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_Journal of Structural Geology 32 (2010) 941e959_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _Tectonic implications for a Cordilleran orogenic base in the Frenchman Cap dome, southeastern Canadian Cordillera_ _F?lix Gervais a,*, Richard L. Brown a, James L. Crowleyb a Department of Earth Sciences, Carleton University, Ottawa, ON, Canada K1S 5B6 b Department of Geosciences, Boise State University, Boise, ID 83725, USA_ _article info_ _Article history: Received 26 December 2009 Received in revised form 6 May 2010 Accepted 13 May 2010 Available online 20 May 2010_ _Keywords: Gneiss dome UePb geochronology Strain gradient Frenchman Cap dome Southern Canadian Cordillera_ _abstract_ _The Frenchman Cap gneiss dome of the Monashee Complex in the Canadian Cordillera sits in basement rocks of the orogen. It records a stepwise downward disappearance of penetrative deformation indicative of a frozen downward-migrating base of easterly verging Cordilleran shearing. This tectonic setting is incompatible with the commonly held views that gneiss domes of the Canadian Cordillera are extensional core complexes and that the presence of gneiss domes in orogens implies vertical ?ow. An important structural-time marker in our study is a widely distributed suite of w1850 Ma granite dykes that allow Cordilleran-aged structures to be distinguished from the older structures. The dykes show that only the uppermost w1.5 km structurally thick carapace of basement gneiss records penetrative Cordilleran strain, whereas the lowermost w5 km thick basement section does not and instead preserves a Paleoproterozoic migmatitic gneissosity. Cordilleran high strain in the upper basement carapace is characterized by penetrative easterly verging shear strain on both the westerly dipping and easterly dipping ?anks of the dome, whereas Cordilleran deformation in the lower basement is limited to a meter-scale, top-to-the-east shear zone and NNE-trending, upright folds. New and previously published UePb data from accessory minerals indicates that the Cordilleran structures formed between 53 and 49 Ma, immediately prior to regional cooling and extension. The dome is interpreted as an incipient upright drag fold developed during top-to-the-east shearing._ _? 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Gneiss domes are common structures of orogens that offer a window into deep crustal processes. In a review of gneiss domes, Whitney et al. (2004) argued that they are a manifestation of crustal ?ow, irrespective of the mechanism that led to their formation (e.g., folding, rolling-hinge, diapirism). Several gneiss domes are exposed in the southeastern Canadian Cordillera, notably the Frenchman Cap and the Thor-Odin domes of the Monashee Complex, and the China, Passmore, and Valhalla domes of the Valhalla Complex. The origin of these domes has generally been related to extension (Carr et al., 1987; Parrish et al., 1988; Vanderhaeghe et al., 1999; Kruse and Williams, 2007), and the Thor-Odin dome was presented as a type-case of a dome formed by vertical ductile ?ow of partially molten middle crust during the gravitational collapse of a thickened orogen (Teyssier and Whitney, 2002; Whitney et al., 2004)._ _* Corresponding author. Present address: Department of Earth & Planetary Sciences, McGill University, Adams Building, 3450 University Street, Montreal, QC, Canada H3A 2A7. Tel.: ?1 514 398 5884._ _E-mail address: felix.gervais@mcgil.ca (F. Gervais)._ _0191-8141 $ e see front matter ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016 j.jsg.2010.05.006_ _Teyssier et al., 2005). However, studies of the adjacent Frenchman Cap dome point to a drastically different history. Crowley et al. (2001) documented a downward-decreasing strain gradient in the dome based on the deformation state of Paleoproterozoic granite and pegmatite dykes (Crowley et al., 2008). These data led to models of progressive downward migration of the base of convergent ductile shearing with time (Parrish, 1995; Gibson et al., 1999; Crowley et al., 2001; Brown, 2004). Inasmuch as studies from the Thor-Odin dome were pivotal to deriving a paradigm of gneiss dome formation and