformation (NE-trending folds, NE-trending, high-angle reverse and oblique-slip faults, E-trending, dextral, strike-slip faults).
The Salinic unconformity, which developed during the Pridolian, is expressed as an angular unconformity, or an erosional surface. The angular relationship between younger Pridolian rocks above older Silurian rocks is found south of the Shickshock Sud Fault (Lachambre, 1987) and in the Restigouche Syncline (Bourque and Lachambre, 1980; Fig. 2⇑) In the northeastern part of the Connecticut Valley–Gaspé Synclinorium (Rivière Saint-Jean Anticline area, northeastern and southeastern limits of the synclinorium, Fig. 2⇑), the erosion cuts into the underlying Silurian rocks and also into Ordovician rocks of the Humber Zone in the northeastern margin of the basin.
The most important NW-trending faults are the Bassin Nord-Ouest (BNOF), Troisième Lac (TLF) and Gastonguay (GF) faults (Fig. 2⇑). Structural analysis of the Lower to Middle Devonian rocks along the Bassin Nord-Ouest and Troisième Lac faults (Béland, 1980; Berger and Ramsay, 1993) and the geological map (Brisebois, 1981) indicate that these faults were the site of dextral strike-slip movement with a vertical component during the Acadian Orogeny (post-Middle Devonian).
Even though these NW-trending faults are viewed as Acadian structures, stratigraphic analysis has demonstrated that normal faulting occurred along these faults during sedimentation of the Silurian and Lower Devonian facies of the Gaspé Belt. This is seen on a transverse seismic profile across the Bassin Nord-Ouest and Troisiéme Lac faults, which shows a thickening of the Chaleurs Group strata toward the Bassin Nord-Ouest Fault (Roksandic and Granger, 1981; Bourque, this issue) and by sedimentological studies of rock units along the faults, namely the Gaspé Sandstones (Rust, 1981; Amyot, 1984), the Upper Gaspé Limestones (Amyot, 1984; Lavoie, 1992), and the Chaleurs Group (Bourque et al., 2000). Bourque et al. (2000) have demonstrated that the Gastonguay and Grande Rivière faults were active during the deposition of the Burnt Jam Brook Formation sediments in the Llandoverian. There is also evidence for a pre-Silurian ductile strike-slip motion along the Bassin Nord-Ouest Fault (Béland, 1980; Berger and Ramsay, 1993) that can be attributed to the Taconian Orogeny (Malo and Kirkwood, 1995). Because the dextral strike-slip motion is also part of the Acadian movement along the fault, it is possible that the strike-slip motion along the NW-trending faults, recognized before and after the sedimentation, was also active during the sedimentation. The synsedimentary normal faulting is therefore related to a transtensional regime of deformation.
In the Connecticut Valley–Gaspé Synclinorium, Acadian regional folds are generally NE-trending, but from the Gastonguay Fault to the east, regional folds in Silurian–Devonian rocks (Fa; Fig. 2⇑) tend to become more east-west oriented (e.g., Rivière Saint-Jean Anticline, Fig. 2⇑). In the northeastern part of the Gaspé Peninsula, NW-trending folds are recognized between the Bassin Nord-Ouest and Troisième Lac faults (Figs. 2⇑, 4⇑). The spatial relationship between NW-trending folds and faults is evident (e.g., the Champoux Syncline; Fig. 4⇑) and it is likely that a genetic link exists between the development of these structures. Analysis of structural trends from air photos in the Silurian–Devonian rocks, south of the Humber Zone (Fig. 4⇑), clearly shows NW-trending folds in the Indian Cove, York Lake–York River formations between the Bassin Nord-Ouest and Troisième Lac faults. It also shows that NE-trending folds (Fa; Fig. 4⇑) close to the Lady Step Complex are superimposed on the northeastern limb of the larger NW-trending fold (Fs), suggesting that the NW-trending fold preceded NE-trending Acadian regional folding. Northwest-trending folds have no cleavage, as opposed to NE-trending folds, which display an axial-planar Acadian cleavage (Béland, 1980; Berger and Ramsay, 1993). I interpret early NW-trending folds as longitudinal extensional folds (Schlische, 1995; Janecke et al., 1998) genetically related to synsedimentary normal faulting along the Bassin Nord-Ouest and Troisième Lac faults.
