Abstract

The Cabonga allochthon (central part of the Grenville Province) is a Proterozoic unit of high-grade metasedimentary and metaigneous rocks resting on top of reworked parautochthonous Archean migmatites of the Réservoir Dozois terrane. The geometry and final assembly of terranes in this area result from the following succession of strains and displacements: (1) thrusting towards the northwest of a Proterozoic terrane; (2) northwest – southeast shortening of the transported terrane as indicated by large-scale northeast-trending buckle folds and by coeval development of a décollement within the upper layers of the Archean footwall; (3) reactivating of this décollement in the form of an east-dipping thrust, the Cabonga thrust (piggy-back sequence); the detached basement and the Proterozoic allochthon forming the Cabonga thrust sheet; (4) northeast-trending sinistral wrenching in the parautochthon, responsible for the sinuous contour of the overlying Cabonga thrust and probably also for the westward thrusting itself.The first two stages are best understood in a regime of northwest-directed shortening tectonics, well documented in other parts of the Grenville Province. The last two stages are correlated with a regional sinistral wrenching affecting the Grenvillian parautochthon and thus the footwall of the Cabonga allochthon. During this episode, the southern part of the parautochthon, sinking towards the northeast, may have acted as a northeast-dipping indenter responsible for the westward thrusting, which can be visualized as a lateral extrusion of the Cabonga thrust sheet. The above succession of events suggests a switch from orthogonal to oblique collision tectonics.

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