LES GRANDS COULOIRS DE DEFORMATION DE LA SODS-PROVINCE DE L' ABITIBI

Special Volume, Vol. SV 43, No. 1990, 1990
Un trait marquant du cadre tectonostratigraphique de la Sous province de l'Abitibi est la presence de nombreux couloirs de deformation d' extension regionale et sub-concordants au grain stratigraphique. La presence de grandes masses granitoides qui perturbent l' allure du grain tectonique est typique de la partie nord de la Sous province de l'Abitibi. Des rides E-W constituees de complexes plutoniques precoces forment des reliefs structuraux agissant comme
des moles resistants qui interferent avec la deformation regionale. Deux types de couloirs de deformation sont distingues, les couloirs de bordure et les discontinuites structurales. Les couloirs de bordure representent des aureoles de deformation elevee qui s' accompagnent generalement d' un metamorphisme au facies amphibolite. lls suivent l'interface entre les roches granitoides et les assemblages supracrustaux. Ces couloirs sont de puissance kilometrique et sont confines, surtout au sein des roches vertes, a l' interface des complexes granitoides d'importance. La limite entre Ia Sous-province de l'Opatica et les roches supracrustales de l'Abitibi est caracterisee par un important couloir de bordure issu possiblement d' un chargement tectonique irregulier et concentre lors du raccourcissement horizontal.
Les discontinuites structurales sont des failles ductiles qui se propagent surtout en direction E-W mais avec des variantes de directions SE et NE. Les discontinuites E-W se caracterisent par la presence d' une lineation d'etirement a fort angle de chute. Les discontinuites SE sont caracterisees par des lineations d'etirement horizontales et directionnelles et les mouvements observes sont systematiquement dextres. Les discontinuites NE sont des failles a comportement beaucoup plus fragile et se distinguent par des composantes de mouvement senestre. Elles sont tardives par rapport aux discontinuites E-W et SE et ne representent pas la symetrique du
systeme SE.
La combinaison des systemes E-Wet SE decrit un motif anastomose a l' echelle regionale. Les failles ductiles se rejoignent de facon asymptotique, isolant des blocs peu ou pas affectes par la deformation. L' anastomose est un phenomene qui se produit a toute echelle et est reliee principalement aux Mterogeneites et aux anisotropies 1) des forces agissantes (champs de contraintes regionale etlocale) et 2) de la reponse del' empilement supracrustal (compositions differentes, fabrique initiate, proprietes mecaniques variees, niveaux
d' alteration).
Dans le but de respecter un raccourcissement horizontal N-S, differents modeles sont proposes; ils mettent en cause soit une transpression dextre, soit des failles inverses reactivees en failles decrochantes dextres soit des failles de chevauchement redressees.

Numerous regional deformation corridors, sub-concordant to the stratigraphic trend, are a fundamental element in the tectonostratigraphic framework of the Abitibi Subprovince. The northern zone of the Abitibi greenstone belt is typified by the presence of large granitoid masses which disturb the tectonic trend. Early plutonic complexes within E-W ridges form structural highs acting as resistant mohls and interfering with the regional stress field. Two types of deformation corridors can be distinguished in the Abitibi Belt: border corridors and structural discontinuities. Border corridors follow the granitoid-supracrustal boundary and represent "contact strain aureole" accompanied largely by amphibolite facies metamorphism. They attain kilometric width and are mostly confined within greenstone rocks near the margin of the large granitoid complexes. The northern limit between the supracrustal sequence of the Abitibi Belt and the OpaticaBelt is characterized by an important high strain zone possibly related to an irregular and concentrated tectonic loading during the horizontal shortening. Structural discontinuities correspond to a series of large eastwest trending shear zones, sometimes curving or branching out in a southeastern and/or northeastern direction. The east-west system is characterized by a strong down dip stretching lineation. The southeast discontinuities are characterized by horizontal stretching lineations and the sense of movement is systematically dextral. The north-east discontinuities are faults with a brittle character and show a component of sinistral movement. They are younger than the E-W and SE discontinuities and are not part of the symmetrical system associated with theSE discontinuities.
Geometric relations between the different systems create an anastomosing pattern where the shear zones join asymptotically, separating blocks which show little or no deformation. Anastomose, which is found at all scales, is a phenomenon essentially related to the heterogeneity and the anisotropy of 1) active forces (regional and local stress fields) and 2) response of the suprascrustal pile ( compositional differences, original fabric, variation in mechanical properties, alteration, etc)
In an attempt to respect a north-south horizontal shortening, different models will be proposed. They will either imply a dextral transpressionnal regime, high angle reverse faults reactivated in dextral transcurrent faults, or verticalized thrust faults .
Keywords: Deformation, Abitibi, Quebec, Failles ductiles, Mines
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