Early folding and growth faulting in the Gaspé Belt rocks are related to extension within the basin, most probably related to the Salinic disturbance (Fig. 3⇑). According to the age of rock units involved in the folding and faulting, both NW-trending structural features were formed over a protracted interval that started mainly during the Pridolian and continued up to the Eifelian (Fig. 3⇑).
An earlier period of NW-trending folding, with no penetrative cleavage, precedes the regional NE-trending folding in the Aroostook–Percé Anticlinorium (Fig. 2⇑). Like those of the northeastern Connecticut Valley–Gaspé Synclinorium, the early NW-trending folds are sometimes spatially associated with NW-trending faults (e.g., Percé and Carleton areas in the Aroostook–Percé Anticlinorium; Fig. 2⇑). On the pre-Devonian palinspastic map (Malo and Kirkwood, 1995), the early NW-trending folds are located mainly in the eastern part of the Gaspé Belt along the St. Lawrence promontory. Malo and Kirkwood (1995) postulated that the earlier NW-trending folds in the southern part of the Gaspé Belt have a common origin with those of the northeastern part of the belt, being associated with the Salinic disturbance and normal faulting during the Late Silurian and Early Devonian. This observation shows that normal (transtensional) faulting was not only a local phenomena in the northeastern region of the basin, but was active also in a larger area located along the eastern margin of the basin.
In the Gaspé Belt, volcanic activity occurred mainly during the Salinic disturbance (Fig. 3⇑). Volcanic rocks are mainly Pridolian to mid-Lochkovian (419–411 Ma) and mid-Pragian to lower Emsian (411-400 Ma), although some of them are locally Wenlockian–Ludlovian (423-419 Ma) (Fig. 3⇑). The Wenlockian–Ludlovian and Pridolian–Lochkovian volcanic rocks are mainly in the Chaleurs Group and consist of intermediate rocks and tholeiitic basalts. The Pragian–Emsian volcanic rocks are made up of transitional alkalic and tholeiitic basalts in the upper Gaspé Limestones and lower Gaspé Sandstones. Both groups of volcanic rocks are intra-plate basalts (Laurent and Bélanger, 1984; Bédard, 1986; Doyon, 1988; Dostal et al., 1993; Doyon and Dalpé, 1993; Whalen et al., 1994; Doyon and Berger, 1997), suggesting an episode of intra-plate extension in the Gaspé Belt during the Salinic disturbance.
Paleogeographic Evolution of the Gaspe Basin During the Salinic disturbance
Paleogeographic maps of three time slices, the Pridolian, Pragian and Emsian (Figs. 5⇓–7⇓), were analyzed to understand better the plate tectonic setting of the Gaspé Basin during the Salinic disturbance and the Acadian Orogeny. Palinspastic maps are simplified versions of those in Bourque et al. (2001, this issue). The southern limit of the Grenville basement was traced on maps based on deep seismic data, gravity and magnetic data in the Gulf of St. Lawrence (Marillier et al., 1989; Durling and Marillier, 1990), and geochemical and isotopic signatures of igneous rocks, which reflect the different nature of the basement underlying the Gaspé Peninsula and New Brunswick (Whalen, 1993). The NW-trending segment of the southern boundary of the Grenville basement corresponds to a long-lived crustal fault zone bordering the St. Lawrence promontory, which was interpreted as a transform fault during the opening of the Iapetus Ocean in the Cambrian (Thomas, 1977). It is also the Grenville–Central lower crustal block boundary (Marillier et al., 1989) or the Bradelle–Shediac basement block boundary (Durling and Marillier, 1990). This NW-trending boundary was the site of dextral strike-slip motion during the Devonian Acadian Orogeny as the result of continued convergence of Avalon terrane in the Québec re-entrant (Stockmal et al., 1990; Malo et al., 1995). It was also the site of dextral shearing during the juxtaposition of Dunnage Zone rocks with those of the Humber Zone along the Baie Verte–Brompton Line during the Taconian Orogeny (Malo and Kirkwood, 1995).
