Ductile Nappe Extrusion in Constrictive Strain at the Origin of Transverse Segments of the Allochthon Boundary Thrust in the Manicouagan Imbricate Zone (Central Grenville Province, Québec)

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Separating the Parautochthonous and Allochthonous belts, the Allochthon Boundary Thrust (ABT) is a major lithotectonic boundary that generally runs parallel with the length of the Grenville Province, but with some transverse segments. In this contribution, we investigate one of these segments, the Manicouagan Imbricate Zone (MIZ), by adding new field-based data supplemented by new U–Pb deformation ages to a compilation of previous research conducted on both sides of the ABT in the region. Our results reveal striking differences between the footwall and hanging wall of the ABT in terms of strain and structures and concludes that the transverse orientation of the ABT is not related to folding. An updated strain analysis allows us to determine that deformation of the MIZ was dominated by a constrictive-type of strain. We build on a previously proposed conceptual model to document ductile extrusion over basement ramps driven by tectonic forcing and lateral density contrast in the upper crust at ca. 990 Ma. The insertion of the hot ductile nappe into partially molten metasediments of the surrounding Parautochthonous Belt was accommodated by constrictive strain within the former and flattening and transverse folding in the latter.