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Paleostress Analysis of Atlantic Crustal Extension in the Quebec Appalachians

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Paleostress Analysis of Atlantic Crustal Extension in the Quebec Appalachians Paleostress Analysis of Atlantic Crustal Extension in the Quebec Appalachians Ste´phane Faure,1 Alain Tremblay,2 Michel Malo,3 and Jacques Angelier4 Consortium de Recherche en Exploration Mine´rale, Universite´ du Que´bec a` Montre´al, C.P. 8888 succ. Centre-ville, Montre´al, Que´bec H3C 3P8, Canada (e-mail: [email protected]) ABSTRACT A paleostress analysis using normal fault slip data and Jurassic dike trends has been conducted in the Quebec and New England Appalachians. Stress tensor analysis of fault slip by a numerical inversion method has distinguished homogeneous and directionally consistent E-W- and NW-SE-oriented extensional stress regimes in the Quebec Ap- palachians and St. Lawrence Lowlands. The directions of extension compare well with the mean direction of exten- sional stress inferred from the Jurassic dolerite dike trend in New England. Both extensions are interpreted as the result of rift-related stress associated with the separation of North America and Africa, early in the Late Triassic– Early Jurassic fragmentation of Pangea. The paleostress field configuration is assumed to be related to the direction of divergent plate motion. The N-S and NE-SW Late Proterozoic faults that bordered the Quebec Reentrant and Taconian-Acadian structures of the Quebec Appalachians appear to have played a significant role in the distribution of later stress regimes in the upper part of the lithosphere. The NNE-SSW-trending regional and mesoscopic normal faults in southern Quebec are interpreted as linked to the formation of Mesozoic rift basins in Massachusetts and Connecticut and synchronous with Early and Middle Jurassic dike emplacement in the Quebec and New England Appalachians. Introduction Structural and sedimentological features related to faults related to Late Triassic–Jurassic extension are the opening of the North Atlantic Ocean are well well-documented offshore. On land however, ex- recorded along the east coast of maritime Canada tensional features are limited to the margin of the and the United States. Seismic and marine geology northeastern edge of North America (fig. 1). In the and oil exploration and deep-sea drilling projects Quebec Appalachians, Jurassic extension is re- have provided essential information concerning the corded by dikes intruded at distances as great as development and evolution of continental crust 300 km west of the continental edge (fig. 1), but no fragmentation processes that led to the formation other type of structures have been clearly assigned of this major ocean. Sedimentary basins and normal to this continental extension. In this article, we address the problem of Atlan- tic-type rifting through an extensive paleostress analysis of normal faults that are chronologically Manuscript received March 21, 2005; accepted December 12, 2005. and mechanically consistent with opening of the 1 Author for correspondence. North Atlantic Ocean. In the Quebec Appala- 2 De´partement des Sciences de la Terre et de l’Atmosphe`re, chians, the presence of such brittle faults and dikes Universite´ du Que´bec a` Montre´al, C.P. 8888 succ. Centre-ville, implies that a widespread intraplate deformation Montre´al, Que´bec H3C 3P8, Canada. 3 Institut National de la Recherche Scientifique–Eau, Terre occurred in Jurassic time, and these faults and dikes et Environnement, Centre Ge´oscientifique de Que´bec, 880 record deformational processes that are otherwise Chemin Sainte-Foy, C.P. 7500, Que´bec, Que´bec G1V 4C7, difficult to document. Our results are compared Canada. with extension directions deduced from the ge- 4 Observatoire Oce´anologique de Villefranche-sur-Mer, Universite´ Pierre et Marie Curie, Port de la Darse, C.P. 48, 06235 ometry of rift basins and dike swarms in north- Villefranche-sur-Mer, France. eastern North America and with paleostress anal- [The Journal of Geology, 2006, volume 114, p. 435–448] ᭧ 2006 by The University of Chicago. All rights reserved. 0022-1376/2006/11404-0003$15.00 435 436 S. FAURE ET AL. Figure 1. Distribution of Late Triassic–Jurassic plutons, dikes, rift-basins, and faults in the northern Appalachians and adjacent regions (symbols on faults indicate dip directions). Precambrian faults reactivated as normal faults in the Mesozoic.AID p Anticosti Island dikes,AD p Avalon dike,CD p Caraquet dike,FB p Fundy basin, HB p Hartford basin,HID p Hog Island dike,LCHV p Lake Champlain–Hudson River valleys,MD p M e´gantic dike; MRF p Metawee River Fault,PVD p Picton and Varty Lake dikes,PS p Palisade sills,SD p Shelburne dike, SLRV p St. Lawrence River valley, andWMMS p White Mountain Magma Series. yses conducted elsewhere in the northern rocks outcropping in the St. Lawrence Lowlands Appalachians. and the Humber and Dunnage zones (St-Julien and Hubert 1975; Williams 1979), Upper Ordovician to Regional Geology Devonian rocks of the Gaspe´ Belt (Bourque et al. 1995), and Carboniferous cover rocks of the Mari- The Quebec Appalachians consist