The Salinic unconformity is best displayed in the basin during the Pridolian, and corresponds with the end of the second regressive cycle (R2, Fig. 3⇑). The unconformity is recognized in several localities in the basin, particularly along the northeastern margin of the basin, but also in the northern and southern parts of the basin, south of the Shickshock Sud Fault and in the Ristigouche Syncline, respectively (Fig. 2⇑). On a Pridolian paleogeographic map, these regions seem to be always close to the cratonward margin of the basin and associated with normal faults (Fig. 5⇑). During the Pridolian, terrestrial redbeds, conglomerate and reef complexes are spatially associated with NW-trending faults in the northeastern and the southeastern parts of the basin, and ENE-trending faults in the northern part of it. The reefs were established at the periphery of faulted and tilted blocks (on footwall blocks of southeast-and southwestward dipping faults), whereas siliciclastic mudstone occurs on hanging-wall blocks (Bourque et al., 2001, this issue). Pridolian volcanic rocks and emerged areas follow a NW trend. This trend was also used for the emplacement of younger Pragian–Emsian tholeiitic basalts (Fig. 6⇓). The oldest volcanic rocks in the Gaspé Belt occur sporadically in the Wenlockian, to the south in the Ristigouche Syncline area (Fig. 2⇑), but they formed more commonly during the Ludlovian (Fig. 3⇑).
During the Pragian (Fig. 6⇑), intra-plate basalts were still locally present, while two major facies were deposited in the basin: deep-water carbonate shelf facies (Upper Gaspé Limestone Group) along the NE- and NW-trending edge of the basin, and siliciclastic basin facies to the south (Fortin Group). The volcanic centres moved farther north, and the two major centres were aligned along a NW trend in the extensional southern limit of the Grenville basement. According to their paleogeographic position, these volcanic magmas probably erupted through the Grenville basement and the overlying accreted Ordovician terranes under the Gaspé Basin, whereas the older Upper Silurian volcanic rocks erupted south of the Grenville basement (Fig. 5⇑) and were deposited above rocks of the oceanic domain of the Dunnage Zone. During the Pragian, there was no deposition in the southern part of the basin, indicating that this region, the actual Chaleurs Bay Synclinorium area, was already uplifted in response to Acadian tectonic activity farther south. The uplifted position of the Chaleurs Bay Synclinorium is also supported by the very low grade burial metamorphism recorded in the Chaleurs Group rocks (Nowlan and Barnes, 1987; Hesse and Dalton, 1991), suggesting that Lower Devonian rocks were not deposited.
During the Emsian (Fig. 7⇓), the northern part of the basin changed from a deep-water carbonate shelf to a terrestrial and continental area, receiving more terrigenous detritus from the growing Acadian orogenic wedge to the south, and probably from a source area to the east (Rust et al., 1989). The Emsian map of Bourque et al. (2001, this issue) is arbitrarily plotted with 50% shortening as a result of the initiation of folding in the orogenic wedge. In Maine, it has been shown that the leading edge of the Acadian foreland basin and deformation front migrated toward the northwest, from the Ludlovian (423 Ma) to the Givetian (383 Ma) (Bradley et al., 1998). It is, therefore, not unrealistic to plot the Emsian map with a shortened basin. The land area in the southern part of the basin during the Pragian had moved to the northwest along the NW-trending edge of the Grenville basement. The orogenic wedge impinged the NE-trending edge of the Grenville basement in the late Emsian, during which much of the Acadian penetrative deformation probably occurred in rocks of the Gaspé Belt (NE-trending cleavage associated with NW-shortening). Acadian folding continued up to the Eifelian, during and after the maximum burial of the Gaspé Belt sequence, which occurred approximately at the end of the Eifelian (Fig. 3⇑; Bertrand and Malo, 2001, this issue). Because surface maturity contours associated with the metamorphic aureole around the Copper and Porphyry mountain stocks at Murdochville crosscut the folded surface maturity contours of regional burial metamorphism (Bertand and Malo, 2001, this issue), it is concluded that the folding was complete by 370 Ma (Famennian, Fig. 3⇑).
