AN UPGRADED STRAIN ANALYSIS METHOD APPLIED TO THE MANICOUAGAN IMBRICATE ZONE, CENTRAL GRENVILLE PROVINCE, QUEBEC, CANADA: EVIDENCE FOR REGIONAL CONSTRICTION

Understanding the tectonic processes at work in an orogen requires the comprehension of the kinematic evolution of its major structures. The Allochthon Boundary Thrust (ABT) is the main tectonic boundary of the Grenville Province: it juxtaposes the Allochthonous Belt, formed of exotic to peri-cratonic rocks that were deformed and metamorphosed during the first phase of the Grenvillian Orogeny (ca. 1090 to 1040 Ma), over the Parautochthonous Belt, formed of basement rocks of the Superior Craton and its metasedimentary sequence that were deformed and metamorphosed in the late phase of the orogeny (ca.1000 - 960 Ma). .

A peculiar feature of the ABT is found northeast of the Manicouagan Reservoir in the Manicouagan Imbricate Zone (MIZ), which encloses one of the rare occurrences of eclogite in the Grenville Province. There, the ABT has a NW-SE strike, perpendicular to its usual orogen-parallel orientation. Different hypotheses have been proposed to explain this geometry, such as late transpressional or transtensional folding or the extrusion and stacking of ductile nappes. One way of discriminating between them is by conducting a strain analysis, as the former two should have developed under a flattening-type of strain, whereas the latter should have developed under a constrictive-type of strain. Our structural and geochronological study in the northeastern part of the MIZ was completed by a compilation of data collected by previous researchers outside our study area. We have adapted a strain analysis method developed by Talbot and Sokoutis in 1995 by using structural elements, such as lineation and foliation, to reconstruct the strain ellipsoid. Our results indicate that the emplacement of the MIZ and the orogen-perpendicular geometry of the ABT was developed by constriction. This supports the previously proposed conceptual model of ductile extrusion and provides crucial constraints towards our understanding of long hot orogens such as the Grenville.

Reference: Talbot, C. J. and Sokoutis, D. (1995) Strain ellipsoids from incompetent dykes: application to volume loss during mylonitization in the Singö gneiss zone, central Sweden. Journal of Structural Geology 17, 921-948.