orogenic collapse (Vanderhaeghe and Teyssier, 2001; Teyssier and Whitney, 2002; Whitney et al., 2004; Teyssier et al., 2005), it is crucial to assess the validity of the contradictory conclusions derived from the Frenchman Cap dome._ _The ?rst aim of this study is to determine whether the granite dykes dated by Crowley et al. (2001, 2008) could be used as time markers to determine the age of deformation throughout the basement of the Frenchman Cap dome of the Monashee Complex. The second aim is to better characterize the strain gradient reported by Crowley et al. (2001) and assess the nature and age of deformation from other parts of the dome, including those at the deepest structural level. Finally, a tectonic model for the dome is proposed._ _942_ _F. Gervais et al. Journal of Structural Geology 32 (2010) 941e959_ _2. Geological setting_ _The Monashee Complex occupies the deepest structural level of the Omineca Belt that forms, along with parts of the Coast Plutonic Belt, the core zone of the Canadian Cordillera (Fig. 1). The belt is east of the Insular, Coast Plutonic and Intermontane belts, which consist of pericratonic and exotic terranes accreted to the western margin of the North American craton in the Jurassic to Early Cretaceous (Evenchick et al., 2007). To the east lies the Foreland Belt, which records 200 km of shortening between 75 and 59 Ma (Price and Sears, 2000). The Canadian Cordillera was thus under compression from the Jurassic to the Paleogene. The end of orogeny in the southern Omineca Belt is marked by 1) 53e48 Ma 40Ar 39Ar mica cooling ages recorded throughout (Scammell, 1993; Bardoux, 1994; Johnson, 1994; Sanborn, 1996; Vanderhaeghe et al., 2003; Ghent and Villeneuve, 2006); 2) the 51e47 Ma bracket for the last increments of ductile shearing along two outward-dipping normal brittle-ductile shear zones that bound the belt (Glombick et al., 2006; Mulch et al., 2006); and 3) the w48 Ma emplacement age of undeformed lamprophyre dykes with chilled margins (Adams et al., 2005). The 53e47 Ma period was thus one of regional cooling accompanied with extension._ _The Monashee Complex consists of two gneiss domes: Frenchman Cap dome (FCD) in the north and Thor-Odin dome (TOD) in the south (Fig. 1). It is fault-bounded, with a westerly dipping, top-to-the-east ductile shear zone termed the Monashee d?collement, on its west ?ank; and an easterly dipping, top-to-the-east brittle-ductile shear zone, termed the Columbia River fault, on its east ?ank (Figs. 1 and 2). The complex is ?anked by the sillimanite-K-feldspar grade rocks of the Lower Selrkirk allochthon in the hanging wall of the Monashee d?collement, and the garnet grade rocks of the Upper Selkirk allochthon in the hanging wall of the Columbia River fault (see Gervais, 2009 for more details about these terms). The domes are cored by Paleoproterozoic gneiss, termed basement, and mantled by metasedimentary rocks, termed cover sequence, that are inferred as having been deposited on the passive margin of North America in the Precambrian (Scammell and Brown, 1990; Crowley, 1999). The base of the cover sequence consists of a quartzite layer that delineates the shape of the domes, and thus constitutes an important structural marker (Fig. 1). The cover sequence is infolded with pervasively deformed basement slices, whereas basement rocks prevail below the basal quartzite (Fig. 1)._ _2007), and zircon dates from migmatites pointed to a major partial melting event at w56 Ma (Vanderhaeghe et al., 1999). In contrast, the Frigg Glacier area (F in Fig. 1), located at one of the deepest structural level of the TOD or w2.5 km structurally below the basal quartzite, consists of basement orthogneiss folded by NeS trending folds with subhorizontal axes and subvertical axial planes that are parallel to leucosome veins (Blattner, 1971). An aplitic dyke from this locality crosscuts all structures except the NeS trending leucosome veins (Kuiper, 2003). Isotope dilution thermal ionization mass spectrometry (ID-TIMS) UePb dating of zircon from this dyke yielded a discordia chord with an upper and lower intercepts of 1.8 and 0.5 Ga. As discussed by Kuiper (2003), if the upper intercept represents the crystallization age of the