Northwest folds are well displayed in the Champoux Syncline, where they are formed by strata of the Upper Gaspé Limestone and Gaspé Sandstone groups (Fig. 4⇑). This indicates that NW-folding is post York Lake–York River Formation or post-Emsian. However, longitudinal folds genetically related to normal faults may have formed during the development of the fault (Schlische, 1995). Because it appears that NW-faulting was highly active during the Pridolian, NW-folding is postulated to have occurred over a long time, starting as early as the Pridolian (Fig. 3⇑).
Tectonic Setting of the Gaspe Peninsula During the Salinic Disturbance
As part of the northern Appalachians, the Gaspé Peninsula has a tectonic history of continued collision from plate convergence between Laurentia and Gondwana during the Paleozoic (Williams, 1979; Williams and Hatcher, 1983; Robinson et al., 1998; van Staal et al., 1998). The paleogeography of the Gaspé Belt during the Silurian and earliest Devonian shows that deposition of the Gaspé Belt sequence occurred in a tectonically active basin and that the Ashgillian–Llandoverian was the only period of quiescence in the basin between the major Taconian and Acadian orogenies (Bourque et al., 2000). During the Late Silurian to Early Devonian, the Gaspé Peninsula was characterized by sedimentation associated with normal faulting, particularly in the northeastern region, whereas several indications exist for compressive tectonics elsewhere in the northern Appalachians (Fig. 3⇑). At the scale of the orogen, extension-related structural features developed in the northeastern part of the Gaspé Peninsula can be viewed as a result of local extension during a global plate-convergent setting. Late Silurian–Early Devonian extension occurred within the Gaspé Basin south of the Taconian nappes and well after tectonic emplacement of the ophiolites at 456 Ma (Lux, 1986; Fig. 3⇑) precluding any association with a late-stage event of the Taconian Orogeny.
Sedimentary basins of middle Paleozoic belts in the northern Appalachians are viewed as successor basins formed after early Paleozoic orogenic events (Williams, 1979, 1995). The Gaspé Basin located in the Québec re-entrant is in agreement with the definition of such a basin (see Ingersoll, 1988). The basin contains different northeast-trending troughs (e.g., Connecticut Valley–Gaspé, Chaleurs Bay, Aroostook–Percé, Merrimack and Fredericton troughs) overlying deformed and intruded lower Paleozoic rocks of the Dunnage and Gander zones (e.g., New Brunswick–Central Mobile Belt, Boundary Mountain Anticlinorium, Munsungum Anticlinorium; Fig. 1⇑). Although the Salinic Orogeny is not well recognized at the scale of the orogen in North America, the Acadian Orogeny is considered to be the most widespread orogenic event in the Appalachians (Williams and Hatcher, 1983; Osberg et al., 1989). In the northern Appalachians, the intensity of Acadian metamorphism and plutonism increased toward the southeast and was more extensive in New England than in adjacent Québec. In New England (Fig. 1⇑), evidence of high-grade metamorphism and polyphase deformation occur in the core of the Boundary Mountain Anticlinorium (Osberg et al., 1989). In northern Maine and northern New Brunswick, polyphase deformation occurred during the Late Silurian to Early Devonian (Hibbard, 1994; van Staal and de Roo, 1995). At the scale of the northern Appalachians, the Gaspé Belt of the Gaspé Peninsula is located in the Acadian foreland area, whereas the Acadian hinterland is located to the south in New England and New Brunswick. With respect to the Acadian-related tectonic features, the Gaspé Basin is a foreland basin in a geographic sense, where sediments accumulated on the cratonic side of an orogenic belt (Dickinson, 1974). The tectonic activity in the Acadian hinterland certainly influenced the basin in the foreland area. The existing plate tectonic models for the region south of the Gaspé Peninsula can explain the following features: the intra-plate volcanism in part localized along the NW-trending eastern edge of the basin; the normal faulting along the northern and eastern margin of the basin; the Pragian paleogeographic distribution of facies with a northern deep-shelf carbonate facies and a silici-clastic basin facies adjacent to a southern land area; and the growing emerged area to the south during the Emsian (all features displayed on paleogeographic maps, Figs. 5⇑–7⇑).
