Atlantic Geology 201 Alleghanian faulting in the southern Gaspé Peninsula of Quebec P. Jutras,1 G. Prichonnet,2 and S. McCutcheon3 1. Department of Geology, Saint Mary’s University, Halifax, NS B3H 3C3 <[email protected]> 2. GEOTERAP, Département des Sciences de la Terre et de l’Atmosphère, Université du Québec à Montréal, C.P. 8888, Succ. Centre-ville, Montréal, QC H3C 3P8 3. New Brunswick Department of Natural Resources, P.O. Box 50, Bathurst, NB E2A 3Z1 Date received: July 11, 2003 ¶ Date accepted: January 7, 2004 ABSTRACT Transcurrent faults and associated compressional structures affecting post-Middle Devonian rocks are recognized for the first time in the southern Gaspé Peninsula (Quebec). This region was previously thought to have experienced only minor normal fault readjustments after the Middle Devonian Acadian Orogeny. Four SW-striking fault systems with post-Acadian sinistral motion that have been identified along the north shore of Chaleur Bay are described here. These faults are kinematically compatible with large NW-striking dextral faults in eastern Gaspé, and suggest that these struc- tures reflect a widespread Alleghanian paleostress system that probably affected the entire Quebec Appalachians. RÉSUMÉ Des failles de décrochement et les structures compressives qui leur sont associées affectent des roches post-Dévonien moyen dans le sud de la Gaspésie (Québec) et sont reconnues pour la première fois. Cette région était auparavant considérée comme n’ayant connu que des réajustements mineurs de failles normales après l’orogenèse Acadienne (Dévonien moyen). Quatre systèmes de failles orientées SW-NE ont connus des déplacements senestres post-acadiens et sont analysés dans cet article. Ces failles sont compatibles avec de grands décrochements dextres orientés NW-SE dans l’est de la Gaspésie et suggèrent que ces structures sont associées à des paléostress alléghaniens ayant possiblement affecté les Appalaches québécoises dans leur totalité. INTRODUCTION GEOLOGICAL SETTING Following identification of post-Acadian (post-Middle Pre-Carboniferous rocks in southern Gaspé range in age Devonian) paleostress indicators in the Quebec Appalachians from Neoproterozoic to Early Devonian (Brisebois et al. 1992). by Faure et al. (1996), significant Alleghanian deformation was Upper Neoproterozoic to Cambrian metasedimentary and vol- recently reported in the eastern Gaspé Peninsula near the town canic rocks of the Maquereau Group were first deformed by of Percé (Jutras et al. 2003) (Fig. 1, inset). The present paper the Late Cambrian to Early Ordovician Gaspesian Orogeny reveals that transcurrent and associated compressive struc- (Ayrton 1967). Middle Ordovician sandstone and mudrock tures also affected the narrow belt of Carboniferous rocks that of the Mictaw Group were first deformed by the Middle is exposed on the northern shore of Chaleur Bay, in southern Ordovician Taconian Orogeny, which for most authors Gaspé (Fig. 1). The eastern Gaspé structures are dextral faults includes the Late Cambrian to Early Ordovician Gaspesian striking NW to NNW, but the southern Gaspé structures dis- deformation as an early phase of the Taconian Orogeny cussed herein sinistral faults striking NE (Figs. 2 and 3). (Rodgers 1967; St-Julien and Hubert 1975; de Broucker 1987). This paper provides a structural analysis of faults affecting Upper Ordovician to Lower Devonian sedimentary rocks of the Carboniferous rocks in southern Gaspé, which contains the Honorat, Matapedia and Chaleurs groups, as well as volcanic northwesternmost rock exposures of the composite Upper rocks of the latter group, were first deformed by the Early to Devonian to Permian Maritimes Basin (Fig. 1; inset). The fault Middle Devonian Acadian Orogeny (Malo and Béland 1989; traces tend to form well defined scarps because of the juxta- Malo et al. 1992, 1995; Malo and Kirkwood 1995; Bourque et position of poorly-indurated post-Acadian rocks with more al. 2000; Kirkwood et al. 1995) (Figs. 2 and 3). resistant pre- and syn-Acadian rocks. Hence, some geomorphic The post-Acadian succession in southern Gaspé starts with descriptions are included, as they help to delineate the faults. the Frasnian Miguasha Group (Brideaux and Radforth 1970; Atlantic Geology 39, 201–211 (2003) 0843-5561|03|03201-11$2.65|o 202 P. Jutras et al. Atlantic Geology 203 Fig. 1 Simplified geology of the southern Gaspé Peninsula (modified from Briseboiset al. 1992) showing only post-Acadian rela- tionships, with map areas of figures 2 and 3 outlined. Inset is modified from Giblinget al. (1992). Hesse and Sawh 1992; Prichonnet et al. 1996), which was gently Carleton area (Fig.2) have been attributed to a ~600 m nor- deformed prior to deposition of the Late Devonian to Early mal splay of that fault