dyke, it would imply that the core of the TOD preserves a Paleoproterozoic migmatitic gneissosity._ _2.2. The Frenchman Cap dome_ _The FCD is divided into four structural levels termed (from top to bottom) upper cover sequence, lower cover sequence, upper basement, and lower basement (Fig. 2). All four structural levels are exposed in the moderately steep west ?ank, whereas the upper cover sequence is not exposed on the gently dipping, east ?ank. The upper cover sequence occurs immediately below the Monashee d?collement, has a structural thickness of w3 km, and comprises two fold nappes. It records east verging, penetrative ductile deformation and kyanite-K-feldspar grade metamorphism between 60 and 55 Ma (Parrish, 1995; Crowley and Parrish, 1999; Gibson et al., 1999; Crowley et al., 2001; Foster et al., 2004). The lower cover sequence has a structural thickness of w1.5 km and comprises the upright, lower limb of the lower fold nappe. It records sillimanite-grade metamorphism and high-temperature east verging deformation until w50 Ma, w5 Ma later than the upper level (Crowley and Parrish, 1999; Crowley et al., 2001). The upper basement consists of Paleoproterozoic para-and orthogneisses that record penetrative Eocene deformation (Journeay, 1986; Crowley et al., 2001), whereas the lower basement consists of the same lithologic units but they do not record penetrative Cordilleran deformation (Crowley et al., 2001, 2008). We will show that the upper basement has a structural thickness of w1.5 km and the exposed lower basem_ Ключевые слова: canadian, southern, foliation, top-to-the-northeast shearing, british columbia, hanging wall, canadian cordillera, passchier, host gneiss, geological survey, granite dyke, dipping, eocene monazite, image, solid earth, locality, dome, shear, attening boudinage, data, zircon, parrish, eocene, structural level, ludwig, upright, bourne granite, crystallization, scammell, strain ellipse, british, gravitational collapse, plotted, strain, transposition foliation, hinchey, field relationship, cap, eds, structural data, upper, easterly, geological, axial plane, cordilleran strain, geological association, melting, northern toe, hornblendebearing leucosome, southeastern, regional cooling, time marker, lineation, level, tectonics, columbia, glacier, structural-time marker, perry river, completely transposed, tectonic setting, strain localization, energy mines, west ank, map, thor-odin dome, wheeler, university, cover sequence, study, cordillera, selkirk allochthon, age, sequence, drag fold, orogenic base, monashee complex, rock, structure, brun, pb, crustal, earth, basement gneiss, experimental deformation, upper basement, monazite, structural, strain pattern, intercept, cordilleran tectonism, crowley, upright fold, geological society, gneiss, rosenberg, ma, augen gneiss, equigranular texture, teyssier, geology, crystallization age, cordilleran deformation, ? ma, structural thickness, subhedral shape, eld relationship, ?ank, thesis, upright folding, structural geology, vanderhaeghe, partial melting, deformation, brown, massachusetts institute, contractional eld, complex, area, carr, grey, gneiss dome, tectonic model, whitney teyssier, fold, wa, verging, perry, gneissosity, price, williams, aleinikoff, dyke, deformed, transition zone, chemical composition, bulletin, kuiper, prob, monashee dcollement, black rim, monashee, whitney whitney, uepb dating, basement, high level, titanite, zone, unit, frenchman cap, pettipiece lake, high angle, granite, bourne, simonetti, scale, paper, sample, high, frisby, journal structural, river, interpreted, grey granite, cover-basement contact, deformation history, grain, migmatitic gneissosity, analysis, leucosome, mineral, fcd, gervais, ductile, pegmatitic leucogranite, metamorphic, westerly dipping, uepb, frisby creek, bourne creek, cover, canada, bourne glacier, model, isoclinal fold, common, chemical, basement ramp, cover rock, journeay, shear zone, cordilleran metamorphism, channel, core, basal quartzite, teraewasserburg plot, suite, whitney, diffusion, basement orthogneisses, journal structural geology, journal, cordilleran, basement rock, stretching lineation, eew extension, shearing, creek, american, frenchman