The Acadian Orogeny involved the collision of the Avalon terrane with Laurentia and its accreted terranes, which have different names along the orogen. The Medial New England terrane in New England (Osberg et al., 1989) is equated with the Miramichi terrane in New Brunswick, which comprises rocks of the Dunnage and Gander zones of the Canadian Appalachians (Robinson et al., 1998; van Staal et al., 1998), and early Paleozoic zones defined in Newfoundland. In this study, the Laurentian plate includes rocks of the Grenville basement and Cambro–Ordovician rocks already accreted to it during early Paleozoic orogenic events.
The tectonic evolution of the Dunnage and Gander zones in New Brunswick predicts extension in the Québec re-entrant during the Late Silurian to Early Devonian (Fig. 3⇑). The Late Silurian terminal collision between Laurentia and its accreted Ordovician arcs (Dunnage Zone rocks) and the Gander margin of Avalon occurred in favour of an oblique collision, and culminated in a sinistral transpression, which was followed by uplift and extensional collapse in the Early Devonian (van Staal and de Roo, 1995). At the same time, the delamination of the descending Avalonian plate to the northwest favoured upwelling of the asthenosphere and volcanism (van Staal and de Roo, 1995). This delamination is also a proposed mechanism for the igneous activity farther southwest in New England (Robinson et al., 1998). The post-Taconian collision processes in the New England Appalachians are related to a southeasterly subduction of the Laurentian plate under the Avalon terrane during the Paleozoic (Robinson et al., 1998). In this scenario, the Late Silurian–Early Devonian volcanism is explained by upwelling of the asthenosphere due to the delamination of the descending Laurentian plate to the southeast, which had started in the Early Silurian (Robinson et al., 1998). The paleogeographic configuration of the Gaspé Belt during the Pridolian neither supports nor rejects these plate tectonic settings for northern New Brunswick and New England. The upwelling of the asthenosphere due to delamination may have helped to create extension and volcanism in the large Québec re-entrant area in the Gaspé Peninsula, as well as in New Brunswick and New England. If the subduction rate had been faster than the convergence rate (Royden, 1993) during the NW-subduction of the Gander margin in New Brunswick, it is possible that extension occurred in the Gaspé Belt to favour sedimentation during the early stages of basin formation in the Ashgillian to Wenlockian, but this mechanism of extension was likely not operative during the Salinic disturbance.
In Newfoundland, the docking of the Avalon terrane occurred in favour of an oblique collision along dextral transcurrent faults, such as the Dover Fault (Williams and Hatcher, 1983). Recent studies along the Gander–Avalon terrane boundary in southwest and northeast Newfoundland (Dunning et al., 1990; O’Brien et al., 1993; Holdsworth, 1994; Lin et al., 1994; Dubé et al., 1995) suggest that collision started during the Silurian. The Silurian collision associated with the Salinic Orogeny (Dunning et al., 1990; O’Brien et al., 1993) occurred in favour of large-scale sinistral transcurrent faults along the Gander–Avalon boundary as opposed to Devonian dextral motion during the Acadian Orogeny (O’Brien et al., 1993; Dubé et al., 1995; Holdsworth, 1994). At the Gander–Avalon boundary, the switch from sinistral to dextral motion is well constrained in time, and dated at 425 Ma (Holdsworth, 1994; Keppie and Dostal, 1994), just after the cessation of the sinistral tectonic regime. Elsewhere in Newfoundland, this switch is not as well constrained, but occurred mainly during the Late Silurian (O’Brien et al., 1993; Holdsworth, 1994) or during earliest Early Devonian (Dubé et al., 1995). This switch is a manifestation of a middle Paleozoic plate reorganization during the continental collision between Laurentia and Avalon (Hibbard, 1994).