of three princi- time Basin (Bradley 1982). In the northern Appa- pal lithotectonic assemblages: Cambro-Ordovician lachians (fig. 1), the present-day location of the Journal of Geology P ALEOSTRESS ANALYSIS 437 Quebec Reentrant and the New York and St. Paleostress Tensor Analysis Lawrence promontories is interpreted as an inher- ited geometry of the western margin of early Pa- To determine the paleostress orientations of minor leozoic Iapetus Ocean (Thomas 1977; Stockmal et fault slip data, the numerical inversion method de- al. 1987). In late Precambrian time, rift-related nor- veloped by Angelier (1984, 1989, 1994) was used. mal faults trending approximately N-S and NE-SW Shear sense indicators on minor brittle faults were developed along the Laurentian craton, parallel to established based on criteria suggested by Hancock the Lake Champlain–Hudson River valleys and to (1985) and Petit (1987). The principle behind this method was first proposed by Carey and Brunier the St. Lawrence River valley, respectively. Since (1974) and was based on the assumptions of Wallace that time, these deep-seated structures have played (1951) and Bott (1959) that the direction and sense a major role in the tectonic evolution of the Ap- of shear stress along a fault correspond to a single palachian Belt. Some of these normal faults were common stress tensor. The results are expressed in reactivated as thrust faults in southern Quebec or terms of reduced stress tensors that give the atti- as strike-slip faults in Gaspe´ and are suggested to tude of the maximum (j1), intermediate (j2), and have served as planes of weakness for the devel- minimum (j3) compressional stress axes (j1 1 opment of major structures related to the Middle- j2 1 j3, with pressure considered positive) and in Late Ordovician Taconian (St-Julien et al. 1983; terms of the F ratio (F p [j2 Ϫ j3]/[j1 Ϫ j3] ), which Spencer et al. 1989; Hayman and Kidd 2002; Se´- quantifies the variation in principal stress magni- journe´ et al. 2003) and/or the Middle Devonian Aca- tudes. The latter ratio varies from 0 (j2 p j3 ) to 1 dian orogenies (Malo and Bourque 1993). After the (j1 p j2 ), which corresponds to prolate (cigar- Acadian Orogeny, the Quebec Appalachians expe- shaped) and oblate (pie-shaped) stress ellipsoids, re- rienced intraplate brittle deformation related to the spectively. The average angle of the largest possible Laurentia-Gondwana collision (Faure et al. 1996a; values of computed shear allows determination of Jutras et al. 2003a) during the Alleghanian Orogeny the quality of the results for each data set, and the (Early Permian), the youngest tectonic pulse of the consistency of each fault slip datum with the mean stress tensor is checked using individual misfit. For Appalachian Orogeny (Williams and Hatcher 1983). a discussion of the calculation methods and their In Mesozoic time, the Appalachian Belt was sub- accuracy, see Etchecopar et al. (1981), Angelier jected to an extensional stress regime associated (1984, 1989, 1990, 1994), Guiraud et al. (1989), Du- with the Late Triassic–Early Jurassic breakup of pin et al. (1993), Gapais et al. (2000), and Yamaji Pangea and related tectonism and magmatism (2003). (McHone et al. 1987; Manspeizer et al. 1989). The In most cases, fault populations at single obser- Ma dikes on Anticosti Island vation sites correspond to a number of distinct- 8 ע existence of178 -stress regimes. Reliable determination of paleo - 1.4 ע Wanless and Stevens 1971) and of a195.5) Ma dike in southern Quebec (Roddick et al. 1992) stress regimes frequently requires the separation of is the only direct evidence for Jurassic extension in fault slip data into subsets that can be attributed the Quebec Appalachians. The Early-Middle Juras- to distinct tectonic events. The recognition of poly- sic White Mountain Magma Series (WMMS) in phase faulting is supported by crosscutting rela- New Hampshire forms a group of alkaline, intra- tionships between different generations of struc- plate, and nonorogenic plutons that outline an tures (faults, veins, folds, and dikes) and by elongated N-S pattern parallel to the Lake Cham- superimposed sets of slickenside lineations indi- cating successive slips on fault planes. Polyphase plain–Hudson River valleys (fig.1; Manspeizer et al. tectonism commonly results in apparently large 1989). Cretaceous magmatism belonging to the misfits. In sites where such inhomogeneous data New England–Quebec Igneous Province occurred sets are found, computed separations of stress ten- in southern Quebec (Monteregian plutons) as well sors and related subsets of fault slip data were per- as in Vermont and Maine (dikes), and it represents formed following a procedure proposed by Angelier the youngest tectonomagmatic event (McHone and Manoussis (1980) and discussed by Angelier 1978; Foland et al. 1986). At present, the St. (1984). Lawrence River and Champlain Lake–Hudson Qualitative determination of j3 axes from dike River valleys are among the most seismically active trends yields complementary information for char- regions in northeastern North America, where neo- acterizing the paleostress directions.
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