Orogenic wedge or channel flow for the tectonic evolution of Parautochthonous Belt? New evidence from the Central Grenville Province

English Abstract:

The tectonic evolution of the Parautochthonous Belt is controversial. An orogenic wedge model was proposed based on detailed fieldwork conducted in the eastern Grenville Province, whereas synconvergent and postconvergent heterogeneous channel flow modes were based largely on geometrical similarities between observed crustal structures and prediction from numerical models. This study presents new field-based structural and geochronological data from the southwestern part of Manicouagan reservoir (Central Grenville Province) that provides important constraints on these hypotheses.

In the Parautochthonous Belt, three deformation phases have been mapped. D₂ structures were developed by oblique-reverse, top-to-the-NNE shearing. Pre- to syn- D₂ axial-planar leucosomes yielded a weighted mean average ²⁰⁶Pb/²⁰⁷Pb age of 992 ±22Ma (LA-ICPMS on zircons) and, one analysis gives a concordia ²⁰⁶Pb/²³⁸U age of 986 ±4Ma (TIMS). A cross-cutting D₂ gabbro, co-magmatic with a pegmatite, has a D₂ lineation defined by retrograde hornblendes. This late D₂ pegmatite gives a weighted average ²⁰⁶Pb/²⁰⁷Pb age of 961 ±22Ma (LA-ICPMS on zircons). D₃ structures have been found locally at upper structural levels of the belt and were formed by oblique-normal, down-to-the-SW shearing. Syn-D₃ leucosome yielded a concordia ²⁰⁶Pb/²³⁸U age of 993 ±3Ma (TIMS on zircons). In the Allochthonous Belt, two deformation phases have been mapped. The main transposition fabric was developed by reverse, top-to-the-NNW shearing associated with the 1080-1030 Ma granulite-facies metamorphism of this belt, whereas the second deformation is the newly-identified Thachic shear-zone (TSZ) developed by oblique-normal, down-to-the-SSE or E shearing. Pegmatitic dykes cross-cutting TSZ but having normal kinematic indicators and a TSZ lineation, yielded a Concordia ²⁰⁶Pb/²³⁸U age of 988 ±3Ma (TIMS on zircons).

Normal-sense shearing at the upper structural level of the Parautochthonous Belt and along the TSZ, was, therefore, synchronous within error with D₂ reverse deformation in the structurally underlying Parautochthonous Belt. These characteristics imply extrusive ductile flow and are thus compatible with a form of channel flow rather than an orogenic wedge.