In the orogen-scale dextral transcurrent system produced during the oblique collision of Avalon along the northeast-trending edge of the Laurentian plate, the NW-trending edge of the Grenville basement represents a releasing bend where transtension and pull-apart basins are expected to form. Pridolian normal block-faulting along the NW-trending southern limit of the Grenville basement and farther north is an indication of this transtension (Fig. 5⇑). Pridolian volcanic rocks are also localized in the same area (Fig. 5⇑), whereas younger Pragian volcanic centres are aligned in the extension of the NW-trending zone of weakness of the southern limit of the Grenville basement (Fig. 6⇑). Volcanism in the Gaspé Peninsula started with the Ristigouche volcanics during the Wenlockian to Ludlovian (428–419 Ma), which corresponds approximately to the Late Silurian switch from sinistral to dextral motion in Newfoundland (Fig. 3⇑). This dextral motion, which induced a local extensional regime in the Québec re-entrant was also proposed by Keppie and Dostal (1994) for the creation of a transtensional rift for the emplacement of Upper Silurian–Early Devonian volcanic rocks in the Gaspé Peninsula. This proposed plate tectonic setting during the Upper Silurian–Early Devonian in the Québec re-entrant agrees with the geological data in the Gaspé Peninsula, namely the age of the volcanism (Fig. 3⇑) and its clear spatial distribution along the NW-trending edge of the St. Lawrence promontory, and the normal block faulting in the same area (Figs. 5⇑, 6⇑). This model, however, does not explain the Pridolian ENE-trending normal faults at the northern limit of the basin.
In the Gaspé Peninsula, the main penetrative deformation is late- to post-Emsian, as an Acadian cleavage is well developed in rocks of the Fortin Group (Fig. 3⇑). However, paleogeographic studies in New England have shown that the Acadian deformation started earlier than the Emsian in that area (Bradley et al., 1998). The Acadian orogenic wedge had already formed during the Ludlovian-Lochkovian, along the present-day coast of Maine. The Chaleurs Bay Synclinorium area was probably uplifted, and included in the Acadian orogenic wedge during the Pragian, with a proximal foreland basin in front (Fortin Group), immediately to the north, and a distal foreland carbonate shelf basin (Upper Gaspé Limestone Group) farther north. The same paleogeographic pattern was present in Maine during the Early Devonian (Bradley et al., 1998). Rocks equivalent to the Fortin Group are interpreted as a foreland basin in the genetic sense, i.e., a peripheral foreland basin (Dickinson, 1974; Ingersoll, 1988). This Acadian foreland basin in Maine (Littleton turbidites, Tomhegan and Fish River sandstones; Bradley et al., 1998) and southern Québec (Compton Formation, Bradley et al., 1998) formed in front of a migrating orogenic wedge that crossed New England between the Lochkovian and the Givetian (Bradley et al., 1998). The Upper Silurian–Early Devonian Piscataquis magmatic belt of Bradley (1983) was emplaced in the Acadian foreland area during coeval flysch sedimentation (Bradley et al., 1998; Robinson et al., 1998). The upwelling of asthenosphere due to the delamination of the southeasterly subducting Laurentian plate supplied magma during the Early Devonian (Fig. 3⇑), while the Laurentian margin was loaded by Avalon terrane to the southeast, inducing lithospheric flexure and normal faulting in the foreland area, to the northwest (Bradley et al., 1998). These normal faults may have been used for syncollisional volcanism but alternative interpretations are possible (see Bradley et al., 1998). The Pragian is a turning point in the evolution of the Gaspé Belt. The paleogeography of the basin during the Pridolian reflected the creation of a local transtensional rift in the Québec re-entrant due to the first docking of the Avalonian and Laurentian plates on the St. Lawrence promontory, whereas the paleogeography during the Pragian clearly shows the effect of the continued Acadian convergence, which resulted in the development of a peripheral foreland basin in front of an uplifted area (orogenic wedge) to the south (Figs. 6⇑, 7⇑).
