DELINEATION AND CHARACTERIZATION OF THE ALLOCHTHON BOUNDARY THRUST IN THE CENTRAL GRENVILLE PROVINCE (NORTH-EAST OF THE MANICOUAGAN RESERVOIR, QUEBEC, CANADA)

Reconstructing the tectonic evolution of ancient orogens requires understanding the kinematic evolution of their tectonic boundaries. In the Proterozoic Grenville Province, the Allochthon Boundary Trust (ABT) juxtaposes the Allochthonous Belt, composed of pericratonic to exotic rocks accreted on Laurentia that were deformed and metamorphosed during the first phase of Grenvillian Orogeny (the Ottawan at 1090-1040 Ma), to the underlying Parautochthonous Belt, composed of rocks of the Superior Province reworked during the latest phase (the Rigolet at 1000-960 Ma). Conceptual tectonic models proposed that the ABT has played a significant role in both phases of the orogeny as well as during the gravitational collapse phase that would have occurred in between, making it an important study target.

The ABT marks the northern boundary of the Manicouagan Imbricate Zone (MIZ), located north of the Manicouagan Reservoir, in Eastern Québec. The MIZ is peculiar in that it constitutes one of the rare occurrence of eclogite-facies rocks in the Grenville Province, which contrast with medium-pressure rocks located at the same structural level in the hanging wall of the ABT south of the reservoir. Furthermore, rather than its typical NE-SW, orogen-parallel strike, the ABT in this area has a NW-SE strike. The scope of our study is to delineate the ABT, determining its kinematics and constrain the timing of deformation in in both its footwall and hanging wall.

Our fieldwork conducted in 2017 allowed us to accurately delineate, for the first time, the ABT in this area. Major differences between Allochthonous and Parautochthonous rocks include a higher qualitative strain and a high amount of modal garnet in all rock types (10-40%) in the former and evidence for melt-present deformation in garnet-free rocks of the latter. We also present results of a macro- and micro-structural analysis that we aim at linking with U-(Th)-Pb geochronology of monazite conducted in-situ with a scanning electron microscope equipped with a field emission gun to constrain ssierthe age of deformation. Because of the key location of the study area (bordering the HP-MIZ and an orogen-perpendicular strike), these results will provide crucial constraints on the tectonic evolution of the Grenvillian Orogeny.