system (Bernard and St-Julien 1986), Mississippian Saint-Jules Formation (Jutras and Prichonnet although no kinematic indicators were documented. Gosselin 2002) (Fig. 4). The latter occupies approximately the same (1988) also postulated that the Bonaventure Formation was stratigraphic position as the Fountain Lake Group of Nova affected by a normal splay of the Grande-Cascapédia Fault Scotia and is also lithologically similar. (Fig. 2) on account of its anomalously steep dip adjacent to the The overlying fanglomerates of the La Coulée Formation of associated fault scarp (Fig. 5). eastern Gaspé (Jutras et al. 1999), which are stratigraphically Gosselin (1988) considered the converging Petit-Montréal, constrained to the Viséan (Jutras et al. 2001), have not been Mont-St-Joseph and Grande-Cascapédia faults (Grande- recognized in southern Gaspé. However, the groundwater cal- Cascapédia fault system) as steeply dipping late Acadian cretization event that is contemporaneous with this formation reverse faults that were active subsequent to regional Acadian affected the southern Gaspé area as well, above the Saint-Jules folding, but also reports evidence for sinistral and, to a lesser Formation and unconformably below the Viséan Bonaventure degree, dextral movement along subordinate structures of the Formation (Jutras et al. 1999, 2001; Jutras and Prichonnet deformation corridor. The possibility that the present disposi- 2002) (Fig. 4). The calcrete locally digested a karstified regolith tion of the Mississippian strata could have been related to that developed in the upper beds of the Saint-Jules Formation, sug- reverse splay, rather than to a subsequent normal splay, has gesting that a significant hiatus separates the Saint-Jules and La never been evaluated. Coulée formations (Jutras and Prichonnet 2002) (Fig. 4). Near Carleton (Fig. 2), the measured dips of Mississippian The youngest sedimentary rock unit in southern Gaspé is beds increase from east to west in the proximity of the Grande- the early Namurian Pointe Sawyer Formation (Fig. 4), which Cascapédia fault system, until they become slightly overturned has very limited exposure (Fig. 3) and which disconformably (Fig. 6a). The strikes of these beds rotate counter-clockwise overlies the Bonaventure Formation. This unit was correlated along with the dip increase (Fig. 2a). with the early Namurian Mabou Group of Nova Scotia on the The subvertically tilted Mississippian beds that are exposed basis of spore-age and lithological similarities (Jutras et al. near the Grande-Cascapédia fault system at Carleton, which is 2001) (Fig. 4). not exposed at its contact with Mississippian rocks, are affected The entire post-Acadian succession of eastern Gaspé, up to by brittle fault planes (n=5) striking NNE plastered with sinis- the Namurian Chemin-des-Pêcheurs Formation (Jutras et al. tral slickensided calcite fibers (Figs. 6b and 2, stereonet 1; Table 2001), which is not exposed in southern Gaspé, was affected by 1). One WNW-striking dextral fault plane with a similar fabric compressive deformation and kilometric strike-slip displace- was also identified (Fig. 2, stereonet 1; Table 1). Moreover, ments (Jutras et al. 2003). Four additional strike-slip fault cor- two minor east-striking reverse fault planes plastered with ridors recognized in the post-Acadian succession of southern calcite slickenfibers were identified in Mississippian rocks of Gaspé are described below. the nearby Saint-Jules-de-Cascapédia quarry (Fig. 2, stereonet 2; Table 1). The described structures are incompatible with the normal THE GRANDE-CASCAPÉDIA fault hypothesis formulated by previous authors (Bernard and FAULT SYSTEM St-Julien 1986; Gosselin 1988). However, they are compatible with sinistral movement along the Grande-Cascapédia fault Steeply dipping strata of the Bonaventure Formation system, with the sinistral and dextral shears (Fig. 2, stereonet along the Petit-Montréal and Mont-Saint-Joseph faults in the 1) being interpreted as, respectively, R and R’ synthetic Riedel 202 P. JutrasP. et al. et Atlantic Geology Fig. 2 Geology of the southwestern Gaspé Peninsula (modified from Jutras and Prichonnet 2002), together with orientations of fault planes (great circles) and slick- enfibers (dots and dotted arrows) in (1) vertically uplifted Mississippian strata near the Grande-Cascapédia fault system, (2) groundwater calcrete of the Saint-Jules quarry, and (3, 4, 5, and 6) the Black Cape Volcanics. Mapping of pre-Mississippian units was by Bourque and Lachambre (1980) and Gosselin (1988). Inset a: sketch of 203 the drag fold near Carleton. Inset b: enlargement of the Black Cape area. Inset c: interpretation of the subsidiary structures of the Black Cape Fault (bold axis). 204 P. Jutras et al. Atlantic