Because of its position in the deepest part of the Québec re-entrant, middle Paleozoic rocks in the Gaspé Peninsula were preserved from penetrative deformation during the Late Silurian and Early Devonian, whereas Late Silurian deformation and metamorphism is recorded on the St. Lawrence promontory, in Newfoundland (Dunning et al., 1990).
The structural features occurring in the Gaspé Belt during the Late Silurian–Early Devonian are: NW-trending synsedimentary normal faults and genetically related NW-trending folds. The erosional Salinic unconformity and the development of reefs on structural highs are directly related to the normal block faulting. Intra-plate volcanism also occurred at the same time. These extensional features can be explained by the tectonic activity farther south. The paleogeography of the basin during Pridolian, Pragian and Emsian times is also a direct result of this tectonic activity. The switch from sinistral to dextral relative motion between Laurentia and the Avalon terrane on the St. Lawrence promontory (Newfoundland) created a transtensional rift in the northeast side of the Québec re-entrant (Gaspé Peninsula). Normal block faulting occurred particularly along the NW-trending eastern margin of the basin, where reefs developed on the footwall of the faults. This NW-trending eastern margin is also a crustal weakness, where intra-plate volcanic rocks were emplaced from the Wenlockian to the Emsian.
Pragian paleogeography shows a northern deep shelf carbonate distal foreland basin and a siliciclastic turbidite proximal foreland basin in front of, and adjacent to, a southern land area. This paleogeographic distribution is compatible with the formation of a peripheral foreland basin in front of the NW-verging Acadian orogenic wedge caused by loading of the Laurentian margin during the convergence of the Avalon terrane within the Québec re-entrant. During the Emsian, the northern deep shelf carbonates were replaced by littoral and continental sands and gravels, reflecting the growing Acadian orogenic wedge to the south and its deeper erosion.
During the Pragian and Emsian, the Gaspé Belt was a true peripheral foreland basin at the orogenic scale, from New England to the Gaspé Peninsula. Orogen-parallel normal faults are expected in this foreland basin. Normal faults active during the Pridolian in the northeastern part of the Gaspé Peninsula (Troisième Lac, Bassin Nord-Ouest and Gastonguay faults) probably continued to be active during the development of the foreland basin.
Should these movements be considered as Salinic or Acadian? There is an overlap between the tectonic features associated with the Salinic unconformity and those related to the migrating Acadian structural front (Fig. 3⇑). This raises the question of how to separate the tectonic events. It is important, however, to realize that all tectonic features in the Gaspé Belt can be explained in terms of a continued, convergent plate tectonic setting during Silurian–Devonian time, and all can be considered, at the limit, as Acadian-related. With the foreland basin tectonic setting, it is also important to consider that regional ENE- to NE-trending faults (e.g., Sainte-Florence, Shickshock Sud, Marcil faults; Fig. 2⇑) may have acted as normal during sedimentation before being inverted by the regional Acadian transpressive deformation.
I would like to acknowledge Pierre-André Bourque, Martin Doyon, Daniel Brisebois and Donna Kirkwood for their continuous collaboration and discussions in the field. Pierre-André Bourque and Donna Kirkwood kindly read the first draft of this paper. Formal reviews by Pierre Cousineau and Brian O’Brien were greatly appreciated. The National Sciences and Engineering Research Council of Canada is acknowledged for an operating grant.
↵1 GIRGAB, Groupe interuniversitaire de Recherches en Géodynamique et Analyse de Bassins.