ELSEVIER Tectonophysics 267 (1996) 239-256 TECTONOPHYSICS Extension versus shortening models for hinterland-directed motions in the southern Quebec Appalachians N. Pinet a, A. Tremblay a, M. Sosson b " bzstitut National de la Recherche Scientifique - G~oressource, 2535 Bd. Laurier, Case postale 7500, Sainte-Foy, Quebec, G1V-4CT, Canada b lnstitut de G~odynamique, CNRS Sophia-Antipolis, Rue A. Einstein, 06560 Valbonne cedex, France Received 8 June 1995; accepted 22 April 1996 Abstract In the Taconian internal zone of southern Qurbec, structures related to hinterland-directed motion are found on both limbs of the Sutton-Notre-Dame mountains anticlinorium (D3we~ t and D3Ea~ t structures) and along the St-Joseph fault. These ductile to brittle-ductile structures were formed during upper- to mid-greenschist grade metamorphic events, thus suggesting that the area occupied a mid-crustal position during hinterland motion. Layer-extensional fabrics along the eastern limb of the Sutton-Notre-Dame mountains anticlinorium can be interpreted either as structures contemporaneous to backthrusting deformation rooted on the western limb of the anticlinorium or as structures formed during normal faulting along the St-Joseph fault. The study of layer-extensional structures found in the Taconian internal zone of the southern Qurbec Appalachians shows that it is not obvious to infer wether normal-sense fabrics that occur in orogenic hinterlands are related to regional crustal extension or to local syn-collisional extensional faulting. 1. Introduction In the internal zones of orogens characterized by polydeformed metamorphic rocks, the distinction be- tween extensional and shortening structures may be problematic because the significance of kinematic indicators can be obscured by tilting of shear planes during subsequent folding (e.g., Wheeler and Butler, 1994). Moreover, the juxtaposition of low-grade metamorphic rocks over higher-grade rocks is not necessarily related to normal faulting and can be ascribed to older tectonic events. The precise deter- mination of the age of motion in the internal zones of most orogenic belts depends on sparsely dis- tributed paleontologic, thermo-barometric and ra- diochronologic data. In the Quebec Appalachians, Cambro-Ordovician rocks belonging to the North American (Laurentian) margin have been mainly affected by Middle to Late Ordovician metamorphism and structures attributed to the Taconian orogeny (St-Julien and Hubert, 1975; Tremblay and Pinet, 1994a) consistent with: (1) the age of olistostromal deposits associated with thrust- ing in the Appalachian foreland (St-Julien and Hu- bert, 1975); (2) radiometric data from correlative rocks of New England (Laird et al., 1984; Sutter et al., 1985); and (3) the presence of major Ordovician unconformities (Pavlides et al., 1968). East of the 0040-1951/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S0040- 1951(96)00096-0